Tropical Storm

Tropical Storm 

The origin of a Tropical storm is actually a Convective cloud or Cumulonimbus clouds which are clouds having massive updraft and simultaneously downdrafts at events and grow vertically instead of growing horizontally. What happens is that in oceans which is a large pool of warm waters such convection (thunderstorm formations) happens. During this period, more and more moist air from the surface rises up creating a surface low pressure. If the Sea Surface Temp or SST is more than 26-28C more latent heat will be released from the cloud. Thus its a fuel for storms. 

Once the initial tropical storm builds due to multiple periods of latent heat release, clouds become bigger and we call Tropical Storm strengthening. 

Conditions for formation: 

1) Warm SST 
2) Good lower convergence 
3) Good upper divergence 
4) less horizontal wind shear 
5) good vertical shears 
6) Coriolis effect 
7) Moist area or good area of moisture 

Source: 
(http://uk.answers.yahoo.com/question/index?qid=20100920121258AAwaxwG)

 

Tropical Revolving Storms (TRS) 

The correct names for the topical storms in either hemispheres is Tropical Revolving Storms (TRS). Other names are usually local to an area eg. Hurricane, Spanish, hurakan, god of the storm, or typhoon from the Chinese dialect, tai fung or big wind. The earth’s rotation, the geostrophic effect, determines the direction of rotation of the TRS and in the northern hemisphere it is anti-clock wise or left handed and in the southern hemisphere it is clock wise or right handed. The storms originate generally between 7 and 15 degrees latitude, south or north. They travel initially in a direction of between west to south-west in the southern hemisphere and west and north-west in the northern hemisphere. They generally recurve (change course about 90 degrees to their original course line) at 25 degrees latitude, may be lower in southern hemisphere, and take a direction of north-east in the northern hemisphere or south-east in the southern hemisphere. The formation of a TRS occurs over the ocean as a result of the differential heating between the air and the sea, this causes spiralling thermal currents which gather intensity resulting in a low pressure system. They always travel away from the equator and therefore never cross it. 

Source: 
(http://www.smh.com.au/news/big-questions/if-a-hurricane-rotates-in-one-direction-and-a-cyclone-rotates-inthe-other-what-happens-if-they-cross-the-equator/2005/09/15/1126750065686.html)

 

Tropical Cyclone 

A tropical cyclone is a storm system characterized by a large low-pressure center and numerous thunderstorms that produce strong winds and heavy rain. Tropical cyclones strengthen when water evaporated from the ocean is released as the saturated air rises, resulting in condensation of water vapor contained in the moist air. They are fueled by a different heat mechanism than other cyclonic windstorms such as nor’easters, European windstorms, and polar lows. The characteristic that separates tropical cyclones from other cyclonic systems is that at any height in the atmosphere, the center of a tropical cyclone will be warmer than its surrounds; a phenomenon called “warm core” storm systems. 

The term “tropical” refers both to the geographic origin of these systems, which form almost exclusively in tropical regions of the globe, and to their formation in maritime tropical air masses. The term “cyclone” refers to such storms’ cyclonic nature, with counterclockwise rotation in the Northern Hemisphere and clockwise rotation in the Southern Hemisphere. The opposite direction of spin is a result of the Coriolis force. Depending on its location and strength, a tropical cyclone is referred to by names such as hurricane, typhoon, tropical storm, cyclonic storm, tropical depression, and simply cyclone. 

In the lower troposphere, the most obvious motion of clouds is toward the center. However tropical cyclones also develop an upper-level (high-altitude) outward flow of clouds. These originate from air that has released its moisture and is expelled at high altitude through the “chimney” of the storm engine. This outflow produces high, cirrus clouds that spiral away from the center. The clouds thin as they move outwards from the center of the system and are evaporated. They may be thin enough for the sun to be visible through them. These high cirrus clouds may be the first signs of an approaching tropical cyclone. As air parcels are lifted within the eye of the storm the vorticity is reduced, causing the outflow from a tropical cyclone to have anti-cyclonic motion. 

Physical Structure: 

– Eye and Center 

A strong tropical cyclone will harbor an area of sinking air at the center of circulation. If this area is strong enough, it can develop into a large “eye”. Weather in the eye is normally calm and free of clouds, although the sea may be extremely violent. The eye is normally circular in shape, and may range in size from 3 to 370 kilometres in diameter. Intense, mature tropical cyclones can sometimes exhibit an outward curving of the eyewall’s top, making it resemble a football stadium; this phenomenon is thus sometimes referred to as the stadium effect. 

There are other features that either surround the eye, or cover it. The central dense overcast is the concentrated area of strong thunderstorm activity near the center of a tropical cyclone; in weaker tropical cyclones, the CDO may cover the center completely. The eyewall is a circle of strong thunderstorms that surrounds the eye; here is where the greatest wind speeds are found, where clouds reach the highest, and precipitation is the heaviest. The heaviest wind damage occurs where a tropical cyclone’s eyewall passes over land. Eyewall replacement cycles occur naturally in intense tropical cyclones. When cyclones reach peak intensity they usually have an eyewall and radius of maximum winds that contract to a very small size, around 10 to 25 kilometres. Outer rainbands can organize into an outer ring of thunderstorms that slowly moves inward and robs the inner eyewall of its needed moisture and angular momentum. When the inner eyewall weakens, the tropical cyclone weakens (in other words, the maximum sustained winds weaken and the central pressure rises.) The outer eyewall replaces the inner one completely at the end of the cycle. The storm can be of the same intensity as it was previously or even stronger after the eyewall replacement cycle finishes. The storm may strengthen again as it builds a new outer ring for the next eyewall replacement. 

– Size 

One measure of the size of a tropical cyclone is determined by measuring the distance from its center of circulation to its outermost closed isobar, also known as its ROCI. 

If the radius is less than two degrees of latitude or 222 kilometres, then the cyclone is “very small” or a “midget”. 

A radius between 3 and 6 latitude degrees or 333 to 670 kilometres are considered “average-sized”. 

“Very large” tropical cyclones have a radius of greater than 8 degrees or 888 kilometres. 

Use of this measure has objectively determined that tropical cyclones in the northwest Pacific Ocean are the largest on earth on average, with Atlantic tropical cyclones roughly half their size. 

Movement and track 

– Steering winds: 

Although tropical cyclones are large systems generating enormous energy, their movements over the Earth’s surface are controlled by large-scale winds—the streams in the Earth’s atmosphere. The path of motion is referred to as a tropical cyclone’s track. 

Tropical systems, while generally located equatorward of the 20th parallel, are steered primarily westward by the east-to-west winds on the equatorward side of the subtropical ridge—a persistent high pressure area over the world’s oceans. In the tropical North Atlantic and Northeast Pacific oceans, trade winds—another name for the westward-moving wind currents—steer tropical waves westward from the African coast and towards the Caribbean Sea, North America, and ultimately into the central Pacific ocean before the waves dampen out. These waves are the precursors to many tropical cyclones within this region. 

In the Indian Ocean and Western Pacific (both north and south of the equator), tropical cyclogenesis is strongly influenced by the seasonal movement of the Intertropical Convergence Zone and the monsoon trough, rather than by easterly waves. Tropical cyclones can also be steered by other systems, such as other low pressure systems, high pressure systems, warm fronts, and cold fronts. 

– Coriolis effect: 

The Earth’s rotation imparts an acceleration known as the Coriolis effect, Coriolis acceleration, or colloquially, Coriolis force. This acceleration causes cyclonic systems to turn towards the poles in the absence of strong steering currents. The poleward portion of a tropical cyclone contains easterly winds, and the Coriolis effect pulls them slightly more poleward. The westerly winds on the equatorward portion of the cyclone pull slightly towards the equator, but, because the Coriolis effect weakens toward the equator, the net drag on the cyclone is poleward. Thus, tropical cyclones in the Northern Hemisphere usually turn north (before being blown east), and tropical cyclones in the Southern Hemisphere usually turn south (before being blown east) when no other effects counteract the Coriolis effect. The Coriolis effect also initiates cyclonic rotation, but it is not the driving force that brings this rotation to high speeds – that force is the heat of condensation. 

– Interaction with the mid-latitude westerlies 

When a tropical cyclone crosses the subtropical ridge axis, its general track around the high-pressure area is deflected significantly by winds moving towards the general low-pressure area to its north. When the cyclone track becomes strongly poleward with an easterly component, the cyclone has begun recurvature. A typhoon moving through the Pacific Ocean towards Asia, for example, will recurve offshore of Japan to the north, and then to the northeast, if the typhoon encounters southwesterly winds (blowing northeastward) around a low-pressure system passing over China or Siberia. Many tropical cyclones are eventually forced toward the northeast by extratropical cyclones in this manner, which move from west to east to the north of the subtropical ridge. 

– Landfall 

Officially, landfall is when a storm’s center (the center of its circulation, not its edge) crosses the coastline. Storm conditions may be experienced on the coast and inland hours before landfall; in fact, a tropical cyclone can launch its strongest winds over land, yet not make landfall; if this occurs, then it is said that the storm made a direct hit on the coast. As a result of the narrowness of this definition, the landfall area experiences half of a land-bound storm by the time the actual landfall occurs. For emergency preparedness, actions should be timed from when a certain wind speed or intensity of rainfall will reach land, not from when landfall will occur. 

– Multiple storm interaction 

When two cyclones approach one another, their centers will begin orbiting cyclonically about a point between the two systems. The two vortices will be attracted to each other, and eventually spiral into the center point and merge. When the two vortices are of unequal size, the larger vortex will tend to dominate the interaction, and the smaller vortex will orbit around it. This phenomenon is called the Fujiwhara effect, after Sakuhei Fujiwhara. 

Dissipation: 

A tropical cyclone can cease to have tropical characteristics in several different ways. One such way is if it moves over land, thus depriving it of the warm water it needs to power itself, quickly losing strength. Most strong storms lose their strength very rapidly after landfall and become disorganized areas of low pressure within a day or two, or evolve into extratropical cyclones. There is a chance a tropical cyclone could regenerate if it managed to get back over open warm water, such as with Hurricane Ivan. If it remains over mountains for even a short time, weakening will accelerate. Additionally, dissipation can occur if a storm remains in the same area of ocean for too long, mixing the upper 60 metres (200 ft) of water, dropping sea surface temperatures more than 5 °C (9 °F). Without warm surface water, the storm cannot survive. A tropical cyclone can dissipate when it moves over waters significantly below 26.5 °C (79.7 °F). This will cause the storm to lose its tropical characteristics (i.e. thunderstorms near the center and warm core) and become a remnant low pressure area, which can persist for several days. This is the main dissipation mechanism in the Northeast Pacific ocean. 

Intensity Classifications 

Tropical cyclones are classified into three main groups, based on intensity: tropical depressions, tropical storms, and a third group of more intense storms, whose name depends on the region. For example, if a tropical storm in the Northwestern Pacific reaches hurricane-strength winds on the Beaufort scale, it is referred to as a typhoon; if a tropical storm passes the same benchmark in the Northeast Pacific Basin, or in the Atlantic, it is called a hurricane. 

Neither “hurricane” nor “typhoon” is used in either the Southern Hemisphere or the Indian Ocean. In these basins, storms of tropical nature are referred to simply as “cyclones”. 

– Tropical Depression 
A tropical depression is an organized system of clouds and thunderstorms with a defined, closed surface circulation and maximum sustained winds of less than 33 knots. It has no eye and does not typically have the organization or the spiral shape of more powerful storms. However, it is already a low-pressure system, hence the name “depression”. 

– Tropical Storm 
A tropical storm is an organized system of strong thunderstorms with a defined surface circulation and maximum sustained winds between 33 knots and 62 knots. At this point, the distinctive cyclonic shape starts to develop, although an eye is not usually present. 

– Hurricane or Typhoon 

A hurricane or typhoon (sometimes simply referred to as a tropical cyclone, as opposed to a depression or storm) is a system with sustained winds of at least 64 knots. A cyclone of this intensity tends to develop an eye, an area of relative calm (and lowest atmospheric pressure) at the center of circulation. The eye is often visible in satellite images as a small, circular, cloud-free spot. Surrounding the eye is the eyewall, an area about 16 to 80 kilometres wide in which the strongest thunderstorms and winds circulate around the storm’s center. Maximum sustained winds in the strongest tropical cyclones have been estimated at about 165 knots. 

Source: 
(http://en.wikipedia.org/wiki/Tropical_cyclone)

 

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Arrow Thunderstorm, Lightening, Tornadoes and Hurricane 

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– Because the Coriolis Force is zero at the equator tropical revolving storms do not form at less than about 5 deg Latitude. When they do form (at latitude 10 to 20 deg) the low latitude results in strong winds as the low deepens. To qualify as a full-scale TRS the windspeed has to reach 64KT sustained. 

– The real diameter can vary between 300km and 1500km. 

– The low-level convergence and convection inside the storm leads to an outflow at height. The extensive Cu and Cb cloud formations tend to become arranged in bands more or less concentric with the centre or eye of the storm. Subsidence takes place between these cloud bands in the outer part of the storm as well as in the eye. The central subsidence produces a clear area of light winds in the eye of the storm which can be from a few kilometres to 100km in diameter. 

– High cirrus, heavy sea swell and continuously falling pressure are classic symptoms of the approach of a TRS. 

– There are never any TRSs in the South Atlantic or in the Pacific off South America because the sea there is too cold. 

– The normal TRS season is the autumn of the hemisphere. about August to October in the North and February to April in the South. 

– In the Typhoon region, the season can start earlier, in June. 

– In the Bay of Bengal and the Arabian Sea the northward passage of the ITCZ in April-May and its southward passage in October-November, triggers TRSs out of the normal timescale, extending the season from May to November.

 

Exam Question Tips 

Arrow Tropical revolving storms do not occur in the south-east Pacific and the south Atlantic because of the low water temperature. 

Arrow Tropical revolving storms tend to develop mostly in the western parts of the tropical oceans because there is a maximum of humidity as a result of the trade winds long sea passage. 

Arrow The region of the globe where the greatest number of tropical revolving storms occur is the north west Pacific, affecting Japan, Taiwan, Korea and the Chinese coastline. 

Arrow The most dangerous zone in a tropical revolving storm is in the wall of clouds around the eye. 

Arrow Over which areas can tropical cyclones occur? (http://www.atpforum.eu/showthread.php?t=3779) 

a) Australia, Gulf of Bengal, Atlantic Ocean at 20 deg S 
b) India, Arabic Sea, Atlantic Ocean at 2 deg S 
c) Caribbean Sea, Indian Ocean at 20 deg S, Pacific Ocean at 2 deg N 
d) Caribbean Sea, Gulf of Bengal, Indian Ocean East of Madagascar 

Answer is (d) 

Tropical cyclones is the generic name for all Tropical Revolving Storms (TRS) A high pressure is an Anticyclone a low pressure is a Cyclone or a depression, so tropical lows – tropical cyclones. 

The area names are : 

Hurricane – USA 
Typhoon – Japan and China 
Severe Cyclone – Bay of Bengal and Arabian sea 
Tropical Cyclone – Indian ocean south of equator 
Cyclone – Western Pacific south of equator 

The option (a) – Australia, Gulf of Bengal, Atlantic at 20 degrees S – is wrong because you don’t get TRS’s in Atlantic south of equator. (Ocean too cold) 

The option (b) – India, Arabic sea, Atlantic Ocean at 2 degrees S – is wrong because you do not get TRS’s in the Atlantic S of the equator, or that close to the equator. (No TRS less than 5 degrees from equator) 

The option (c) – Caribbean Sea, Indian Ocean at 20 degrees S, Pacific Ocean at 2 degres N – is wrong (No TRS less than 5 degrees from equator) 

That makes option (d) – Caribbean Sea, Gulf of Bengal, Idian Ocean east of Madagascar – the only viable answer. 

Arrow Over the Indian Ocean and the Bay of Bengal tropical cyclones are occasionally observed, on an average 12 per year. 

Arrow Tropical revolving storms are NOT formed in South Atlantic Ocean