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Tropical cyclones


Tropical cyclones

A tropical cyclone is the generic term for a low pressure system over tropical or sub-tropical waters, with organised convection (i.e. thunderstorm activity) and winds at low levels circulating either anti-clockwise (in the northern hemisphere) or clockwise (in the southern hemisphere). The whole storm system may be five to six miles high and 300 to 400 miles wide, although sometimes can be even bigger.

At its very early and weak stages it is called a Tropical Depression. When the winds reach 39 m.p.h. it is called a Tropical Storm. If the wind should reach 74 m.p.h. or more the tropical storm is called a Hurricane in the Atlantic and the north-east Pacific or a Typhoon in the north-west Pacific. In other parts of the world, such as the Indian Ocean and South Pacific the term Cyclone or Tropical Cyclone is used.

Tropical cyclones form between approximately 5° and 30° latitude and initially move westward (owing to easterly winds) and slightly towards the poles. Many tropical cyclones eventually drift far enough from the equator to move into areas dominated by westerly winds (found in the middle latitudes). These winds tend to reverse the direction of the tropical cyclone to an eastward path.

In the northern hemisphere most tropical cyclones occur between June and November with a peak in September.

How it forms?

In the tropics there is a broad zone of low pressure which stretches either side of the equator. Within this area of low pressure the air is heated over the warm tropical ocean. This air rises in discrete parcels, causing thundery showers to form.

This creates a flow of very warm, moist, rapidly rising air, leading to the development of a centre of low pressure, or depression, at the surface.

The Coriolis force caused by the rotation of the Earth helps the spin of this column of rising air. The development of the surface depression causes an increase in the strength of the trade winds. The spiralling winds accelerate inwards and upwards, releasing heat and moisture as they do so.

As the depression strengthens it becomes a tropical storm and then a hurricane or typhoon. A mature hurricane or typhoon takes the form of a cylinder of deep thundercloud around a centre that is relatively free from clouds.

There is a relatively small area of intense horizontal winds at the surface, often well over 100 m.p.h., while air rises strongly above, maintaining the deep cumulonimbus clouds.

At the centre of the tropical cyclone, air is subsiding, which makes it dry and often cloud free, and there is little or no wind at the surface. This is called the eye of the storm.

What conditions do need tropical storm to build?

  1. A source of warm, moist air derived from tropical oceans with sea surface temperatures normally in the region of, or in excess, of 27 °C;
  2. Winds near the ocean surface blowing from different directions converging and causing air to rise and storm clouds to form;
  3. Winds which do not vary greatly with height – known as low wind shear. This allows the storm clouds to rise vertically to high levels;
  4. Sufficient distance from the equator to provide spin or twist.

How do tropical cyclones get their names?

Tropical cyclones are named to provide ease of communication between forecasters and the general public, regarding forecasts, watches and warnings. Since the storms can often last a week or even longer, and more than one can be occurring in the same region at the same time, names can reduce the confusion about what storm is being described.

In most regions pre-determined alphabetic lists of alternating male and female names are used. However, in the north-west Pacific the majority of names used are not personal names.

While there are a few male and female names, most are names of flowers, animals, birds, trees, foods or descriptive adjectives. The names are also not allocated in alphabetical order, but are arranged by the name of the Asian country which contributed the name.

Structure of a tropical cyclone

A fully developed tropical cyclone has a central cloud free region of calm winds, known as the “eye” of the cyclone with diameter varying from 10 to 50 km.

Surrounding the eye is the “wall cloud region” characterised by very strong winds and torrential rains, which has the width of about 10 to 150 km. The winds over this region rotate around the centre and resemble the “coils of a snake”.

Wind speed fall off gradually away from this core region, which terminate over areas of weaker winds with overcast skies and occasional squall .There may be one or more spiral branch in a cyclone where higher rainfall occurs. The vertical extent of the cyclone is about 15 km.

Why there are fewer cyclones over the Arabian Sea as compared to the Bay of Bengal?

Cyclones that form over the Bay of Bengal are either those develop insitu over southeast Bay of Bengal and adjoining Andaman Sea or remnants of typhoons over Northwest Pacific and move across south China sea to Indian Seas. As the frequency of typhoons over Northwest Pacific is quite high (about 35 % of the global annual average), the Bay of Bengal also gets its increased quota.

The cyclones over the Arabian Sea either originate in-situ over southeast Arabian Sea (which includes Lakshadweep area also) or remnants of cyclones from the Bay of Bengal that move across south peninsula. As the majority of Cyclones over the Bay of Bengal weaken over land after landfall, the frequency of migration into Arabian Sea is low.

In addition to all the above the Arabian Sea is relatively colder than Bay of Bengal and hence inhibits the formation and intensification of the system.

Disaster response system

IMD has established linkages/institutional arrangements with disaster management agencies both at the centre and in the states. During normal weather conditions two bulletins are transmitted to Control Room of National Disaster Management Division (NDM).

In a case of depression develops over north Indian Ocean which has the potential to affect Indian coast, special bulletins at-least three times a day are issued to NDM.

When the system intensifies into a cyclonic storm, the cyclone warning bulletins are every three hourly. At present 4 stage warning procedure as discussed earlier is followed for issuing bulletins to NDM Control Room.

Cyclone Warning Dissemination System (CWDS)

This is a unique scheme not tried anywhere in the world. The scheme has been extremely successful during the cyclones for last 24 years and gained considerable confidence of the public of this country.

  • Designed by ISRO and implemented by IMD in the mid-eighties, the CWDS is used all these years to disseminate cyclone warnings effectively.
  • Selective addressing (Separate messages for each district) is done by transmitting a digital code followed by the actual warning message.
  • CWDS is one-way communication system and will be complimentary to other systems of cyclone warning dissemination. Facility of acknowledgement is available in the upgraded (Digital) version of CWDS
  • The present CWDS network covers 252 stations spread over coastal areas of maritime districts along the east and e west coast.



Future plans of IMD to strengthen the Cyclone warning setup?

  • Strengthening of surface observational network with the state-of-the-art automatic weather stations (AWSs) models.
  • A dense network of Satellite reporting rain gauges in the coastal region.
  • Deployment of Wind Profilers and Cyclone Warning dissemination system.
  • Increased S-Band Doppler Weather Radar network in the coastal region
  • The up gradation of the computing facility in IMD that will place a computing platform capable of running high-resolution global and regional models. It will be used for development of models for better prediction of tropical cyclone track and intensity.
  • Augmentation of Cyclone Warning Dissemination System (CWDS) with state-of-the-art Digital CWDS
  • Supply of satellite radio receivers to fishermen to receive cyclone warnings.

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