Why do natural disasters occur? Part 2

What do you call this natural disaster in the image above? Cyclone? Typhoon? Hurricane? Well, they are all correct. What is interesting is the place you mainly reside in determines the label you describe this dangerous weather phenomenon.
But why though?
Why are there 3 different names for the same meteorological disaster appearance-wise from satellite images?
Why couldn't the meteorologists worldwide agree to assign a single name as are a majority of other natural disasters?
Storms are mother nature's most destructive weather events. Not only it can damage infrastructure, inundate communities, displace residents, lift heavy objects off the ground with ease and fling them long distances at speed but also kill every organism unfortunate to be in its path. To many, they can be the most terrifying and unpredictable, and the best cause of action is to evacuate within a given timeframe between the first reported sighting and predicted landfall and intensity. However, there are some people who would stay behind and weather the storm for news reporting or some illogical reason risking serious injury/death. But how dangerous are these cyclonic storms actually?

C. Meteorological Disasters 

i. Cyclones / Hurricanes / Typhoons 

https://en.wikipedia.org/wiki/Cyclone 
https://en.wikipedia.org/wiki/Tropical_cyclone
https://en.wikipedia.org/wiki/Typhoon

                                       

• In meteorology, a cyclone is a tremendous mass of air that rotates around a powerful centre of low atmospheric pressure. 
• The term 'cyclone' was coined by Henry Piddington in his 40 papers discussing tropical storms from Calcutta between 1836 and 1855 in the Journal of the Asiatic Society. Its word roots is defined as "coil of a snake". 
• Depending on its location and intensity, a cyclone may be referred to as hurricane, typhoon, tropical cyclone, cyclonic storm, tropical depression, or simply cyclone. 

Describe the structure of cyclones 

Tropical cyclones area regions of relatively low pressure in the Earth's troposphere, with the greatest pressure perturbations situating at low altitudes near the ocean surface. Symonds (2003) recorded the pressures at the centres of tropical cyclones are among the lowest ever at sea level. In 2007, the National Oceanic and Atmospheric Administration (NOAA) determined the temperature near the centre of tropical cyclones is greater than the surroundings at all altitudes, hence they are referred to as "warm core" systems. 

-- Wind field 

• The wind field of a tropical cyclone is characterised by air rotating swiftly around a centre of circulation, as well as drifting radially inwards. 
• Although the air may be calm at the outer edge of the storm, Earth's rotation makes the air have non-zero absolute angular momentum. 
• As air flows radially inward, it starts to rotate cyclonically (anti-clockwise in the Northern Hemisphere, and clockwise in the Southern Hemisphere) to conserve angular momentum. 
• Air at the inner radius starts to rise to the top of the troposphere. The National Hurricane Center (NHC) (2016) described the powerful near-surface winds at the storm's inner radius of the eyewall or "radius of maximum winds". 
• The United States Naval Research Laboratory (USNRL) found air drifts away from the eye, creating a shield of cirrus clouds. This develops into a virtually axisymmetric wind field, which involve slow wind speeds at the centre, then accelerate with distance outwards to the radius of maximum winds, before deteriorating gradually with increasing radius. 
• Frank (1977) discovered the wind field demonstrates variability spatially and temporally because of the effect of localised phenomena, including thunderstorm activity and horizontal flow instabilities. In the vertical direction, winds are fastest near the surface and deteriorate with height within the troposphere. 

-- Eye (centre)

• Mayday episode: Into the Eye of the Storm 
  https://www.dailymotion.com/video/x7tqav0
• At the centre of a mature tropical cyclone, air plummets rather than elevates. In a sufficiently strong storm, the air may descend over a layer sufficiently deep to quash cloud formation, hence producing a clear centre (eye). 
• The National Weather Service (2005) described the weather in the eye as calm and relatively clear, and the sea as extremely rough. 
• Pasch et al. (2016) measured the eye as typically 30-65 km (19-40 mi) in diameter, though it can range between 3-370 km (1.9-230 mi). 
• The eyewall is the cloudy outer edge of the cyclone, which expands outward as it grows, which resembles an arena football stadium (i.e. stadium effect). In this section, the winds are the fastest, the air elevates most rapidly to their highest altitude, and precipitation is the heaviest. The National Weather Service (2005) recorded the devastating wind damage as the tropical cyclone’s eyewall hits landfall. 
• The American Meteorological Society (2011) observed the eye of weaker cyclones can be blocked by the central dense overcast. This central dense overcast is the upper-level cirrus shield linked to a dense area of powerful thunderstorm activity adjacent to the eye.
• Outer rain bands may transform into an outer ring of thunderstorms gradually moving inward, removing moisture and angular momentum from the primary eyewall. 
• The NOAA (2006) noted the tropical cyclone weakens along with the weakening of the primary eyewall. Furthermore, the outer eyewall eventually replaces the primary eyewall at the conclusion of the cycle, which may restore the storm’s original intensity. 

-- Rapid intensification 

• Rapid intensification is a meteorological event where a tropical cyclone strengthens significantly in a short space of time. 
• The NHC defined this event as an increase in the maximum sustained winds of a tropical cyclone of at least 55 km/h (30 knots, 35 mph) in a 24-hour period.

What conditions are required for rapid intensification?

• In order for rapid intensification to occur, certain conditions need to coincide with each other. 
• Water temperatures need to be near or above 30 ^oC  (86 ^oF), and the water at this temperature need to be sufficiently deep for waves unable to force deeper cooler waters up to the surface. 
• Low wind shear. 
• Moist air 
• An anticyclone in the upper layers of the troposphere above the storm needs to exist for produce significantly low surface pressures. This occurs when air converges towards the low pressure at the surface which drives the air rapidly upwards in the eyewall. Moreover, the wind diverges at the top of the troposphere due to the conservation of mass. 

-- Size 

How do cyclones form? 

https://en.wikipedia.org/wiki/Tropical_cyclogenesis

https://en.wikipedia.org/wiki/Cyclogenesis

This map illustrates the cumulative tracks of all tropical cyclones during the 1985-2005 time period. The Pacific Ocean west of the International Date Line sees more tropical cyclones than any other basin, while there is virtually no activity in the southern hemisphere between Africa and 160^oW. 

Cyclogenesis is an umbrella term that encompasses all weather phenomena demonstrating the development or strengthening of cyclonic circulation in the atmosphere. 
The criteria for a tropical cyclone to form include: 

- Sufficiently warm sea surface temperatures 

- Unstable Atmosphere 

- High humidity in the lower to middle levels of the troposphere 

- Sufficient Coriolis force to form a low-pressure centre 

- A pre-existing low-level focus or disturbance 

- Low vertical wind shear. 

Every year, an average of 86 tropical cyclones of tropical storm intensity develop globally, with 47 of them reaching hurricane / typhoon intensity, and 20 of them becoming intense tropical cyclones (i.e. Category 3 on the Saffir-Simpon Hurricane Scale). 

This collage of GOES 13 satellite images captures the development of the nor-easter over several days.

-- Meteorological scales 

-- Times 

-- Factors 

• Water temperatures of at least 26.5 ^oC (79.7 ^oF) are required to down to a depth of at least 50 m (160 ft). This destabilises the overlying atmosphere sufficiently to enable the sustainability of convection and thunderstorms. e.g. Hurricane Ophelia (2017) 
• Rapid cooling with height releases the heat of condensation that fuels a tropical cyclone. 
• More than 555 km (345 mi) or 5 degrees of latitude away from the equator. This allows the Coriolis effect to deflect the winds blistering towards the low pressure centre and producing a circulation. 
• Finally, a formative tropical cyclone requires a pre-existing system of disturbed weather, since tropical cyclones don't form spontaneously. 
• Kikuchi et al. (2009) found the low-latitude and low-level westerly wind bursts associated Madden-Julian oscillation generates favourable conditions for tropical cyclogenesis by initiating tropical disturbances. 

-- Locations 

• Many researchers observed most tropical cyclones develop in a worldwide band of thunderstorm activity near the equator, referred to as the Intertropical Front (ITF), the Intertropical Convergence Zone (ITCZ), or the monsoon trough. 
• Avila & Pasch (1995) discovered atmospheric instability in tropical waves, which contribute to the development of about 85% of intense tropical cyclones in the Atlantic Ocean and become a majority of the tropical cyclones in the Eastern Pacific. 
• Henderson-Sellers et al (2006) reported most develop between 10 and 30 degrees of latitude away from the equator, and 87% of them develop no farther away than 20 degrees north or south. 
• Neumann (2006) discovered tropical cyclones rarely develop or drift within 5 degrees of the equator, where the effect is weakest, due to the Coriolis effect initiates and maintains their rotation. 

a. Typhoons 

-- Frequency 

Tropical storms and typhoons by month, for the period 1959-2015 (Northwest Pacific) 

• Around 1/3 of the world's tropical cyclones develop within the western Pacific, making it the most active basin on Earth. 

-- Pathways 

This map illustrates the paths of all tropical cyclones in the northwestern Pacific Ocean between the 1980 and 2005. The vertical line to the right is the International Date Line. 

• Most tropical cyclones develop on the side of the subtropical ridge near the equator, then progress poleward past the ridge axis before turning north and northeast into the main belt of the Westerlies. 
• A majority of typhoons develop in a region in the northwest Pacific known as 'typhoon alley', where Earth's most devastating tropical cyclones frequently form. 
• When the subtropical ridge moves due to El Niño, this shifts the common cyclone paths. This means regions west of Japan and Korea experience fewer tropical cyclone impacts in September-November season during El Niño & neutral years. 
• Wu et al. (2003) found the break in the subtropical ridge situating near 130 ^oE, favours the Japanese archipelago during El Niño. In contrast, during La Niña years, both cyclogenesis and the subtropical ridge move westward across the western Pacific Ocean, increasing the landfall risk to China and cyclone intensity to Philippines. 
• Nemeth (1987) reported cyclones forming near the Marshall Islands drift towards Jeju Island, Korea. 
• Elsner & Liu (2003) outlined 3 paths of tropical cyclones: 

- Straight track = General westward path towards the Philippines, southern China, Taiwan and Vietnam. 

- Parabolic recurving track = Cyclones recurve toward eastern Philippines, eastern China, Taiwan, Korea, Japan, and the Russian Far East. 

- Northward track = From point of origin, heading north towards a number of small islands. 

 

-- Naming 

List of Western Pacific tropical cyclone names 

• Typhoons are not named after people, but rather named after animals, flowers, astrological signs, and a number of personal names. 
• Philippines (PAGASA) retains its own naming list, which contains human names. 
• The naming of typhoons on this list is used from top to bottom, from left to right. When all names on this list have been utilised, it will resume naming from the top-left corner. 

-- Records 

• The highest estimated maximum sustained winds recorded for a typhoon was Typhoon Haiyan at 314 km/h (195 mph) shortly before its landfall in the central Philippines on November 8, 2013. 
• The most intense storm in terms of minimum pressure was Typhoon Tip in the northwestern Pacific Pacific Ocean in 1979, with 870 hectopascals (26 inHg) and maximum sustained wind speeds of 165 knots (85 m/s, 190 mph, 310 km/h). 
• The deadliest typhoon of the 20th century was Typhoon Nina, with an estimated 100,000 deaths in China in 1975 due to a flood triggered by the failure of 12 reservoirs. 
• Around midnight on August 8, 2009, Typhoon Morakot flooded the following places in Taiwan with record-breaking rainfall: Chaiyi, Tainan, Kaohsiung, Pingtung, Taitung and Nantou. 

What are the different types of cyclones?

a. Surface-based types 

-- Extratropical cyclones 

https://en.wikipedia.org/wiki/Extratropical_cyclone

An extratropical cyclone over the North Pacific Ocean in January 2018. It contains an eye-like feature and a long cold front that extends to the tropics.  

• Also known as mid-latitude cyclones or wave cyclones, extratropical cyclones are low-pressure areas that drive the weather over Earth. 
• The adjective 'extratropical' describes cyclones forming outside the tropics and in the middle latitudes of Earth between 30o and 60o latitude. 
• The adjective 'mid-latitude' denotes cyclones forming within those latitudes, or post-tropical cyclones if a tropical cyclone enters the mid-latitudes. 
• Maue (2004) classified extratropical cyclones as 'baroclinic', since they develop along zones of temperature and dewpoint gradient known as frontal zones. Later in the life cycle, they become 'barotropic' as heat distribution around the cyclone becomes relatively uniform with its radius. 

The yellow bands mark the approximate areas of extratropical cyclogenesis worldwide.

How do extratropical cyclones form?

- Extratropical cyclogenesis 

• Extratropical cyclones develop along linear bands of temperature / dewpoint gradient with powerful vertical wind shear, hence are categorised as 'baroclinic' cyclones. Initially, cyclogenesis situates along frontal zones near a favourable quadrant of a maximum in the upper level airstream known as a jet streak. 
• Carlyle et al. (1990) noted the favourable quadrants are typically the right rear and left front quadrants, where divergence occurs. This divergence drives air out of the top of the air column. 
• Since mass in the column is decreased, atmospheric pressure at surface level also decreases, which subsequently intensifies the cyclone. The decreased pressure sucks in air, produces convergence in the low-level wind field. 
• The presence of low-level convergence and upper-level divergence indicated upward motion within the column, giving the cyclones a cloudy appearance. As the cyclone intensifies, the cold front surges towards the equator and curls around the back the of the cyclone. 
• Meanwhile, its associated warm front moves more slowly, while the cooler air ahead of the storm is denser, hence more difficult to displace. 
• Afterwards, the cyclones occlude as the poleward section of the cold front overtakes a section of the warm front, manifesting a tongue, or trowal, of warm air aloft. Ultimately, the cyclone barotropically cools and loses energy. 
• The presence of strong upper level forces on the system can rapidly decrease the atmosphere pressure. If pressures decrease more than 1 millibar (0.030 inHg) per hour, it undergoes a process called explosive cyclogenesis. 
• Under favourable conditions (such as near a natural temperature gradient e.g. Gulf Stream, or a preferred quadrant of an upper-level jet streak, pressures can rapidly decrease below 980 millibars (28.94 inHg). 
• Sienkiewicz et al. (2005) found hurricane-force extratropical cyclones are likely to develop in the northern Atlantic and northern Pacific oceans during December and January. 
• On December 14 and 15 1986, an extratropical cyclone reported near Iceland deepened to under 920 millibars (27 inHg), a pressure equivalent to a category 5 hurricane.

This diagram illustrates an upper-level jet streak. DIV areas are regions of divergence aloft, which will lead to surface convergence and augment cyclogenesis.  

- Extratropical transitioning 

• Hart & Evans (2001) outlined how tropical cyclones often become extratropical at the end of their tropical existence, usually between 30o and 40o latitude. This region is where sufficient drive from upper-level troughs or or shortwaves riding the Westerlies for the process of extratropical transition to proceed. 
• The NHC (2011) described the process of a cyclone in extratropical transition (i.e. known across the eastern North Pacific and North Atlantic oceans as the post-tropical stage) invariably developing or combining with adjacent fronts/troughs consistent with a baroclinic system. This increases the storm system's size, whereas the core weakens. 
• Hart & Evans (2001) noted the cyclone may restore its intensity due to baroclinic energy, depending on the environmental conditions encompassing the system. Furthermore, the cyclone's shape can warp, decreasing in symmetry over time. 
• During extratropical transition, the cyclone starts to slant back into the colder airmass with height, and the cyclone's primary energy source transforms from the release of latent heat from condensation to baroclinic processes. Hart et al. (2006) explains the low pressure system eventually loses its warm core and transforms into a cold-core system. 
• Guishard et al. (2009) observed the peak time of subtropical cyclogenesis in the North Atlantic is between September and October. This period is when the difference between the aloft air temperature and sea surface temperature is the biggest, which leads to the greatest potential for instability. 
• Evans & Guishard (2009) reported extratropical cyclones rarely transition to tropical cyclones unless they are in a region of warm ocean waters and low vertical wind shear. e.g. 1991 Perfect Storm. 
• The Joint Typhoon Warning Center utilises the extratropical transition (XT) technique to subjectively estimate the probability of the intensity of tropical cyclones transitioning to extratropical based on visible and infrared satellite imagery. 
• Velden et al. (2006) noted the Dvorak technique can fail if central convection decreases in transitioning tropical cyclones, which yields unrealistically low estimates. 
• A 2012 report combined aspects of the Dvorak technique and aspects of the Hebert-Poteat technique, which estimate tropical cyclone intensity and subtropical cyclone intensity respectively. This newly amalgamated technique is utilised for the interaction of a tropical cyclone with a frontal boundary or the loss of the cyclone's central convection as its forward speed or acceleration remains consistent. 

Describe the structure of extratropical cyclones 

- Surface pressure and wind distribution 

This QuikSCAT Image illustrates a typical extratropical cyclones over the ocean. Note the maximum winds are on the outside of the occlusion. 

• The wind field of an extratropical cyclone contracts with distance in relation to surface level pressure, with the lowest pressure being located near the centre, and the fastest winds usually on the cold / poleward side of warm fronts, cold fronts, and occlusions, where the pressure gradient force is greatest. 
• The area poleward and west of the cold and warm fronts associated with extratropical cyclones is referred to as the 'cold sector', whereas the area towards the equator and east of its associated cold and warm fronts is referred to as the 'warm sector'. 
• Due to the Coriolis effect, the wind flow around an extratropical cyclone is anticlockwise in the northern hemisphere, and clockwise in the southern hemisphere. 
• Near the centre, the pressure gradient force (from the eye compared to the outside of the cyclone) and the Coriolis force need to be approximately balance in order to prevent the cyclone from collapsing in on itself caused by the pressure difference. 
• As the cyclone matures, its central pressure decreases, whereas the sea-level pressure outside the cyclone remains consistent. 
• Meteorologists observed that the cold fronts ahead of a majority of extratropical cyclones develop into warm fronts, transforming the frontal zone into a wave-like shape. 
• In the northern hemisphere, an occluded cyclone is followed by a trough of warm air aloft (trowal) This trowal is a result of strong southerly winds on its eastern periphery twisting aloft around its northeast, and ultimately into its northwestern periphery (i.e. warm conveyor belt). 
• This drives a surface trough towards the cold sector on a corresponding curve to the occluded front. The trowal develops a section of an occluded cyclone known as its 'comma head', since it resembles a comma-like shape of the mid-tropospheric cloudiness. A 203 St. Louis University study suggested the trowal may be a focus of locally heavy precipitation, with sufficient instability possibly triggering thunderstorms. 

- Vertical structure 

• Lang (2006) found extratropical cyclones drift back into cooler air masses and intensify with height occasionally exceeding 9 km (30,000 ft) in length. 
• Above the Earth's surface, the air temperature near the cyclone's centre is increasingly freezing than the surrounding environment. Hart (2003) pointed out these characteristics are contrary to those observed in tropical cyclones, occasionally referred to as "cold-core lows". 
• A number of charts can be studied to verify the characteristics of a cold-core system with height, such as the 700 millibars (20.67 inHg) chart, which corresponds to 3,048 metres (10,000 ft) altitude. 

Describe the evolution of extratropical cyclones 

- Norwegian Cyclone model 

https://en.wikipedia.org/wiki/Norwegian_cyclone_model

• This model was developed during and after World War I within the Bergen School of Meteorology by Jacob Bjerknes, who proposed the polar front theory. 
• The theory suggested the main inflow into a cyclone focused along 2 lines of convergence; 1 ahead of the low and the other trailing the low. 
• The convergence line ahead of the low transforms into either the steering line or the warm front, whereas the convergence zone trialing the cyclone is referred to as 'the squall line' or 'cold front'. 

1. Initial stage 
2. Wave forms on front
3. Wave intensifies
4. A mature low pressure system
5. Dissipating stage of cyclone

1. A wave along a frontal boundary appears as an upper level disturbance drifts towards this section of the boundary. 
2. Precipitation starts to develop ahead of the surface low, within the cold sector of the cyclone poleward of the warm front. 
3. As the low decreases in pressure, both the cold and warm fronts around the low become more apparent, exerting its presence. 
4. As the low matures, it combines with the upper level disturbance shifting into the cyclone's cold sector. Then the cold front reaches the westward section of the warm front, developing an 'occluded front'. 
5. Finally, the cyclone piles on top of the upper level disturbance, segregating within the cold sector, and weaken as it shifts away from the original temperature discontinuity along the cold and warm fronts. This creates a 'cold-core low', whilst the frontal boundary weakens and surrounds the equatorward section of the cyclone, awaiting the next upper level disturbance to develop a novel low pressure area. 

• In meteorology, a conveyor belt is a flow of stream of warm moist air that develops within the warm sector of an extratropical cyclone ahead of the cold front that inclines above and north of the surface warm front. 
• The University of Oklahoma first conceptualised the conveyor belt in 1969. 
• A 2007 study described the path of a cold conveyor belt beginning north of the warm front and shifts in a clockwise trajectory (in the northern hemisphere) into the main belt of the westerlies aloft. However, there is contradictory evidence with respect to its actual evidence. 

- Shapiro-Keyser model 

This is a comparison of the Norwegian model with the Shapiro-Keyser model of cyclogenesis. Both schematics describe lower-tropospheric geopotential height (e.g. 850 hPa) and potential temperature.

• Developed in 1990, this theory suggested the cold front fractures, recognised warm-type occlusions and warm fronts as identical, projected the cold front to drift through the warm sector perpendicular to the warm front. 

- Warm Seclusion 

• A warm seclusion is defined as the mature phase of the extratropical cyclone lifecycle. This phenomenon was conceptualised by a 1980s ERICA field experiment that observed intense ocean cyclones producing an absurdly warm low-level thermal structure, surrounded by a warm-front and a concurrent chevron-shaped band of strong surface winds. 
• They may have features lacking clouds and a cyclone eye at their centre, with considerable pressure decreases, hurricane-force winds, and moderate to strong convection. 
• Maue (2006) evaluated the most powerful warm seclusions often achieve pressures less than 950 millibars (28.05 inHg) with an established lower to mid-level warm core structure. 
• They situate at latitudes poleward at the tropics, since they are part of the baroclinic life cycle. 
• Schultz & Werli (2001) found a large proportion of warm seclusion events situate over the oceans, thus mainly impacting coastal nations with hurricane force winds and torrential rain. 
• Pasch & Blake (2006) found the extratropical transition of a tropical cyclone may re-intensify into a warm seclusion in all tropical basins except the Northern Indian Ocean. e.g. Hurricane Maria (2005).

- Precursors for development 

https://en.wikipedia.org/wiki/Frontogenesis

In surface weather analysis, a pre-existing frontal boundary is needed to form a mid-latitude cyclone. A 2000 study described the beginning of cyclonic flow situating around a disturbed area of the stationary front due to an upper level disturbance. Schemm & Sprenger (2015) discovered the growth of extratropical cyclones can be hindered by intensified along-frontal stretching rates in the lower troposphere. 

- Vertical motion affecting development 

• When temperatures cool polewards and pressure perturbation lines slant westward with height, this leads to cyclogenesis. Wallace & Hobbs (2006) pointed out cyclogenesis likely occurs in areas of cyclonic vorticity advection, downstream of a westerly stream. 
• Upward motion around the flow manifests in a combination of vorticity advection and thermal advection generated by the temperature gradient and low pressure centre. A sufficiently substantial temperature gradient would elevate temperature advection and drive vertical motion, which intensify the cyclone. 

- Modes of development 

• When topography drives a surface low to develop as baroclinic waves sweep over a mountain barrier, a low develops on the leeward side of the mountains. The COMET Program (2002) described this process as "lee cyclogenesis". 
• Chu (2006) noted cold fronts move faster than warm fronts and approach it since the slow erosion of higher density airmass situates ahead of the cyclone and the higher density airmass sweeps it behind the cyclone. This leads to a warm sector being narrowed. 
• A St Louis University study (2006) found an occluded front develops where the warm air mass is driven upwards into a trough of warm air aloft, known as a trowal (a trough of warm air aloft). 
• Norris (2005) highlighted all developing low pressure areas demonstrated upward vertical motion within the troposphere. This decreases the mass of local atmospheric columns of air, which decreases surface pressure. 

- Maturity 

• Ahn et al. (2005) defined the maturity of an extratropical cyclone as 'after the time of occlusion when the storm has approached peak strength and cyclonic flow has reached peak intensity. 
• Woodruff (2006) observed the atmosphere stabilises and the storm's centre of gravity sinks as the occlusion process occurs and the warm air mass is driven upwards over a cold air mass. 

How do extratropical cyclones move? 

This diagram is a zonal flow regime. Notice the dominant west-to-east flow as shown in the 500 hPa height pattern. 

• Extratropical cyclones are typically steered by deep westerly winds heading eastwards across both the Northern and Southern hemispheres via a "zonal flow regime". They likely drift slower northwards or southwards as its motion shifts from a zonal to a meridional pattern. A meridional flow regime typically features powerful amplified, troughs and ridges. 
• When a cyclone interacts with other low pressure systems, troughs, ridges, or anticyclones, it alters their direction, become hindered, becomes weaker, diverts the cyclone towards the anticyclone's periphery, or a combination of the aforementioned effects. If the obstructing anticyclone or ridge weakens, it would re-intensify the extratropical cyclone. 
• If 2 extratropical cyclones interact with each other, they amalgamate to become a binary cyclone, with 2 vortices rotating around each other. 

This is a radar image of a large extratropical cyclone at its peak over the USA during February, 2007.

What are the effects of extratropical cyclones? 

This map illustrates the preferred region of snowfall in an extratropical cyclone. 

a. General 

• Extratropical cyclones brought about mild weather with showers and surface winds of 15-30 km/h (9.3-18.6 mph), o torrential rain and gale-forced winds faster than 119 km/h (74 mph). 
• In mature extratropical cyclones, an area of considerable precipitation, thunderstorms, and thundersnowstorms situated on the northwest periphery of the surface low is referred to as a 'comma head. 

b. Severe weather 

• In 1999, the University of Illinois found defined bands of thunderstorms known as 'squall lines' develop ahead of cold fronts and lee troughs. This is caused by considerable atmospheric moisture and powerful upper level divergence, resulting in hail and gale winds. 
• NOAA (2002) implied the possibility of tornado development since powerful directional wind shear exists ahead of the cold front in the existence of an upper-level jet stream. 
• The Met Office (2007) described the 'Great Storm of 1987' that struck Great Britain demonstrated a greatly low pressure of 953 millibars (28.14 inHg) and winds up to 220 km/h (140 mph). It resulted in 19 fatalities, 15 million downed trees, widespread destruction to homes, and an estimated cost of £1.2 billion (US$2.3 billion). 

c. Climate and general circulation 

• Edward Lorenz discovered the mechanism of extratropical cyclones involves converting potential energy generated by temperature gradients (from the pole to equator) to eddy kinetic energy. As energy is transferred towards to the pole to heat up the higher latitudes, the pole-equator temperature gradient weakens. 

What were notable extratropical cyclones? 

• November 14, 1854, Europe, during the Crimean War 
• Columbus Day Storm of 1962, USA, Oregon 
• April 10, 1968, Wahine storm, New Zealand -- Named after the ferry TEV Wahine which struck a reef and sunk at the entrance of Wellington Harbour. 
• November 10, 1975, Lake Superior -- Sunk the SS Edmund Fitzgerald near the Canada-US border, 15 nautical miles northwest of the entrance to Whitefish Bay. 
• October 11, 1984, Vancouver Island, Canada 
• Braer Storm of January 1993, northern Atlantic Ocean 
• Great Storm of 1703, UK 
• October 29, 2012 -- Hurricane Sandy, New Jersey, USA 
• August 23-25, 2005-- Montevideo, Uruguay 

-- Polar lows 

https://en.wikipedia.org/wiki/Polar_low

A polar low over the Sea of Japan in December 2009

• A polar low is a small, temporary atmospheric low pressure system situated over the regions of ocean poleward of the main polar front in both the Northern and Southern Hemisphere, as well as the Sea of Japan. 
• Their horizontal length is less than 1,000 km (620 mi) and their timespan may last several days. 
• Conventional weather reports struggle to detect polar lows, which may pose a hazard to high-altitude operations such as shipping and gas and oil platforms. 
• They were first identified in the 1960s on the meteorological satellite imagery. It discovered the most active polar lows over several ice-free maritime regions in or near the Arctic during the winter, such as the Norwegian Sea, Barents Sea, Labrador Sea, and Gulf of Alaska, as well as the Sea of Japan and the Sea of Ohkhotsk.

 

Describe the structure of polar lows 

• The spiraliform polar low comprises of several cloud bands enveloping the centre of the low. 
• The comma-shaped polar low tends to situate with systems adjacent to the polar front. 
• Some polar low share the appearance of a tropical cyclone, with deep thunderstorm clouds circling the cloud-free 'eye'. 

How do polar lows form? 

• Polar lows can form on horizontal temperature gradients through baroclinic instability, appearing as small frontal depressions. 
• They can also form in cumulonimbus clouds, which associate with cool pools in the mid- to upper-troposphere. 
• Rasmussen & Turner (2003) found deep convection develops as cold-core lows with temperatures in the mid-levels of the troposphere approach -45 ^oC (-49 ^oF) and shift over open waters, which result in the formation of polar low. 

Where do polar lows often occur? 

• Polar lows tend to occur in Greenland and the Canadian Arctic, at any time throughout the year, with summer lows being weaker than winter lows. 
• They can also develop over the Sea of Japan due to the Japan-Sea Polar-Airmass Convergence Zone (JCP) resulting from the cold-core low aloft and the warm Tsushima Current. This severely affects the populous Japanese communities with powerful winds and heavy snowfall. 
• However, they are poorly understood and rarely devastating since they situate in sparsely populated areas. 
• Polar lows often affect oil and gas rigs throughout the Antarctic ocean (Southern Ocean), which can directly affect cargo and shipping traffic. 

-- Subtropical cyclones 

https://en.wikipedia.org/wiki/Subtropical_cyclone

A satellite image of Subtropical Storm Alpha in September 2020, shortly before an unusual landfall in Portugal. 

• A subtropical cyclone is a weather phenomenon that shares certain features with a tropical and extratropical cyclone. 
• Throughout the 1950s and 1960s, scientists previously used labels such as semi-tropical and quasi-tropical before settling on subtropical.to describe such cyclones. 
• In 1972, the National Hurricane Center (NHC) recognised these storms as subtropical cyclones, and hence updated the hurricane database to include subtropical cyclones detected between 1968 and 1971. 

How are subtropical cyclones named? 

• In the North Atlantic basin, subtropical cyclones were initially named from the NATO phonetic alphabet in the early 1970s. 
• Between 1975 and 2001,  subtropical cyclones were either named from the traditional list and recognised as tropical in real-time, or from a separate numbering system. 
• Between 1992 and 2001, 2 different numbers were allocated to subtropical cyclones; one for public use, and the other for NRL and NHC reference. e.g. Hurricane Karen (2001) was initially known as Subtropical Storm One and AL1301 (13L). 
• In 2002, the NHC started allocating numbers to subtropical depressions and names to subtropical cyclones from the same sequence as tropical cyclones. e.g. Subtropical Depression 13L --> Subtropical Depression Thirteen 
• Examples of subtropical cyclones include Hurricane Gustav (2002), Subtropical Storm Nicole (2004). 

How do subtropical cyclones form? 

This intensity map shows the formation of Subtropical Storm Andrea in May 2007.

• Landsea (2008) recognised subtropical cyclones develop in an extensive band of latitude, typically south of the 50th parallel in the northern hemisphere. 
• Landor (2004) observed subtropical cyclones frequently situate across the North Atlantic rather than the northwestern Pacific Ocean. 
• In the eastern half of the north Pacific Ocean and north Indian Ocean, subtropical cyclones are generally weaker which develop beneath a mid to upper-tropospheric low separated from the main belt of the westerlies during winter. 
• Hastenrath (1991) reported subtropical cyclones developing in the north Indian Ocean brought the monsoon rainfall during the wet season. 
• The World Meteorological Organisation (2006) found subtropical cyclones in the southern hemisphere situate across the southern regions of the Mozambique Channel. 

• A majority of subtropical cyclones develop as a deep cold-core extratropical cyclone descends into the subtropics. A high altitude ridge occludes the storm and then remove its frontal boundaries as its source of cool and dry air from the higher altitudes deflects away from the storm. 
• Scientists noted the temperatures differences between the 500 hPa pressure level and sea surface temperatures initially overshoot the dry adiabatic lapse rate. This leads to a series of thunderstorms developing at a distance east of the centre. 
• Since the air temperatures are quite cold, sea surface temperatures need to be at least 20^oC (86^oF) to initiate thunderstorms. This subsequently increases the moisture in the environment around the low, which destabilises the atmosphere by decreasing the lapse rate required for convection. 
• Guishard et al. (2009) evaluated the average sea temperature associated with subtropical cyclogenesis is around 24^oC (75^oF). Landsea (2011) noted persistent and deep thunderstorm activity leads to deepening of its initial low level warm core, hence tropical cyclogenesis. 
• Guishard et al. (2007) observed the focal point of subtropical cyclogenesis in the North Atlantic is generally over the ocean water, which explains the frequent impact on the island of Bermuda. 
• Evans & Braun (2012) suggested a mechanism for subtropical cyclogenesis in the South Atlantic involves lee cyclogenesis in the area of the Brazil Current.

- Transition from extratropical storms 

• If an extratropical low attains tropical aspects, it may transition into subtropical depression or storm, then into a tropical depression or storm, and ultimately a hurricane. 
• This transition requires considerable instability in the atmosphere, and temperature differences between the shrouded ocean and the mid-level of the troposphere being over 38^oC (68^oF). 

Examples include: 

- Tropical Storm Gilda (1973) 

- Subtropical Storm Four (1974) 

- Tropical Storm Jose (1981) 

- Hurricane Klaus (1984) 

- Tropical Storm Allison (2001) 

- Tropical Storm Lee (2011) 

- Hurricane Humberto (2013) 

- Tropical Storm Ian (2016) 

- Typhoon Jelawat (2018) 

- Tropical Storm Gaemi (2018) 

What are the characteristics of subtropical cyclones? 

• Intense winds sweeping a considerable distance from the centre compared to a tropical cyclone and a lack of weather fronts associating directly to the centre of circulation. 
• Subtropical depressions have wind speeds less than 18 m/s (65 km/h, 35 knots, 39 mph), whereas subtropical storms have faster winds. 
• Subtropical cyclones exert hurricane-force winds of 33 m/s (119 km/h, 64 knots, 74 mph) or greater. 

What are the different types of subtropical cyclones? 

- Upper-level low 

• This features circulation towards the surface layer and maximum sustained winds situating at a radius of about 160 km (99 mi) or more from the centre. 
• The NHC (2009) found these upper-level cold lows have a relatively widespread zone of maximum winds situated farther from the centre, as well as an asymmetric wind field and distribution of convection. 

- Mesoscale low 

• This initiates in or near a frontolysing zone of horizontal wind shear, referred to as a 'dying frontal zone', with a radius of maximum sustained winds less than 50 km (31 mi), as well as an entire circulation with an initial diameter of less than 160 km (99 mi). 
• Dorst (2007) described these temporary systems as either cold core or warm core as a 'neutercane'. 

- Kona storm 

• These deep cyclones develop during the cold season of the central Pacific Ocean. During the 1970s, changes to its definition categorised most kona storms as extratropical cyclones. 
• Hastenrath (1991) categorised kona storms situating across the northeast Pacific Ocean as subtropical cyclones in the presence of a weak surface circulation. 
• Morrison & Businger (2002) stated kona is a Hawaiian term for leeward, based on the alterations in wind direction sweeping across the Hawaiian Islands from easterly to southerly during the presence of a kona storm. 

- Australian East Coast low 

• They are extratropical cyclones that develop between 25^o South and 40^o South and within 5^o of the Australian coastline, usually during the winter months.
  They vary in size from mesoscale (10 - 100 km) to synoptic scale (~ 100 - 1000 km).

-- Tropical cyclones (Hurricanes) 

https://en.wikipedia.org/wiki/Tropical_cyclone

A satellite image of Hurricane Isabel (2003) during Expedition 7 of the International Space Station. Notice the eye, eyewall, and surrounding rainbands, characteristics of tropical cyclones in the narrow sense.

• A tropical cyclone is a rapidly rotating storm system denoted by a low-pressure centre, a closed low-level atmospheric circulation, powerful winds, and spiralling thunderstorms that precipitate heavy rain or squalls. 
• Depending on its location and strength, they can be referred by different names. Tropical cyclones situating in the Atlantic Ocean and northeastern Pacific Ocean are denoted hurricane, while tropical cyclones situating in the northwestern Pacific Ocean are denoted typhoon. Those occurring in the south Pacific or Indian Ocean are simply denoted as tropical cyclones or severe cyclonic storms. 

-- Typhoons 

https://en.wikipedia.org/wiki/Typhoon

A satellite image of Typhoon Yutu in 2016

• The term typhoon belongs to tropical cyclones situating in the northwest Pacific region. The first signs of its definition was around 1504 in the French language as typhon meaning 'whirlwind or storm'. 
• The Oxford Dictionary cited the Hindustani word ṭūfān and the Chinese word tai fung gave rise to many early variations in English, e.g. "touffon", "tufan", "tuffon", as early as 1588. 
• From 1699, "tuffoon" and later "tiffoon" were derived from Chinese with spelling roots from older Hindustani roots. 
• The modern spelling "typhoon" dates back as early as 1820, preceded by "tay-fun" in 1771 and "ty-foong", were all derived from the Chinese spelling. 

- Chinese: Tai fung or Tai feng = 台风

- Hindustani:  ṭūfān = توفان/طوفان

RMSC Tokyo's Tropical Cyclone Intensity Scale 

What are common paths of typhoons? 

This diagram illustrates the tracks of all tropical cyclones in the northwestern Pacific Ocean between 1980 and 2005. The vertical line to the right is the International Date Line.

• A majority of develop in the northwest Pacific area called 'typhoon alley', where Earth's most intense tropical cyclones often form. 

There are 3 general paths of typhoons: 

1. Straight track (straight runner) = A general westward path towards the Philippines, southern China, Taiwan and Vietnam. 
2. Parabolic recurving track = Recurvature towards eastern Philippines, eastern China, Taiwan, Korea, Japan, and the Russian Far East. 
3. Northward track = From point of of origin, then heading north towards smaller islands. 

b. Upper-level types 

-- Polar cyclones 

https://en.wikipedia.org/wiki/Polar_vortex

A powerful polar vortex configuration in November 2013

A weaker polar vortex on January 5, 2014

• A polar vortex is a gigantic, upper-level low-pressure area, less than 1,000 km (620 mi) in diameter, rotating anti-clockwise at the North Pole and clockwise at the South Pole. 
• They were first described in an 1853 article called "Air Maps", but it wasn't discovered until 1952 by radiosonde observations at altitudes higher than 20km when sudden stratospheric warming (SSW) occurs during the winter in the Northern Hemisphere. 

Where do polar vortices occur? 

i. Northern hemisphere (Arctic) 

• Usually there is a giant vortex in the Arctic with a jet stream confined near the polar front. When it weakens, it segregates into 2 or more smaller vortices, the strongest of which situate near Baffin Island, Canada and the other over northeast Siberia. 
• Then the flow of Arctic air disintegrates, and cold Arctic air drifts towards the equator, which results in drastic cooling. 
• In late January 2019, a polar vortex impacted the northern portion of the USA and a majority of Canada causing a deep freeze, increasing the risk of frostbite. 

ii. Southern hemisphere (Antarctic) 

• The Antarctic vortex is described as a low-pressure zone situated near the edge of the Ross ice shelf, near 160o west longitude. The strength of the polar vortex is found to correlate with the mid-latitude Westerlies' strength. 
• A 2001 study found a weakened polar vortex allows high-pressure zones of the mid-latitudes to force the polar vortex, jet stream and polar front towards the equator. This leads to an interaction between the cold dry air and the warm, moist air of the mid-latitudes, producing a drastic change of weather  known as a "cold snap". 
• A 2020 study explained the Australian polar vortex, also referred to as a "polar blast" or "polar plunge" carries cold air from Antarctica to Australian shores. This results in showers, snowfall, icy winds, and hail impacting the southeastern sections of Australia, such as Victoria, Tasmania, southeast coast of South Australia and the southern half of New South Wales.  

How are polar vortices identified? 

• The polar front can be identified by the interface between the pole's cold dry air mass and the warm moist air mass situated south. It is generally centred, approximately at 60^o latitude. The temperature between the equator and the poles determines the polar vortex's strength, being stronger in the winter and weaker in the summer. 
• Hartmann & Schoeberl (1991) found the stratospheric polar vortex forms at latitudes above the subtropical jet stream. Cavallo et al. (2009) measured most polar vortices have a horizontal radius of less than 1,000 (620 mi). 
• Kolstad et al. (2010) measured the 50 hPa surface is the typical location in the stratosphere where polar vortices situate, as well as down into the mid-troposphere. In the troposphere, the strength of a polar vortex can be determined by the closed contours of potential temperature. 

Describe the duration and strength of polar vortices 

This diagram illustrates the polar vortex and weather impacts due to stratospheric warming.

• When extratropical cyclones ascend, coinciding with a weak polar vortex, it disintegrates the single vortex into smaller vortices within the polar air mass, which can last more than a month. 
• Rodock (2000) observed polar vortices manifested by volcanic eruptions in the tropics during the winter can last as long as 2 years. Mitchell (2004) described the Arctic oscillation as a magnitude gauge of the polar vortex's strength in the northern hemisphere. 
• A strong Arctic vortex appears elongated with 2 cyclonic centres, 1 over Baffin Island in Canada, and other over northeast Siberia. 
• On the other hand, a weak Arctic vortex involves subtropic air masses infiltrating the poles, leading to the Arctic air masses drifting towards the equator. 
• The air mass movement and heat transfer in the polar region influences the development and dissipation of a polar vortex. 
• Nash (2012) found the circumpolar winds during the autumn (fall) accelerate and the polar vortex ascends to the stratosphere. This manifests in an organised rotating air mass. 
• During the winter, the vortex core cools, the winds decelerate and the vortex energy dissipates. Thus, intense weather systems transfers segments of the vortex air into lower latitudes. 
• In the stratosphere's lowest level, intense potential velocity gradients exist. Furthermore, a large proportion of air is restricted within the polar air mass into December in the Southern Hemisphere, and April in the Northern Hemisphere. 

• Reicher et al. (2012) found that as the air in the stratosphere increases, it reverses the rotation in the Arctic Polar Vortex from anti-clockwise to clockwise. This has flow-on effects in the troposphere beneath it. Moreover, the changing intensity of the polar vortex influences the sea circulation under the waves. 
• Limpasuvan et al. (2005) found the polar vortex can be strengthened by particular storms systems within the troposphere that cool the poles, as well as climate anomalies related to La Niña. 
• This leads to changes in relative humidity since dry air from the stratosphere enters the vortex core, as well as long wave cooling manifested by a reduction in water vapour near the vortex. 
• Cavello & Hakim (2013) found the decreased water vapour concentration is due to a lower tropopause within the vortex, which lets the dry stratospheric air settle above the moist tropospheric air. 
• Hartmann & Schoeberl (1991) found displacement of the vortex tube induces instability of the vortex rings, which makes it subject to be influenced by planetary waves. Although the planetary wave activity in both hemispheres varies, it generates a proportional change in the polar vortex's strength and temperature. 
• Widnall & Sullivan (1973) found a correlation between the number of waves around the vortex's perimeter, as well as an inverse relationship between the vortex core and the number of waves. 
• Since the polar night jet under the Arctic Polar Vortex is at its weakest in the winter, it doesn't steer any descending polar air, which subsequently coalesces with air in the mid-latitudes. 
• Manney et al. (1994) highlighted that coalescence is minimised in the late winter since air parcels descend to a certain extent. 
• Waugh et al. (2012) noted the ex-vortex air dissipates into the middle latitudes approximately a month after the vortex disintegrates. 
• On a rare occasion, if a sufficiently large chunk of a disintegrating polar vortex survives, it may drift into Canada as well as the USA Midwest, Central, Southern and Northeastern regions due to the polar jet stream's displacement. 

How does climate change associate with polar vortices? 

Meanders of the northern hemisphere's jet stream developing (a,b) and detaching a "drop" of cold air (c); orange indicates warmer masses of air; pink indicates the jet stream. 

• Baldwin (2001) discovered weather patterns can be influenced by air circulation in the stratosphere. Furthermore, other scientists discovered a statistical association between weak polar vortices and severe cold weather in the Northern Hemisphere. 
• Many studies recognised associations between climate change and Arctic sea ice decrease, depleting snow cover, evapotranspiration patterns, NAO anomalies or weather anomalies associated with polar vortices and jet streams. 
• It's known decreased snow cover and sea ice means less reflected sunlight, thence increased evaporation and transpiration. This shifts the pressure and temperature gradients of the polar vortex, which weakens it. 
• When the jet stream amplitude grows over the northern hemisphere, this leads to Rossby waves propagating southwards or northwards. This sequentially carries warmer air to the north pole and polar air down to the lower altitudes. 
• Therefore, the jet stream amplitude becomes greater as the polar vortex weakens, meaning the probability of weather systems being hindered increases. 

What are the effects of polar vortices? 

Southern Hemisphere Ozone Concentration, February 22, 2012 

• When nitric acid in polar stratospheric clouds reacts with chlorofluorocarbons to form chlorine, it catalyses the photochemical depletion of ozone. 
• Müller (2010) found chlorine concentrations peaks in the winter, and the consequent ozone depletion is significant in the spring when sunlight returns. 
• Mohanakuma (2008) stated ozone depletion is more significant at the south pole than the north pole because of the substantial air exchange between the Arctic and the mid-latitudes. 
• A BBC Online report stated the 2011 Arctic polar vortex caused sufficient chemical ozone depletion to form an "ozone hole". 

-- TUTT cell (Upper tropospheric cyclonic vortex) 

https://en.wikipedia.org/wiki/Upper_tropospheric_cyclonic_vortex

A satellite image of a TUTT cell in the western North Pacific. 

• An upper tropospheric cyclonic vortex is a circulation with a visible centre that drifts from east-northeast to west-southwest that often situates in the Northern Hemisphere during warmer seasons. 
• Its circulations' minimum height is roughly 6,080 metres (19,950 ft). 

What are the characteristics of TUTT cells? 

• It has a cold core, implying its strength aloft than near the Earth's surface, or in regions of the troposphere with decreased pressures. 
• If an upper tropospheric cold-core low is in phase with a lower tropospheric easterly wave, along with the easterly wave near or east of the upper level cyclone, then thunderstorms may develop. 
• If the aforementioned weather events are out of phase, along with the tropical wave being west of the upper level circulation, this results in suppression of convection. This is due to convergence aloft manifests in downward motion over the tropical wave in the easterlies. 
• A 2010 study found the low-level convergence generated by the cut-off low lead to squall lines and rough seas. Furthermore, the low-level spiral cloud bands produced by the upper level circulation are positioned parallel to the low-level wind direction. 

Describe the climatology of TUTT cells

• In the Northern Hemisphere, the TUTT cell typically situates between May and November, peaking between July and September. 
• Sadler & Bosart demonstrated the TUTT cells are caused by the mid-latitude disturbance moving around the western side of the TUTT. 
• Carlson & Salder (1968) detected TUTT cyclones over the eastern Caribbean Sea in October 1965, specifically over the Azores before moving southwest towards a latitude of 20^oN 
  and 40^o of longitude. 
• The lowest level of closed circulation beneath the TUTT is approximately between 700 and 500-hectopascal level (3,000 m / 9,800 ft to 5,800 m / 19,000 ft above sea level). 
• During the summer, the TUTT often form over the trade wind regions of the North Atlantic Ocean, Gulf of Mexico, and Caribbean Sea.

-- Mesocyclone 

https://en.wikipedia.org/wiki/Mesocyclone

• A mesocyclone is a vortex the size of a cyclonic storm, usually around 3.2 to 9.7 km (2 to 6 mi) in diameter, within a thunderstorm. 
• In the northern hemisphere, it is found in the right rear edge of a supercell. 

                                          

How do mesocyclones form? 

• They form when drastic changes of wind speed and/or direction with height ("wind shear") cause sections of the atmosphere's lower portion to spin in invisible tube-like rolls. 
• The University of Illinois (2006) described the convective updraft of a thunderstorm lifts this spinning air, tilts the toll's orientation upwards and causes the updraft to rotate as a vertical column. 
• As the updraft rotates and absorbs cool moist air from the forward flank downdraft (FFD), it develops into a wall cloud. That is a spinning layer of clouds dropped from ambient storm cloud base beneath the mid-level mesocyclone. 
• As it descends, a funnel cloud develops near its centre, verifying the first visible stage of tornadogenesis. 

Wind shear (red) causes air to spin (green).

The updraft (blue) lifts the spinning air upright.

The updraft begins rotating.

How are mesocyclones detected? 

• The Doppler weather radar is often used to detect mesocyclones. Nearby high values of the opposite sign within the velocity data indicate the presence of mesocyclones. They are often identified by hook echo rotation on a weather radar map. Visual cues such as a rotating wall cloud or tornado may indicate mesocyclone activity. 

Mesocyclone detection algorithm output on tornadic cells in northern Michigan on July 3, 1999.

Describe the physics and energetics of cyclones 

Tropical cyclones exhibit an overturning circulation where air inflows at low levels near the surface, rises in thunderstorm clouds, and outflows at high levels near the tropopause. 

The 3D wind field in a tropical cyclone has 2 components: "primary circulation" and "secondary circulation".  

-- Primary circulation 

• The primary circulation is the rotational component of the flow, where it is circular. It is larger in magnitude, which dominates the surface wind field, and causes the largest proportion of damage. 
• The primary rotating flow in a tropical cyclone is caused by the conservation of the angular momentum. Absolute angular momentum on a rotating planet (M) is calculated by: 

M = 0.5*f*r² + v*r 

-- f = Coriolis parameter 

-- v = Azimuthal (i.e. rotating) wind speed 

-- r = Radius to the axis of rotation 

-- 0.5*f*r² = Component of planetary angular momentum projecting onto the local vertical (i.e. axis of rotation). 

-- v*r = Relative angular momentum of the circulating air with respect to the axis of rotation 

• As air flows radially inward at low levels, it starts rotating cyclonically to conserve angular momentum. As rapidly rotating air drifts radially outward near the tropopause, its cyclonic rotation decreases and changes direction at a sufficiently large radius, resulting in an upper-level anti-cyclone. 
• This leads to a vertical structure characterised a cyclone at low levels and anti-cyclone near the tropopause. Due to thermal wind balance, this correlates with a system with higher air temperatures at its centre than in the surrounding environment at all altitudes (i.e. warm core). 
• Due to hydrostatic balance, the warm core results in lower pressure at the centre at all altitudes, with the maximum pressure drops located at the surface. 

-- Secondary circulation: a Carnot heat engine 

• The secondary circulation is the overturning (in-up-out-down) component of the flow, in which it flows in radial and vertical directions. It is slower than the primary circulation and dictates the energetics of the cyclone. 
• The tropical cyclone's primary energy source is heat produced by evaporating warm ocean water, heated by sunlight. Emanuel (1986) analogised the energetics of this storm system to an atmospheric Carnot heat engine. 

1. Inflowing air near the surface acquires heat mainly by water evaporation (i.e. latent heat) at the temperature of the warm ocean water. 
2. The warm air elevates and cools within the eyewall, while conserving total heat content (i.e. latent heat converts to sensible heat during condensation). 
3. Air outflows and dissipates heat via infrared radiation to space at the temperature of the cold tropopause. 
4. Air subsides and warms at the cyclone's outer edge while conserving total heat content. 

• Steps 1 and 3 are virtually isothermal, while steps 2 and 4 are virtually isotropic. 
• The NOAA estimated a tropical cyclone releases heat energy at the rate of 50-200 exajoules (10¹⁸) daily, equivalent to about 1 PW (10¹⁵ Watts). 

-- Maximum potential intensity 

• Because of surface friction, the inflow only partially conserves angular momentum. Therefore, the sea surface lower boundary acts as both a source (evaporation) and sink (friction) of the cyclone's energy. 
• This results in a theoretical upper bound on the strongest wind speed that may be attained by the cyclone. Since evaporation increases linearly with wind speed, there is a positive feedback on energy input into the system known as the Wind-Induced Surface Heat Exchange (WISHE) feedback. 
• This feedback is offset when frictional dissipation, which is directly proportional with the cube of the wind speed, becomes sufficiently large. 
• This upper bound is called the "maximum potential intensity", denoted v_p

-- Ts = Temperature of the sea surface

-- To = Temperature of the outflow ([K]) 

-- Δk = Enthalpy difference between the surface and the overlying air ([J/kg]) 

-- Ck & Cd = Surface exchange coefficients (dimensionless) of enthalpy and momentum, respectively 

-- Δk = ks* - k

-- ks* = Saturation enthalpy of air at sea surface temperature and sea-level pressure 

-- k = Enthalpy of boundary layer air overlying the surface 

• Emanuel (2000) explained the maximum potential intensity of a cyclone is a function of the background environment alone, therefore this value determines which regions on Earth can support tropical cyclones of a specific intensity, and how these regions evolve over time. 

- Derivation 

• You can view a tropical cyclone as a heat engine that converts input heat energy from the surface into mechanical energy, which is subsequently used to do mechanical work against surface friction. 
• At equilibrium, the rate of net energy production in the system equals the rate of energy loss due to frictional dissipation at the surface, i.e. 

W_in = W_out

The rate of energy loss per unit surface area from surface friction (W_out) is determined by: 

W_out = C_d*ρ*|u|³

-- ρ = Density of near-surface air ([km/m³]) 

-- |u| = Near surface wind speed ([m/s]) 

The rate of energy production per unit surface area (W_in) is determined by: 

W_in = ε*Q_in 

-- ε = Heat energy efficiency 

-- Q_in = Total rate of heat input into the system per unit surface area.

If we assume a tropical cyclone operates similarly to a Carnot heat engine, then the Carnot heat engine efficiency can be calculated by:

ε = (T_s - T_o)/T_s

Heat (enthalpy) per unit mass is determined by: 

k = C_p*T + L_v*q

--  C_p = Heat capacity of air 
-- T = Air temperature 
-- L_v = Latent heat of vaporisation
-- q = Concentration of water vapour 
-- C_p*T = Sensible heat
-- L_v*q = Latent heat

There are 2 sources of heat input; (1) at the surface due to evaporation, and (2) internal sensible heat produced by frictional dissipation situating near the surface within the tropical cyclone and constantly recycled.
The bulk aerodynamic formula for the rate of heat input per unit area at the surface (Q_in:k )

Q_in:k = C_k*ρ*|u|*Δk

Therefore the total rate of net energy production per unit surface area is determined by: 

Setting W_in = W_out and taking |u| ~ v (i.e. the rotational wind speed is dominant) leads to the solution for vp provided above. This solution assumes that total energy input and loss within the system can be estimated by their values at the radius of maximum wind.
The term Q_in:friction multiplies the total heat input by the factor T_s / T_o . Mathematically, this substitutes T_s with T_o in the denominator of the Carnot efficiency.
An alternative definition of the maximum potential intensity is:

-- CAPE = Convective Available Potential Energy
-- CAPE_s* = CAPE of an air parcel lifted from saturation at sea level relative to the environmental sounding.
-- CAPE_b = CAPE of the boundary layer air
Both values are calculated at the radius of maximum wind.

- Characteristic values and variability on Earth 

• On Earth, a typical temperature value for T_s is 300K and for T_o is 200 Km which corresponds to a Carnot efficiency of ε = 1/3.
• The ratio of the surface exchange coefficients (C_k / C_d) is usually 1. Powell et al. (2003) suggested that the drag coefficient (C_d) varies with wind speed and decreases at high wind speeds within the boundary layer of a mature hurricane.
• Furthermore, Bell et al. (2012) implied C_k may vary at high wind speeds due to the effect of sea spray on evaporation within the boundary layer. 
• A typical value of the maximum potential intensity is 80 m/s (180 mph, 290 km/h). Nevertheless, Bister (2002) thought this value varies considerably across space and time, especially within the seasonal cycle, spanning a range of 0 to 100 m/s (0 - 224 mph, 0 - 360 km/h). 
• This variability is mainly influenced by variability in both the surface enthalpy disequilibrium (Δk) and the thermodynamic structure of the troposphere, which are governed by the large-scale dynamics of the tropical climate. 
• Emanuel & Sobel (2013) outlined these processes are mediated by a number of factors such as the sea surface temperature, background near-surface wind speed, and the vertical structure of atmospheric radiative heating. 
• Woolnough et al. (2000) found a correlation between the variability in the maximum potential intensity and sea surface temperature perturbations from the tropical mean in smaller scales. This is due to regions containing relatively warm water attaining thermodynamic states that are capable of sustaining a tropical cyclone compared to regions containing relatively cool water.

-- Interaction with the upper ocean 

This chart illustrates the decrease in surface temperature in the Gulf of Mexico as Hurricane Katrina and Rita passes over. 

• The passage of a tropical cyclone over the ocean significantly cools the upper layers of the ocean, which may impact subsequent cyclone development. This effect is mainly triggered by the wind-driven blending of deeper cold water with the warm surface water. 
• This leads to the negative feedback process inhibiting advanced development or weakening of the cyclone. 
• Ocean cooling may also be caused by falling raindrops, and overcast weather (cloud cover), meaning lack of sunlight. 
• Fedorov et al. (2010) discovered the blending of the ocean waters lead to heat being shoved into deeper waters, with potential effects on global climate. 

How do tropical cyclones move?

-- Environmental steering 

• It is defined as the storm's movement driven by prevalent winds and other extensive environmental conditions. 
• A 2012 study by the Atlantic Oceanographic and Meteorological Laboratory (AOML) found east-to-west trade winds steer tropical cyclones westwards from the equatorial side of the subtropical ridge. 
• A 2006 AOML study also found trade winds steer tropical easterly winds westward from the African coast over the North Atlantic & Northeast Pacific oceans towards the Caribbean Sea, North America, and central Pacific Ocean. Avila & Pasch (1995) highlighted these waves act as precursors to numerous tropical cyclones in this region. 
• DeCaria (2005) stated tropical cyclogenesis in the Indian Ocean and Western Pacific in both hemispheres are heavily impacted by the seasonal shift of the Inter-tropical Convergence Zone and the monsoon trough. 

-- Beta drift 

• When a tropical cyclone moves sluggishly poleward and westward, it is undergoing "beta drift". This occurs because of the vortex superposing onto an environment where the Coriolis force varies with latitude, such as on a sphere or beta plane.

-- Fujiwhara effect (Multiple storm interactions) 

https://en.wikipedia.org/wiki/Fujiwhara_effect

This diagram illustrates the Fujiwhara effect by which 2 tropical cyclones interact with each other.

• Named after Sakuhei Fujiwhara in 1921, the Fujiwhara effect is a phenomenon upon the interaction of 2 adjacent cyclones spin around each other and narrow the distance between the circulations of their corresponding low-pressure areas. 
• Binary interaction of smaller vortices can merge into one cyclone, or produce a larger cyclone. 
• This occurs when 2 tropical cyclones are within 1,400 km (870 mi) of each other, or when 2 extratropical cyclones are within 2,000 km (1,200 mi) of each other. 
• The rotation rates within the binary pairs accelerate when 2 tropical cyclones are within 650 km (400 mi) of each other, or when 2 extratropical cyclones are 1,100 km (680 mi) of each other other. 

-- Interaction with the Westerlies 

• When a tropical cyclone changes direction towards the poles or east either through westward movement from the subtropical ridge axis or interaction with the mid-latitude flow (e.g. jet stream or an extratropical cyclone), it is described as "recurvature". 
• Since the western edge of the major ocean basins contains jet streams directed poleward, tropical cyclone recurvature tends to occur there. 

-- Landfall 

• When a storm's eye drifts over a coastline, it is considered landfall. 
• NOAA declared a tropical cyclone has imposed a "direct hit" on a location when it is located within the radius of maximum winds, regardless of the cyclone's eye position relative to the shore. 

-- Changes caused by El Niño-Southern Oscillation 

• When the subtropical ridge position shifts driven by El Niño, the preferred tropical cyclone tracks also shifts. Therefore, regions west of Japan and Korea experience fewer tropical cyclone events between September & November during El Niño and neutral years. 
• Wu et al. (2004) evaluated the break in the subtropical ridge during El Niño years is roughly 130^oE, which favours the Japanese archipelago. On the other hand, during La Niña years, tropical cyclogenesis along the subtropical ridge position moves westward across the western Pacific Ocean. This results in elevated landfall risks to China and significantly cyclone intensity in the Philippines. 
• ENSO (2008) stated Guam's risk of tropical cyclone impact increases by a third compared to the long-term average during El Niño years. 
• Rappaport (1999) explained how increased vertical wind shear across Guam during El Niño years results in depressed activity over the tropical Atlantic Ocean. 

How do cyclones dissipate? 

-- Factors 

• A tropical cyclone's movement over land or mountains, therefore starving it of the warm water required to fuel its strength. This results in disordered regions of low pressure within a day or 2, or the transition into extratropical cyclones. 
• Movement over waters colder than 26.5 ^oC (79.7 ^oF), resulting in a remnant low-pressure area. 
• Vertical wind shear forces the convection and heat engine away from the centre, causing cessation tropical cyclogenesis. 
• Interaction of the Westerlies' main belt can lead to tropical cyclones transitioning to extratropical cyclones.

-- Artificial dissipation 

• During the 1960s and 1970s, the United Stated government set up Project Stormfury in their attempt to weaken hurricanes by seeding silver iodide into hand-picked storms. They believed seeding of silver iodide would result in freezing of supercooled water, leading to the collapse of the eyewall and subsidence of the winds. 
• However, the project was halted due to the unpredictability of the hurricane's behaviour, and the eyewall replacement cycles occurring naturally in intense hurricanes. 
• A 2006 study found silver iodide is unlikely to elicit an significant effect since the supercooled water is situated at the lower echelons of the tropical cyclone's rainbands. 

What are the effects of cyclones? 

https://en.wikipedia.org/wiki/Effects_of_tropical_cyclones

i. At sea 

• A 2000 study detected a mature tropical cyclone releases heat at a rate of roughly 6 x 10¹⁴ Watts. Roth & Cobb (2001) reported tropical cyclones over the ocean induce giant waves, torrential rain, and gale winds, which disrupts international shipping, and cause shipwrecks. 
• Since hurricanes maintain the global heat balance by shifting warm, tropical air to the mid-latitudes and polar regions, it also impacts ocean heat transport. 

- North American colonisation 

• Powerful tropical cyclones have caused shipwrecks that shaped the course of history, as well as influence art and literature. For instance, in 1565, a hurricane helped the Spanish triumph over the French for control of Fort Caroline, hence the Atlantic coast of North America. 
• In 1609, a hurricane caused the Sea Venture shipwreck near Bermuda, which resulted in the Bermuda colonisation. This inspired Shakespeare to write The Tempest. 

- Shipping 

• To safely navigate around tropical cyclones, sailors dissect tropical cyclones according to their direction of motion, and manoeuvre to avoid the cyclone's right segment in the Northern Hemisphere (left segment in the Southern Hemisphere). 
• The dangerous semicircle contains the heaviest rain and strongest winds, and the cyclone's transition speed and additive rotational wind. 
• The navigable semicircle contains subtractive weather conditions, but potentially hazardous nonetheless. 
• It is advisable for ships avoid sailing through the dangerous semicircle and its forecast path. For ships sailing in the dangerous semicircle, it is recommended to allow the true wind blow on the starboard bow and achieve as much headway as possible. 
• For ships sailing through the navigable semicircle, it is recommended to allow the true wind blow on the starboard quarter and achieve as much headway as possible. 

This diagram illustrates a dangerous semicircle at the upper-right corner, with the arrow indicating the direction of motion of a Northern Hemisphere storm. Note that typhoons are asymmetrical, and a semicircle is a misnomer. 

ii. Upon landfall 

-- Strong winds 

• Damages or destroys vehicles, buildings, trees, bridges, personal property, as well as accelerating loose debris into deadly flying projectiles. 
• Land sea (1998) estimated hurricanes account for 21% of all land-falling tropical cyclones, but induce 83% of damage in the USA. Writer (2005) stated it causes power outages, severs vital communications and hampers rescue efforts. 
• Shultz et al. (2005) noted tropical cyclones complicate the efforts to transport food, clean water, and medical supplies to the areas affected. Moreover, it leads to significant economic damage to the impacted region, and to the diaspora of the population residing in the impacted region. 

-- Storm surge

• Known as the increase in sea level, it accounts for 90% of tropical cyclone fatalities. It floods numerous homes, severs escape routes, destroys infrastructure, and disturbs the waters of coastal estuaries. 

-- Heavy rainfall 

• This increases the risk of potential flooding, mudslides, and landslides, as well as disrupt marine life in coastal estuaries. 
• Shultz et al. (2005) stated the wet environment manifested by the tropical cyclone, along with the warm tropical climate and the destruction of sanitation facilities, introduces epidemics of infectious diseases. Wading in waters polluted with sewage can amplify the infections of cuts and bruises of those affected. 
• It's known large stretches of flood waters lead to mosquito-borne illnesses. As evacuees gather in emergency shelters or evacuation centres, the risk of disease propagation increases. 
• Application of moisture to typically dry regions, averting droughts and cancelling water deficits.

-- Tornadoes

• A 2006 study found the considerable rotation of a land-falling tropical cyclone tend to spawn tornadoes, especially in the right front quadrant. 

iii. Natural resources 

-- Geomorphology 

Tropical cyclones reorganise the coastal geology by eroding sand from the beach and offshore area, rearrange coral, and alter dune configuration offshore. 

(a) Coastal ridges 

• Nott (2003) stated waves and storm surges carried by tropical cyclones undersea sands, erode shell deposits, detach corals from shore reefs, and drag debris towards the land in a rolling wave of material deposited onshore. 
• Nott (2007) found a number of Category 5 tropical cyclones moving across northeast Australia's tropical coastline since the previous considerable change in sea levels (roughly 5 millennia ago) has installed ridges of sand, shell and coral within the costal landscape. 
• e.g. In March 1918, a tropical cyclone moving over the northeast Australian (Queensland) town of Innisfail deposited a ridge of pumice 4.5 metres (15 ft) to 5.1 metres (7.1 ft) high. 

(b) Limestone cave stalagmites 

• Nott (2007) found tropical cyclones deposit layers of calcium carbonate of 'light' composition (i.e. unusual isotopic ratio of Oxygen-18 and Oxygen-16) via rainfall onto stalagmites in limestone caves up to 300 km (190 mi) from the cyclone's path. 
• e.g. The Chillagoe limestone caves located in northeast Australia (130 km, 81 mi inland from Cairns) is thought to have accumulated isotropically light rainfall over approximately 800 years. 

-- Landscapes 

• Turton & Dale (2007) found intense tropical cylones defoliate tropical forest canopy trees, shed vines and epiphytes from the trees, fracture tree crown sterns, and uproot trees. Regarding the tree's size and type, cyclonic wind speeds exceeding 42 m/s (150 km/h; 94 mph) can damage trees. 
• A 1998 study stated shed trees and scattered forest debris acted as fuel for potential wildfires. In 1989, a wildfire lasted 3 months and scorched 460 square miles (1,200 km) of forest shed by Hurricane Gilbert. 
• Unwin et al. (1968) provided the following typology for delineating the variable impacts cyclones have on tropical rainforest landscapes along their paths: 

1. Severe & Extensive = Closest to the cyclone's eye (centre); multidirectional impact and crowns of trees snapped, destroyed or blown off. 

2. Severe & Localised = Closer to the cyclone's eye (centre); Direction of the gale winds can be clearly identified, and severe canopy destruction limited to the windward of forest areas. 

3. Moderate Canopy Disturbance = Closer to the cyclone edge; a majority of tree stems unscathed, minimal uprooting of trees, damage is mainly defoliated canopy and broken branches 

4. Slight Canopy Disturbance = Closest to cyclone edge; occasional stem fall or broken branches, damage mainly foliage loss on forest edges only, followed by leaf damage and heavy leaf little falling. 

iv. Societal impact 

• Destruction wrought by tropical cyclones prompts significant redevelopment, readily increasing local property values. This incites new residents to live in dangerous regions with a high risk of future tropical storms. e.g. Hurricane Katrina after Hurricane Camille. 
• In isolated areas with small populations, a large proportion of the death toll accounts for the founder's effect as survivors repopulate the areas affected. e.g. In 1775, a typhoon struck Pingelap Atoll, along with a subsequent famine, significantly decreased the island's population. 
• Generations after the event, roughly 10% of Pingelapese attained a genetic form of colour-blindness aclled achromatopsia. Sheffield (2000) explained one of the survivors of the depopulation had the mutated gene and the population bottleneck drove its genetic expansion in successive generations.

v. Death toll 

A pie graph of American tropical cyclone casualties by cause from 1970-1999.

Deaths per year by tropical cyclones: 

• Adler (2005) estimated about 1.9 million humans perished to tropical cyclones worldwide in the past 2 centuries, and about 10,000 deaths per year. 

US hurricane fatalities by decade. US fatalities in landfalling hurricanes from 1900 to 2009, aggregated by decade. Source: Pielke et al. (2008) and NHC annual summaries. Note that Hurricane Katrina accounts for 93% of the total 1,963 fatalities in the 2000s. 
https://www.researchgate.net/figure/US-hurricane-fatalities-by-decade-US-fatalities-in-landfalling-hurricanes-from-1900-to_fig1_227282076

How are cyclones categorised?

-- Nomenclature & Intensity Classifications 

https://en.wikipedia.org/wiki/Tropical_cyclone_scales

i. Saffir-Simpson Scale (Atlantic, Eastern & Central Pacific) 

ii. ESCAP / WMO Typhoon Committee's Tropical Cyclone Intensity Scale (Western Pacific) 

iii. Indian Meteorological Department Tropical Cyclone Intensity Scale (North Indian Ocean) 

iv. Southwest Indian Ocean Tropical Cyclone Intensity Scale (Southwest Indian Ocean) 

v. Australian Tropical Cyclone Intensity Scale (Australia & Fiji) 

-- Naming 

https://en.wikipedia.org/wiki/Tropical_cyclone_naming

• Tropical cyclones are allocated an identification code comprising of a 2-digit number and a suffix letter by the warning centres that monitor them. The codes begin at 01 every year and are allocated in order to systems, with potential to develop further, eventually bringing destruction to life and property. 

- IDL = International Date Line 

- n = number 

- y = year 

• Tropical cyclones are were historically named after places or objects they impact before their formal naming process. Between 1887 and 1907, the Queensland Government Meteorologist Clement Wragge first used personal names for weather systems. 
• However it was discontinued years after Wragge's retirement, before it was revived in the latter part of World War II for the Western Pacific. Neal (2012) stated formal naming schemes for tropical cyclones occurring in other common basins began to emerge. 
• Presently, tropical cyclones are officially named by 1 of 11 meteorological services and keep their names in order to ease communication between forecasters and the general public concerning forecasts, watches, and warnings, as well as minimise confusion in terms of description. 
• Depending on the basin it develops in, tropical cyclones are provided names in order from predetermined lists with 1, 3 or 10-minute sustained wind speeds of more than 65 km/h (40 mph). You can find those list of names in the list above. 

When and where were the deadliest cyclones / typhoons / hurricanes? 

https://en.wikipedia.org/wiki/List_of_tropical_cyclone_records

https://en.wikipedia.org/wiki/Lists_of_Atlantic_hurricanes

https://en.wikipedia.org/wiki/List_of_Pacific_hurricanes

https://en.wikipedia.org/wiki/List_of_the_most_intense_tropical_cyclones

https://www.wunderground.com/hurricane/articles/deadliest-tropical-cyclones

List of tropical cyclone records: 

Hurricane Katrina: 

https://www.youtube.com/watch?v=HbJaMWw4-2Q

viii. Thunderstorms 

https://en.wikipedia.org/wiki/Thunderstorm 

• Also known as an electric storm or a lightning storm, a thunderstorm is a type of storm system distinguished by lightning and its acoustic effect on the Earth's atmosphere, thunder. 

Describe the life cycle of thunderstorms

• Deng (2005) stated warm air ascends and cool air descends to the bottom because warm air is more dense than cool air. As warm air carries moisture into higher altitudes within cooler air, this leads to formation of clouds. 
• A 2007 study stated rising moist air cools and water vapour within that rising air begins to condense. Mooney (2007) explained the condensation of moisture releases energy known as 'latent heat of condensation', which results in reduced cooling of ascending air compared to the cooler surrounding air driving the ascending clouds. 
• Sufficient atmospheric instability for longer periods leads to the formation of cumulonimbus clouds and develop lightning and thunder. 
• Blanchard (1998) outlined meteorological indices such as convective available potential energy (CAPE) and the lifted index help determine the potential upward vertical cloud development. 
• There are 3 criteria for the formation of thunderstorms: (1) Moisture; (2) An unstable airmass; (3) A lifting force (heat). 
• The average thunderstorm is approximately 24 km (15 mi) diameter, with each of the 3 stages (explained below) lasting at least 30 minutes. 

(1) Developing stage 

• Also known as the cumulus stage, it involves moisture ascending into the atmosphere caused by solar illumination, convergence of 2 winds, or the wind blowing over terrain of increasing elevation. 
• The moisture then cools to condense into liquid water due to cooler temperatures at high altitude, giving the appearance of cumulus clouds. This subsequently releases latent heat, reduces its density relative to the surrounding air. 
• As the air ascends in an updraft via convection, it produces a low-pressure zone within and under the developing thunderstorm. 
• Vidali (2009) estimated a typical thunderstorm elevates 500 million kg of water vapour into the Earth's atmosphere. 

(2) Mature stage 

• The warm air continues to ascend until it approaches an area of warmer air where it plateaus, referred to as the 'tropopause'. 
• The air dissipates and transforms the storm into an anvil shape, generating a cloud called a cumulonimbus incus. 
• The water droplets then coalesce to form larger and heavier droplets before they freeze into ice particles. Due to their higher mass, gravity forces these ice particles to fall and interaction with the warmer air melts them to produce rain. 
• A sufficiently strong updraft that keeps droplets aloft adequately long to enlarge to a point they don't melt completely and fall as hail instead of water droplets. The falling rain drags the surrounding air downwards, forming a downdraft. 
• This cycle of updrafts and downdrafts indicates the thunderstorm's mature stage and generates cumulonimbus clouds. 
• A 2009 article in the Aviator's Journal reported cumulonimbus clouds produces internal turbulence in the form of gale winds, lightning, and tornadoes. 
• Mogil (2007) thought a minimal amount of wind shear drives the storm's progression to the next stage and 'rain itslef out'. Zeitler & Bunkers (2005) described an adequate shift in wind speed or direction leads to the segregation of downdrafts and updrafts, forming a supercell. 

(3) Dissipating stage 

• This stage involves downdrafts pushing downwards out of the thunderstorm, contact the ground and dissipate, known as a 'downburst'. They are hazardous weather conditions for aircraft to fly through because rapid changes in wind speed and direction can impact on airspeed and subsequent loss of lift in the aircraft. 
• Cool air dragged to the ground by the downdraft severs the inflow of the thunderstorm, causes the updraft to dispel, and allows the thunderstorm to dissipate. 
• As thunderstorms protrude an outflow boundary radially, they weaken due to a lack of wind shear. This disrupts its inflow of relatively warm, moist air, and hampers the thunderstorm's growth.   

What are the different types of thunderstorms? 

(i) Single-cell (Air-mass) 

https://en.wikipedia.org/wiki/Air-mass_thunderstorm

• Also known as an air-mass thunderstorm, single-cell thunderstorms are relatively weak storm systems that develop in environments with Convective Available Potential Energy (CAPE), exhibiting little wind shear and helicity. 
• They appear in temperate zones, as well as in areas with cool unstable air followed by a cold front passage drifting from the sea during winter. 
• When the storm system becomes severe temporarily, it is referred to as a 'pulse severe storm'. Due to their disorderly nature caused by a lack of vertical wind shear and its random nature in time and space, they are difficult to forecast. 
• The National Severe Storms Laboratory (2006) estimated single cell thunderstorms last approximately between 20 and 30 minutes. 

How do they move? 

1. Advection of the wind 

2. Propagation along outflow boundaries towards areas of concentrated heat and moisture

What are their effects? 

- Convective rain = Includes graupel and hail; heavy rainfall deteriorates the microwave transmission exceeding 10 GHz (Gigahertz), which increases in severity with frequencies above 15 GHz. 

- Lightning = Increases the risk of wildfires; total lightning rate is directly proportional with the thunderstorm's size, its updraft velocity, and level of graupel over land. 

- Wind shear = Manifested by the formation of an outflow boundary

- Haze = Poses visibility difficulties for pilots 

- Hail 

(ii) Multi-cell clusters 

https://en.wikipedia.org/wiki/Multicellular_thunderstorm

• Multicellular thunderstorms are namely composed of multiple cells, each being at a different stage in the thunderstorm's life cycles. 
• With each individual cell lasting about 20 to 60 minutes, a multicellular cluster lasts for several hours. 
• A typical holograph illustrates a linear relationship between wind shear and altitude. This implies moderate vertical wind shear results in an asymmetric surface convergence associated with the thunderstorm outflow. 
• The National Weather Service (2017) measured the movement of indivuals cells along the wind shear to be at 30^o, at 70% of the mean wind speed in the layer. 

A holograph of a multicell thunderstorm

This diagram illustrates the radar reflectivity in the clouds. The arrow indicates the vertical motion. 

What are their effects?

• Severe hail 
• Rain 
• Transition into a Mesoscale Convective System (MCS) or a squall line. 
• Intense outflow of straight-line winds 
• Tornadoes 
• Squall lines can elongate, accelerate and become a derecho. 

(iii) Multi-cell lines (Squall lines) 

https://en.wikipedia.org/wiki/Squall_line

A squall line over 1,600 km (1,000 mi) long across the Gulf of Mexico and Eastern USA detected on 30th January 2013. Note the radar coverage originated from ground radars, thus the middle image doesn't cover the portion over the Gulf. The bottommost image is captured a few hours after the other 2, illustrating the strongest portion of the line as it passes through Florida, Georgia and South Carolina.   

• Also known as a quasi-linear convective system, a squall line is a string of thunderstorms that develop along or ahead of a cold front. 
• This weather event brought heavy precipitation, hail, lightning, intense straight-line winds, as well as tornadoes and water spouts. 
• During World War I,  Jacob Bjerknes proposed his polar front theory based on observations from a network of sites in Scandinavia. It suggested the main inflow into a cyclone converged along 2 lines, 1 ahead of the low and the other trailing behind the low. He labelled the trailing convergence zone as the squall line or cold front. 
• The University of Oklahoma (2004) conceptualised the 3D structure of the cyclone after the development of the upper air network. 

Describe the life cycle of squall lines 

Typical development of (a) into a bow echo (b,c) and into a comma echo (d). The dashed line indicates axis of greatest potential for downbursts. The arrows denote wind flow relative to the storm. Area C is most susceptible to tornado development.  

1. Organised sections of thunderstorm activity strengthen pre-existing frontal zones, leading them to outrun cold fronts. This situates within the westerlies in a conformation where the upper level divide into 2 streams. 
2. This leads to a mesoscale convective system (MCS) developing at the upper level separation in the wind configuration in the region of low level inflow. 
3. The convection then drifts eastwards and equatorward into the warm sector, parallel to low-level thickness lines. NOAA depicted a MCS with a linear or curved convection as a squall line, which situates at the leading edge of the strong wind shift and pressure increase.
4. Squall lines developing over desolate areas create a dust storm known as a 'haboob'. Meanwhile, wake lows can form on the back edge of the rain shield behind the squall line. Since the descending air mass heats up, it results in a heat burst. 
5. Former cumulonimbus clouds disintegrate into cumulus, stratocumulus, cirrus, altocumulus or cirrocumulus ahead of the main squall line, as the shear force weaken.

What are the characteristics of squall lines?

Cross-section of a squall line depicting precipitation, airflow and surface pressure. 

- Updrafts = It ascends from ground level to the highest extensions of the troposphere, condenses water, and constructs an ominous cloud shaped like an anvil. 

- Pressure perturbations = They are generated by layers of updrafts and downdrafts as buoyancy is swift within the lower and mid-levels of a mature thunderstorm, resulting in a pressure mesocentres.  

- Wind shear = In low to medium wind shear environments, there are moderate levels of downdrafts, resulting in a leading edge lifting mechanism, i.e. the gust front. In high wind shear environments, there are updrafts and resultant downdrafts demonstrate high intensity. 

What is a derecho? 

https://en.wikipedia.org/wiki/Derecho

• Deriving from the Spanish word "derecho" meaning "straight", this weather event is an extensive and lasting, powerful straight-line storm induced by convection. It relates with a rapid band of thunderstorms shaped like a bow echo. 
• Their winds are sustained and increase in intensity behind the gust front in the direction of their associated storms' movement. 
• They appear mainly in the summer months in the Northern Hemisphere between May and August in the Northern Hemisphere. Corfidi et al. (2005) found derechoes occur at any time of the year and occur more frequently at night compared to the day time. 
• Characteristics include sustained wind speeds of 93 km/h (58 mph) during the storm, a bow echo on radar, rear inflow notch and bookend vortex, as well as downbursts. 
• They often situate in North America, though they can potentially appear in other places such as Asia. 

(iv) Supercells 

https://en.wikipedia.org/wiki/Supercell

• A supercell is an uncommon but incredibly powerful thunderstorm distinguished by the presence of a mesocyclone. 
• They form in the warm sector of a low pressure system propagating northeastwards in line with its cold front. 
• Supercells may deviate left or right from the mean wind shear, or develop 2 separate updrafts with opposing rotations. 
• They manifest in a range of sizes from small to large, and produce significant hail, torrential rainfall, powerful winds and downbursts. 
• Although they can occur anywhere around the world under suitable conditions, it wasn't until 1962 that the first supercell thunderstorm was observed in Wokingham, England by Keith Browning and Frank Ludlam. 
• Studies implied supercells often occur in the Great Plains of central USA and southern Canada extending into the southeastern USA and northern Mexico, as well as east-central Argentina, some regions of Uruguay, Bangladesh, eastern India, South Africa, and eastern Australia. 

Describe the anatomy of supercells 

• The modern conceptual model of a supercell was depicted by Leslie R. Lemon and Charles A. Doswell III in Severe Thunderstorm Evolution and mEsocyclone Structure as Related to Tornadogenesis. 
• Moisture flows in from the side of the precipitation-free base and coalesces into a stream of warm updraft where the top of the thunderstorm is capped by shear winds. 
• The strong wind shear changes the vorticity's angle from horizontal to vertical, and causes the ascending clouds to spin, resulting in a mesocyclone. 
• A cap develops where shear winds prevent further updrafts temporarily before a weakness leads to a protrusion of an overshooting top. 
• The cap applies a warm-over-cold layer above a cold-over-warm boundary layer, which blocks the ascent of warm surface air. This leads to warming or humidifying of the air below the cap and cooling of air above the cap. 
• While the cool and dry air propagate into the warm, moist moisture laden inflow, the cloud base produces a cloud wall as well as decrease in pressure. 
• This generates a warm, moist layer under a cool layer, which leads to instability due to warm air having decreased density and the tendency to ascend.

Describe the structure of supercells 

- Overshooting top = The dome-shaped feature appears above the intense updraft location on the storm's anvil. It is produced by a strong updraft that protrudes the upper levels of the troposphere into the lower stratosphere. 

- Anvil = This feature is created by the collision between the storm's updraft and the upper levels of the troposphere and forced to flow in one direction dictated by the laws of thermodynamics. Temperatures in this region are relatively cold, precipitation is bare and moisture is scarce, meaning winds are freely moving. When the ascending air reaches altitudes of at least 15,200 metres (50,000 ft), the cloud formation takes on an anvil shape.  

- Precipitation-free base = Situated under the main draft, it is the main area of inflow. Despite the lack of precipitation, hail can form here. 

- Wall cloud = This feature develops near the downdraft / updraft interface, which is the region between the precipitation area and precipitation-free base, due to rain-cooled air from the downdraft being dragged by the updraft. The moist, cooler air rapidly saturates as the updraft elevates it, producing a cloud falling from the precipitation-free base. 

- Mammatus clouds = They are bulb-shaped clouds that extend from under the thunderstorm's anvil. They are produced by cold air in the anvil descending to warmer air beneath it. 

- Forward Flank Downdraft (FFD) = This feature has the heaviest and most extensive precipitation. Most supercells contain a precipitation core surrounded by a shelf cloud on its leading edge. This is generated by rain-cooled air within the precipitation core spreading outward and integrating with warmer, moist air from the supercell's exterior. 

- Rear Flank Downdraft (RFD) = Although this feature is not well-understood, it is known to be usually occur within typical, as well as HP and LP supercells. Scientists postulated the RFD is involved in tornadogenesis by contracting rotation within the mesocyclone. It is thought to be a result of supercell's mid-level winds interacting with the updraft column and manoeuvring it in all directions. 

- Flanking line = It is defined as a configuration of smaller cumulonimbi or cumulus that develop in the warm ascending air drawn in by the main updraft. Convergence and updrafts along the feature may result in landspouts appearing on the outflow boundary. 

What are the variations of supercells? 

i. Low Precipitation (LP) 

• These supercells unleash a light precipitation (rain or hail) core independent from the updraft. Rasmussen & Straka (1986) stated they develop within powerful mid-to-upper level storm-relative wind shear, however atmospheric environment driving this phenomenon requires more research. 
• Its main feature include cloud striations in the updraft base or an updraft shaped like a corkscrew, which is generated within drier and moister environments. 
• They were first described by Howard Bluestein in the early 1980s, despite earlier observations by storm chasers in the 1970s. In 2008, Bluestein explained how classic supercells maintain updraft rotation as they shrink, transitioning to the LP supercell. This process is known as "downscale transition". 

ii. High Precipitation (HP) 

• This feature has a heavier precipitation core that envelops around the mesocyclone. They bring about torrential rain, downbursts, tornadoes, relatively small hail, and cloud-to-ground and intracloud lightning 

iii. Mini-supercell (Low-topped supercell) 

• Identified by Jon Davies in the early 1990s, these mini-supercells are smaller than classic supercell thunderstorms. 

What are the effects of supercells? 

• Hailstones with an average diameter of 5.1 cm (2 inches)
• Wind speeds over 110 km/h (70 mph) 
• Tornadoes between EF3 and EF5 intensity (assuming wind shear and atmospheric instability are sufficient) 
• Heavy floods due to torrential rainfall 
• Frequent lightning 

Where and when were the strongest supercells occur? 

(v) Severe thunderstorms 

• If wind speeds reach at least 93 km/h (58 mph), fallen hail is 25 mm (1 in) in diameter or larger, or if funnel clouds are observed, then this is officially a 'severe thunderstorm' in the USA. 
• If rainfall rate exceeds 50 mm (2 in) in 1 hour, or 75 mm (3 in) in 3 hours, then it is classified as a severe thunderstorm in Canada. 

(vi) Mesoscale convective systems 

https://en.wikipedia.org/wiki/Mesoscale_convective_system

• A mescoscale convective system (MCS) is a network of thunderstorms that is larger than a single thunderstorm but smaller than extratropical cyclones. 
• They contain weather systems such as tropical cyclones, squall lines, lake-effect snow events, polar lows, and mesoscale convective complexes (MCC). 
• Mogil (2007) stated a majority of MCS form overnight and last throughout the following day. 
• Haerter et al. (2020) found they develop when the surface temperatures changes by more than 5^oC (9^oF) between day and night. 
• Morel & Senesi (2002) noted MCS develop over land during warmer months across Asia, Europe and North America, with activity peaking during the late afternoon and evening hours. 
• Some forms of MCS appear in the tropics in either the Intertropical Convergence Zone or monsoon troughs, during the warmer months in spring and autumn.

How do thunderstorms move? 

• (1) Advection of the wind and (2) propagation along outflow boundaries towards warmer and moister environments. 
• Weaker thunderstorms are driven by winds nearer to the Earth's surface than powerful thunderstorms, whereas long-lasting thunderstorm cells move at right angles with respect to the vertical wind shear vector. 
• If the leading edge of the outflow boundary drifts ahead of the thunderstorm, the storm accelerates in tandem. 
• A back-building thunderstorm, or a training thunderstorm, develops on the upwind side that either makes the storm stationary or propagate backwards. 
• Corfidi (2015) described this phenomenon as a multi-cell storm with novel, intense cells developing on the upwind side, superceding older cells.  

What are the hazards of thunderstorms? 
i. Cloud-to-ground lightning 

• Risk of sparking wildfires -- Rakov (1999) stated the light rainfall from LP thunderstorms fails to prevent the dry vegetation from igniting since lightning generates a prodigious amount of heat energy. 
• Risk of acid rain -- During a thunderstorm, nitric oxide forms due to the oxidation of atmospheric nitrogen. When it interacts with water molecules in the precipitation, it leads to acid rain. This can damage infrastructure composed of calcite, dissolve vegetation tissue, and increase the acidification process in bodies of water and in soil, causing death in marine and terrestrial organisms. 

ii. Hail 

• Hailstorms frequently occur along mountain ranges since mountains force horizontal winds upwards (i.e. orographic lifting), therefore strengthening the updrafts within thunderstorms and increasing the likelihood of hail. 
• Damages automobiles, aircraft, skylights, glass roofs, livestock, and farmers' crops (e.g. wheat, corn, and soybeans). 
• They frequently occur in mountainous northern India, Colorado, Nebraska and Wyoming in North America, and Canada. 

iii. Tornadoes & Water spouts 

• Details on these topics in the tornadoes section of the blog. 

iv. Flash flood 

• It involves torrents of water rapidly submerging stretches of urban landscape. They associated with torrential rainfall and occur more swiftly and localised than seasonal river flooding and areal flooding. 
• It poses a risk on infrastructure such as bridges, and poorly constructed buildings, as well as plants and crops in agriculture, automobiles stationed within impacted areas, cause soil erosion and possibly landslides.

v. Downburst 

• They are intense wind patterns that are often misunderstood for wind speeds generated by tornadoes. 
• Poses a risk to unstable, incomplete, or poorly constructed infrastructures and buildings. 
• Damages and uproots agricultural crops, and other plants in impacted environments. 
• Mogil (2007) described downburst winds develop in environments with high pressure air systems of downdrafts fall and displace the air mass beneath it, as it's more dense. When the downdrafts approach the surface, they propagate and transform into destructive, horizontal winds. 

vi. Thunderstorm asthma 

• It is an asthma attack by environmental conditions directly caused by thunderstorms. Studies found pollen particles absorb moisture and then rupture into minute fragments that get dispersed by the wind. Although larger pollen particles are filtered by nose hairs, smaller pollen particles avoids those hairs and infiltrate the lungs, triggering asthma attacks. 

How often do thunderstorms occurs? 

https://en.wikipedia.org/wiki/United_States_rainfall_climatology

https://en.wikipedia.org/wiki/Distribution_of_lightning

• Although thunderstorms can occur anywhere in the world, they often form in tropical rainforest regions. Studies estimated around 2,000 thunderstorms occur on Earth worldwide at any given time, with areas such as Kampala and Tororo (Uganda), Darwin (Australia), Caracas (Venezuela), Manila (Philippines) and Mumbai (India). 
• The National Weather Service (2008) discovered a correlation between thunderstorm frequency and monsoon seasons worldwide, which involve the rain bands of tropical cyclones. 
• Roth (2006) found thunderstorms are highly prevalent during spring and summer months in temperate regions, though they can exist along or ahead of cold fronts at any time of the year. 
• In the USA, thunderstorms frequently occur in the Midwest and the Southern states, and rarely in the West Coast states. 

The global map of lightning frequency-strikes/km sqr/year. The highest lightning areas are on land located in the tropics. Area with almost no lightning are the Arctic and Antarctic, closely followed by the oceans which have only only 0.1 to 1 strikes/km sqr/year. 

ix. Tornadoes 

https://en.wikipedia.org/wiki/Tornado

https://en.wikipedia.org/wiki/Fujita_scale

• A tornado is a violently rotating column of air that contacts both Earth's surface and a cumulonimbus cloud or the base of cumulus cloud. It is often referred to as a twister, or whirlwind. 
• The word tornado originates from the Spanish word of the same spelling (past principle of 'to turn', or 'to have torn'). 

What are the different types of tornadoes? 

-- Funnel cloud 

https://en.wikipedia.org/wiki/Funnel_cloud

• A funnel cloud is a rotating column of wind that extends from the (cumulonimbus or cumulus) cloud's base that pertains a funnel-shaped cloud of condensed water droplet, but it doesn't contact the ground or a water surface. 
• Cold-air funnel clouds (vortices) are relatively weaker than the vortices generated by supercells. Studies stated they associate with partly cloudy skies in the wake of cold fronts, especially particular low pressure systems, or atmospheric boundaries such as a lake and sea breeze or outflow boundaries. 

-- Tornado outbreaks & families 

https://en.wikipedia.org/wiki/Tornado_family

https://en.wikipedia.org/wiki/Tornado_outbreak_sequence

• A tornado family is defined as a series of tornadoes generated by a thunderstorm supercell. They emerge as a line of parallel tornado paths, which travel a range of distances. 
• Studies defined a period of consecutive days with tornado outbreaks in a regular region as a tornado outbreak sequence, or an extended tornado outbreak. 
• e.g. Hesston-Goessel, Kansas tornadoes of March 1990, Tri-State Tornado of March 1925. 

-- Multiple vortex 

https://en.wikipedia.org/wiki/Multiple-vortex_tornado

• A multiple-vortex tornado comprises of several vortices (subvortices or suction vortices) rotating around, within, and as part of the main vortex. 
• e.g. May 2011 EF5 Joplin, Missouri tornado; May 31, 2013 El Reno, Oklahoma 

-- Waterspout 

https://en.wikipedia.org/wiki/Waterspout

• It is a powerful columnar vortex that situates over a body of water. They appear under different cloud forms such as cumulus, cumuliform, and cumulonimbus. 
• They can form anywhere in the world, mainly in the tropics and subtropical areas, as well as in Europe, Australia, New Zealand, Middle East, the Great Lakes, USA and Antarctica. 

How do water spouts form? 

• They form in highly moist environments as their parent clouds begin development. Scientists hypothesise they rotate during its ascent towards the surface boundary from the horizontal shear adjacent to the surface. 
• Subsequently, they elongate vertically towards the cloud as the low level shear vortex orients with a developing cumulus cloud or thunderstorm. 

What are the different types of water spouts? 

i. Non-tornadic (Fair-weather)

Non-tornadic water spouts observed from a beach at Kijkduin near The Hague, the netherlands on 27 August 2006.

• The most frequent type of water spouts, but they aren't related to the rotating updrafts typically found in a supercell thunderstorm. 
• They frequently develop over coastal waters and form under developing convective cumulus clouds. 
• Choy & Spratt (2006) estimated this type of water spout's life cycle lasts around 20 minutes. 
• The National Weather Office (2008) reported the intensity of fair-weather water spouts register no higher than EF0 on the Enhanced Fujita scale, with wind speeds less than 30 m/s (67 mph, 108 km/h). 
• NOAA (2007) found they commonly situated in tropical and sub-tropical climates, with approximately 400 annually spotted in the Florida Keys. 
• Smith (2007) observed their movements were relatively slow due to its horizontally static attachment to the cloud, manifested by vertical convective action. 

ii. Tornadic 

A tornadic water spout off the coast of Punta Gorda, Florida due to a severe thunderstorm, spotted on 15 July 2005.

• They are a product of mesocyclones that is reminiscent of land-based tornadoes associating with severe thunderstorms.
• They are generally tornadoes moving over bodies of water instead of land. 
• They can occur, albeit rarely, in landlocked regions of the USA, as well as Adriatic, Aegean and Ionian Seas. 

iii. Snow spout 

A snow spout over Lake Ontario, off the shore of Whitby, Ontario spotted on 26 January 1994.

• Also known as a winter waterspout, snow devil, ice spout, ice devil or snownado, it is an uncommon type of water spout that develops under the base of snow squall. 
• The criteria for the development of snow spout include extremely low air temperatures over a sufficiently warm body of water that produces fog, as well as winds concentrating down the axis of long lakes. 

Describe the climatology of water spouts 

• They tend to appear in tropical areas, well as temperate areas, across the European western coast, British Isles, and regions of the Mediterranean and Baltic Seas. 
• They also appear on lakes and rivers such as the Great Lakes and the St. Lawrence River. 
• NOAA (2009) recorded over 66 water spouts across 7 days in 2003 on the Great Lakes during the late summer and early autumn (fall). 
• Joseph Banks (1997) described several water spouts occurring off Australia's east coast during the Endeavour's voyage.
• Studies also recorded water spouts along the southeast US coast (i.e. southern Florida and the Keys), as well as a number of European countries such as Spain, Italy, the Netherlands, and United Kingdom. 

Describe the life cycle of water spouts 

1. A circular, light-coloured disk appears on the water's surface, encompassed by a prodigious darker area. 
2. A dense annulus of sea spray (or a cascade) appears around a dark area that resembles an eye. 
3. A water spout then appears as an observable funnel from the water surface to the overhead cloud. 
4. The spray vortex ascends to a height of several 100+ metres, which generates a visible wake and a wave front as it drifts. 
5. Smith (2007) states the funnel and spray vortex starts to dissipate once the inflow of warm air into the vortex weakens, completing the life cycle.

What are the effects of water spouts? 

• Marine hazards posing risks to water craft, air craft and people. e.g. 1555 Malta water spout impacted the Grand Harbour of Valletta, which sunk 4 galleys, many boats, and killed many lives. 
• Anything within 90 cm (1 yard) of the water spout, such as fish, will be dragged into the air. 
• Even if the water spout halts its rotation, the fish can remain aloft in the atmosphere until the currents dissipate. Cosier (2006) reported incidences of raining fish as far as 160 km (inland) after a water spout. 

-- Land spout 

https://en.wikipedia.org/wiki/Landspout

• Coined by atmospheric scientist Howard B. Bluestein in 1985, it is defined as "a colloquial expression describing tornadoes occurring with a parent cloud in its growth stage and with its vorticity originating in the boundary layer." 
• They develop during the growth stage of the cumulus congestus cloud by stretching the boundary layer vorticity upwards and into the cloud's updraft. 
• Wilson (1989) reported land spouts were smaller and weaker than supercell tornadoes and don't develop from a mesocyclone or pre-existing rotation in the cloud. Hence, they are hardly detected on Doppler weather radar. 

-- Gustnado 

https://en.wikipedia.org/wiki/Gustnado

• Its etymology is a portmanteau by elision of "gust front tornado", since they develop due to non-tornadic straight-line wind characteristics in the downdraft (outflow). 
• It is defined as a short-lived, shallow surface-based vortex that develops within the downburst originating from a thunderstorm. 
• They usually last a few minutes, with wind speeds equivalent to an EF0 and EF1 tornado (180 km/h, 110 mph). 

-- Dust devil 

https://en.wikipedia.org/wiki/Dust_devil

• They are powerful and relatively momentary whirlwinds of dust and debris that vary in size.
• Though they are relatively harmless, they can propagate to sufficiently large sizes to be considered a risk to both life and property. 
• In Australia, they are referred to as "willy-willy".
• In Ireland, they are referred to as "shee-gaoithe" or "fairy wind". 

How do dust devils form?

• When a patch of hot air near the ground ascends rapidly through cooler air above it, it creates an updraft. Under suitable conditions, this updraft may start to rotate. 
• The ascension of hot air stretches the air column vertically, thus shifting mass closer to the axis of rotation, amplifying the spinning effect by conservation of angular momentum, therefore creating a dust devil. 
• Eventually, the dust devil cools, loses its buoyancy and begins to dissipate once the surrounding cooler is absorbed. 

The formation of dust devils require 3 conditions: 

- Flat barren terrain, desert or tarmac

- Clear skies or lightly cloudy conditions 

- Light or no wind and cool atmospheric temperatures. 

What are the characteristics of dust devils? 

• Numerous dust devils tend to small and weak, with diameters less than 1 metre (~3 ft) and maximum average wind speeds of 70 km/h (45 mph), lasting less than a minute. 
• On occasion, dust devils can reach diameters of up to 90 metres (300 ft) with winds exceeding 100+ km/h (60 mph), lasting up to 20 minutes. 

What are the effects of dust devils? 

• Damage to infrastructure, facilities and property. e.g. Coconino County Fairground in Flagstaff, Arizona on September 14, 2000.
• Can cause injury or death on the ground or in the air e.g. skydivers and paragliding pilots. 
• Radio noise and electrical fields stronger than 10,000 volts per metre due to electrically charged particles via triboelectrification. It also generates a fluctuating magnetic field between 3 and 30 times per second. 
• Studies found electric fields aim in the dust devil's ability to elevate 1 gram of dust per second from each square metre (10 lb/s from each acre) of ground it traverses. 
• Sand pillars are referred to as minuscule dust devils that traverse deserts during the summer and elevate 3 times as much dust, influencing the dust composition of the atmosphere. 

Dust devils on Mars 

• The Viking orbiters in the 1970s and the Mars Pathfinder lander in 1997 captured shots of dust devils occurring on Mars. 
• These dust devils can up to 50 times wider and 10 times taller than terrestrial dust devils, which may risk damage to terrestrial technology delivered to the Martian surface. 

A Martian dust devil photography by the Mars rover Spirit. The counter in the bottom-left corner indicates times in seconds after the first photo was captured in the sequence. The final frames illustrate a visible trail on the Martian surface. 

-- Fire whirls 

https://en.wikipedia.org/wiki/Fire_whirl

• Also known as a fire devil or fire tornado, a fire whirl is a fire-induced whirlwind that also comprises of ash or flame. 

How do fire whirls form? 

• Fortofer (2012) described a fire whirl comprising of a burning core and a revolving section of air, with temperatures approaching 1,090 ^oC (2,000 ^oF). 
• They increase in prevalence when a wildfire generates its own wind, as well as in bonfires and inside the laboratory. 
• Umshield et al. (2006) outlined their development sparked by a warm updraft and convergence during the wildfire. Its height ranges from 10 to 50 metres (33 to 164 ft), its width is roughly a few metres (several feet), and its timespan lasts a few minutes. 
• Grazulis (2003) found fire whirls can reach heights over 1 km, blow winds up to 200 km/h (120 mph), and last for more than 20 minutes. 
• Billing et al. (1983) stated fire whirls can uproot trees up to 15 metres or more (49 ft) tall, which may augment the formation of spot fires, hence propagating and beginning new fires as they elevate flammable materials such as tree bark. 
• Bogdan et al. (2016) found fire whirls could form within the region of a plume during a volcanic eruption. Studies observed these fire whirls can develop into cumulonimbus flammagenitus (cloud), which spawn land spouts and water spouts. Rarely, they form mesocyclones, which spawn tornadoes reminiscent of those formed by supercell thunderstorms.

What are the different types of fire whirls? 

Forman (2009) outlined 3 widely recognised categories of fire whirls. 

Type 1 = Stable and centred over burning area 

Type 2 = Stable or transient, downwind of burning area 

Type 3 = Stead or transient, centred over a sparse area adjacent to an asymmetric burning area with the wind. 

Examples of fire whirls

• 1871 Peshtigo fire, Williamsonville 
• 1923 Great Kantō earthuquake, Japan - a fire whirl affected the Hifukusho-Ato region of Tokyo 
• 7th April, 1926, San Luis Obispo, California 
• 2003 Canberra, Australia 
• 2017 Port Hills, Christchurch, New Zealand 
• July 2018, Carr Fire in Redding, California 
• August 2, 2018, Sacrementa California 
• August 15, 2020, Loyalton, California 

-- Steam devils 

https://en.wikipedia.org/wiki/Steam_devil

• It is a small whirlwind rotating over water that absorbs fog into the vortex, making it visible to the naked eye. 
• They were first investigated by Lyons and Pease in 1972 after they observed them at Lake Michigan, Wisconsin, USA in January 1971. They published their findings in an attempt to convince the National Oceanic and Atmospheric Administration (NOAA) to include steam devils in the International Field Year for the Great Lakes, which imminently eventuated the following year. 

Steam devils on Lake Michigan photographed on 31st January 1971, from the paper which first named and reported the phenomenon.

Describe the appearance of steam devils 

• Vortices range between 50 and 200 metres in diameter, and up to 500 metres in height. 
• They generally last no longer than 3 or 4 minutes, while smaller ones over hot springs last several seconds. 
• Smaller steam devils forming over hot springs have diameters of up to a metre, heights between 2 and 30 metres, and rotations of 60 revolutions per minute (rpm). 
• Holle (2007) stated steam devils originating over small bodies of water can separate from their base and drift downstream pushed by the wind. 

How do steam devils form? 

• Steam devils development require a layer of moist air on the water with the foggy air being sucked upwards into non-rotating columns of steam fog. 
• The production of fog requires thawing of frozen water, as well as a wind composed of cold, dry air. This cold air is heated by the water and subsequently humidifies through evaporation. 
• Allaby described the ascending warm air then cools adiabatically due to decreasing pressure, this leads to the condensation of water vapour out into fog streamers. 
• Other prerequisites of steam devil formation include chilling air above a body of water, winds of dry air greater than 25 mph (40 km/h) blistering across the water's surface, and significant temperature differences between the water and the air. 
• Under the correct conditions, the air ascends vigorously increasing instability in air flow, leading to the formation of vortices. 

Where do steam devils often occur?

• On the Great Lakes of North America in early winter 
• In the Atlantic off the coast of the Carolinas when the cold air sweeps across the Gulf Stream 
• Yellowstone Park e.g. Grand Prismatic Spring in the Yellowstone Midway Geyser Basin 

What are the characteristics of tornadoes? 

-- Size & Shape 

• Tornadoes are generally shaped like a funnel with a cloud of debris adjacent to the ground. 
• Tornadoes that are virtually cylindrical and relatively short are referred to as "stovepipe" tornadoes. 
• Giant tornadoes that are at least as wide as their cloud-to-ground height are referred to as "wedge" tornadoes. 
• When tornadoes begin to dissipate, they appear as narrow tubes or ropes, occasionally referred to as "rope tornadoes". This lengthens the funnel, weakens the winds within the funnel due to the conservation of angular momentum. 
• Lyons (2007) found tornadoes in the USA have average diameters of 150 m (500 ft) and average distance covered around 8km (5 mi), though they can appear in a range of diameters and travel a range of distances. 
• e.g. A wedge tornado that impacted El Reno, Oklahoma on 31st May 2013 was 4.2 km (2.6 mi) in diameter. On 18th March, 1925, the Tri-State Tornado impacted sections of Missouri, Illinois and Indiana, known to traverse 352 km (219 mi). 

-- Appearance 

• Depending on the environment they develop in, tornadoes can appear in a variety of colours. 
• Virtually invisible tornadoes form in drier environments, with only the swirling debris at the base of funnel indicating its existence. 
• Condensation funnels that lift little or no can appear grey or white, whereas water spouts can appear blue. 
• Slow-moving tornadoes that consume a significant amount of dust and debris may appear dark brown, or generally shares the same colour as the dust's colour. 
• A tornado viewed in front of the sun can appear quite black, whereas a tornado viewed with the sun behind the observer can appear grey or white. 
• Tornadoes forming near subset can appear in a range of colours such as yellow, orange, and pink. 
• The visibility of tornadoes is influenced by the dust picked up by the winds of the parent thunderstorm, torrential rain, hail, and the darkness of night. 

-- Rotation 

• Tornadoes typically rotate cyclonically anti-clockwise in the northern hemisphere, and clockwise in the southern hemisphere. 
• The influence of the Coriolis effect is negligible on thunderstorms and tornadoes due to their small size, represented by their large Rossby numbers. 

-- Sound 

• Tornadoes emit a broad spectrum of sounds produced by a multitude of mechanisms. Descriptions of such sounds involve whooshing and roaring winds, including a freight train, raging rapids or waterfalls, etc. 
• Witnesses report funnel clouds and smaller tornadoes whistle, whine, hum or buzz reminiscent of bees and electricity, or more or less harmonic. 

-- Electromagnetic, lightning, and other effects 

• Studies reported emissions on the electromagnetic spectrum, such as sferics (radio atmospheric signals) and E-field effects. 
• Perez et al. (1997) found powerful tornadoes and thunderstorms manifests greater and irregular dominance of positive polarity cloud-to-ground (CG) discharges. Overall CG lightning activity reduces as a tornado contacts the ground and eventually returns to baseline as a tornado dissipates. 
• Tornadoes also elicit changes in atmospheric variables such as temperature, moisture, and pressure. e.g. In 2003, a probe located in South Dakota, USA, measured a decrease in pressure by 100 millibar (hPa) (2.95 inHg). This coincided with an approaching vortex, which was subsequently followed by a considerable pressure decrease towards 850 millibar (hPa) (25.10 inHg) in the tornado's core. Then the pressure increases sharply as the vortex drifts away, producing a V-shaped pressure trace. 

How are intensity and damage of tornadoes measured? 

• The Fujita scale rates the tornadoes by damage caused, which has been upgraded to an Enhance Fujita Scale.

• Each tornado is categorised based on information collected by a ground or aerial damage survey, or both; as well as circumstances, ground-swirl patterns (cycloidal marks), weather radar data, witness testimonies, media reports and damage imagery, and photogrammetry or videogrammetry (if motion picture recording is available). 
• This scale was introduced in 1971 by Ted Fujita of the University of Chicago, in collaboration with Allen Pearson, who was head of the National Severe Storms Forecase Center (NSSFC). 

This schematic was produced by Dr. Ted Fujita (1920-1998) which explains the technical details of the Fujita tornado intensity scale when he introduced it. 

Describe the climatology of tornadoes 

This map highlights the areas worldwide where tornadoes are likely to occur, indicated by orange shading.

• Dotzek (2003) concluded a majority of tornadoes occur in the USA, which is roughly 4 times more than estimated in all of Europe, except water spouts. 
• Since North America's geography extends from the tropics north towards arctic regions, with a lack of east-west mountain range to prevent air flow between these 2 regions. 
• In the middle latitudes, the Rocky Mountains prevent moisture and destabilise atmospheric flow, which allow downsloped winds to steer dry air at the troposphere's mid-levels, and produce a low pressure area downwind to the east of the Rocky Mountains. 
• Cai (2001) found a dry line develops when westerly air flow off the Rocky Mountains increase and exhibits strength aloft, as well as the Gulf of Mexico provides low-level moisture to the southerly flow heading east.

This map illustrates intense tornado activity in the USA. The darker-coloured areas describe the area referred to as 'Tornado Alley'. 

• A significant portion of tornadoes develop in a defined area of central USA called 'Tornado Alley'. It also extends into Canadian regions such as Ontario and the Prairie provinces, southeast Quebec, interior of British Columbia, and western New Brunswick. 
• Dayna (2016) estimated around 1,200 tornadoes occur in USA every year, and around 62 occur in Canada every year. 
• Holden & Wright (2003) found the Netherlands recorded the highest average number of tornadoes per area of any country (more than 20, or 0.00048 per km2 , 0.0013 per sq mi), annually, with the UK coming second (~33, or 0.00013 per km2 , 0.00035 per sq mi). 
• Bimal & Rejuan stated the deadliest tornadoes occur in Bangladesh, which kill an average 179 people every year. Factors contributing to the death toll include the region's high population density, deplorable construction quality, and lack of tornado safety knowledge. 
• Grazulis (1993) concluded tornadoes frequently appear in spring months and rarely in winter months. More intense winds, wind shear, and atmospheric instability coincides with the spring months. 
• Many studies concluded a majority of tornadoes occur in the late afternoon, between 3 pm and 7 pm local time, peaking around 5 pm, though they can occur at any time of day. e.g. 1936 Gainesville Tornado occurred at 8:30am local time. 

Describe the life cycle of tornadoes 

1. Formation 

• As a mesocyclone descends below the cloud base, it draws in cool, moist air from the downdraft area of the storm. 
• As warm air converges with cool air in the updraft, it produces a rotating wall cloud. Meanwhile, the rear flank downdraft (RFD) concentrates the mesocyclone's base, allowing it to absorb air from a minuscule area on the ground. 
• As the updraft strengthens, it generates an area of low pressure at the surface. This subsequently causes the mesocyclone to descend producing a visible condensation funnel. 
• Both the funnel and RFD descend and contact the surface, blowing dust outwards and generating a devastating gust front. 

2. Maturity 

• At the beginning, the tornado is fuelled by warm, moist flowing inward, augmenting its growth until it matures to an extent. 
• Lasting between several minutes and over an hour, this is the most destructive phase of the tornado. 
• The NOAA (2003) found the RFD starts to encompass the tornado, which severs the inflow of warm air that usually fuels the tornado.

3. Dissipation 

• This stage involves the tornado's vortex weakening, thence shrinking and becoming a rope tornado.
• Studies state the winds of the parent storm determines the dissipating tornado's shape and ultimate pattern. 
• Singer (1985) described the dissipating tornado becoming ropey due to the conservation of angular momentum, at which point winds can accelerate. 
• Its associated mesocyclone also weakens as the RFD severs the inflow warm air fuelling it. 

Where and when were the deadliest tornadoes? 

This map shows all tornadoes in the Contiguous United States from 1950 to 2013, plotted by midpoint, highest F-scale on top, Alaska and Hawaii negligible (hence not included). Source: NOAA Storm Prediction Center

https://en.wikipedia.org/wiki/List_of_F5_and_EF5_tornadoes

https://en.wikipedia.org/wiki/Tornado_records

Top 10 deadliest tornadoes in USA history 

• Deadliest tornado in world history = Daultipur-Salturia Tornado, Bangladesh - April 26, 1989: Killed approximately 1,300 people. 
• Most extensive tornado outbreak on record = 2011 Super Outbreak - 360 confirmed tornadoes over southeastern USA, 216 of them within a single 24-hour period. This overtook the previous record of 147 tornadoes set by 1974 Super Outbreak. 
• Highest wind speed measured by a tornado = 302 + 20 mph (486 + 32 km/h) - F5 El Reno, Oklahoma, USA on 31st May, 2013.
• Costliest tornado in terms of damage = $2.8 billion - $3.182 billion USD - Joplin, Missouri tornado on 22nd May, 2011 

The next part will be about the hottest natural disasters.