Tropical Cyclones & IMD Warning

1. Tropical cyclone and its annual frequency

The tropical cyclones develop over the warm waters of the Bay of Bengal and the Arabian Sea during the months of May-June and October-November with severe intensity.

The average annual frequency of tropical cyclones in the north Indian Ocean (Bay of Bengal and Arabian Sea) is about 5. The frequency is more in the Bay of Bengal than in the Arabian Sea, the ratio being is 4:1.

The terms “hurricane” and “typhoon” are region specific names for a strong “tropical cyclone”. Tropical cyclones are called “Hurricanes” over the Atlantic Ocean and “Typhoons” over the Pacific Ocean.

A tropical cyclone (TC) is a rotational low-pressure system in tropics when the central pressure falls by 5 to 6 hPa from the surrounding and maximum sustained wind speed reaches 34 knots (about 62 kmph). It is a vast violent whirl of 150 to 800 km, spiraling around a centre and progressing along the surface of the sea at a rate of 300 to 500 km a day.

2. What is the energy potential of a Tropical Cyclone?

Tropical Cyclone can be compared to a heat engine. The energy input is from warm water and humid air over tropical oceans. Release of heat is through condensation of water vapour to water droplets/rain. Only a small percentage (3%) of this released energy is converted into Kinetic energy to maintain cyclone circulation (wind field). A mature cyclone releases energy equivalent to that of 100 hydrogen bombs.

  • Wind speed criteria for different rain bearing systems form over the ocean
SystemAssociated wind speed Knots (Kmph)
Low pressure area<17(<31)
Depression17-27 (31-49)
Deep Depression (DD)28-33 (50-61)
Cyclonic Storm (CS)34-47 (62-88)
Severe        Cyclonic       Storm (SCS)48-63 (89-117)
Very       Severe         Cyclonic Storm (VSCS)64-89 (118-166)
Extremely Severe Cyclonic Storm (ESCS)90-119 (167-221)
Super Cyclonic Storm≥ 120 (≥ 222)
Wind speed criteria for different rain bearing systems form over the ocean

3. How do the cyclones form and intensify?

In the tropics, weak pressure waves move from east to west. These are  called easterly waves. Under favourable situation, a low pressure area forms over the area of an easterly trough. This gives rise to low level convergence. If the sea is warm (sea surface temperature ≥ 26.50 C) and there is sufficient upper level divergence i.e air is blown off at higher levels from the area of low pressure, the pressure gradually falls. Low level convergence coupled with upper level divergence gives rise to vertical motion taking moist air upwards. These moistures condense at higher levels (middle troposphere) and give out latent heat of condensation. Due to release of heat of condensation the area warms up resulting into further fall in pressure. This process continues and a low pressure system gradually intensifies into a cyclonic storm.

4. What is the size of a tropical cyclone over the north Indian Ocean?

The size of a Tropical Cyclone over Indian seas varies from 50-100 km radius to 2000 km with an average of 300 –600 km.

5. What is the life period of cyclones?

Life period of a tropical cyclone over the north Indian Ocean is 5-6 days. It will have hurricane intensity for 2-4 days as against 6 days of global average.

6. What is the normal movement of a Tropical Cyclone?

Tropical cyclones move as a whole. They casually move west- northwestwards or northwestwards in the northern hemisphere. The average speed is 15-20 kmph (360-480 km per day). They may change their direction of movement towards north. During this change their speed of movement decreases to 10 kmph or even less. A larger fraction of such storms later turn towards northeast and move northeastwards very fast at a speed of 25 kmph or more.

7. How are Tropical Cyclones monitored by IMD?

IMD has a well-established and time-tested organization for monitoring and forecasting tropical cyclones. A good network of meteorological observatories (both surface and upper air) is operated by IMD, covering the entire coastline and islands. The conventional observations are supplemented by observational data from automatic weather stations (AWS), radar and satellite systems. INSAT imagery obtained at hourly intervals during cyclone situations has proved to be immensely useful in monitoring the development and movement of cyclones.

8. How track prediction is done in IMD?

IMD has one of the best forecasting system for predicting tropical cyclones, using high resolution weather prediction models including global, regional and cyclone specific models.

The ensemble forecast systems introduced in the recent past provides probabilistic guidance for track and intensity of cyclones and depressions. Regional models like Hurricane Weather Research Forecast (HWRF) have also been implemented to generate forecasts at very high resolution of 2km. IMD is also planning to make the ocean atmosphere coupled Hurricane Weather Research & Forecast model operational which has the potential to predict intensity more accurately.

9. What is our accuracy of landfall prediction?

Probability of correct forecast decreases with increasing forecast validity period. Mean forecast errors for 12, 24, 48 and 72 hours are about 49, 81,129 and 165 km respectively, which are comparable to corresponding figures of other centres like National Hurricane Centre, Miami, which monitor Atlantic Hurricanes; Typhoon Warning Centre, Tokyo, which monitors Typhoons of Northwest Pacific etc.

10. What is the organizational set up in IMD for Cyclone forecasting and Warning?

The Cyclone Warning Organization in India has a 3-tier system to cater to the needs of the maritime States. These are: Cyclone Warning Division set up at IMD Head Quarters to co-ordinate and supervise cyclone warning operations in the country and to advise the Govt. at the apex level; Area Cyclone Warning Centres at Chennai, Mumbai and Kolkata and Cyclone Warning Centres at Visakhapatnam, Ahmedabad, Bhubaneswar and Thiruvanathapuram

11. What is 4-stage warning system for Tropical Cyclones?

Expectations of Disaster Managers are longer lead time and improved accuracy of landfall forecast. But the present state of art has limitations to make the above requirements go hand in hand. Lead time depends on the formation and duration of cyclone itself which may vary considerably from one cyclone to another. However, since pre-monsoon cyclone season of 1999, IMD introduced a 4-Stage warning system to issue cyclone warnings to the disaster managers. They are as follows:

(1)    Pre-Cyclone Watch

Issued when a depression forms over the Bay of Bengal irrespective of its distance from the coast and is likely to affect Indian coast in future. The pre-cyclone watch is issued by the name of Director General of Meteorology and is issued at least 72 hours in advance of the commencement of adverse weather. It is issued at least once a day.

(2)    Cyclone Alert

Issued atleast 48 hours before the commencement of the bad weather when the cyclone is located beyond 500 Km from the coast. It is issued every three hours.

(3)    Cyclone Warning

Issued at least 24 hours before the commencement of the bad weather when the cyclone is located within 500 Km from the coast. Information about time /place of landfall are indicated in the bulletin. Confidence in estimation increases as the cyclone comes closer to the coast

(4)    Post landfall outlook

It is issued 12 hours before the cyclone landfall, when the cyclone is located within 200 Km from the coast. More accurate & specific information about time /place of landfall and associated bad weather indicated in the bulletin. In addition, the interior distraction is likely to be affected due to the cyclone are warned in this bulletin.

12.  Who are the recipients of Cyclone Warnings?

Warnings are issued for general public, fishermen, farmers and different categories of users such as central and state government officials responsible for disaster mitigation and relief, industrial and other establishments located in the coastal areas, ports, coastal shipping, railways, aviation, transport, communication and power authorities.

13.  How a common man gets information about a cyclonic storm?

Local AIR broadcast hourly (or more frequently) bulletins in local language as well as in Hindi and English. The bulletins give the location of the Cyclonic storm, its direction of movement, place and time of landfall and details of adverse weather expected over the areas likely to be affected by the storm. AIR, New Delhi issues bulletins thrice in a day giving similar information. Apart from that, the cyclone warning messages are sent to the collectors of the districts likely to be affected and the chief secretary of concerned state. The state Govt. takes necessary steps to inform the local population through their machinery such as police wireless etc. They make necessary arrangement for evacuation from coastal area and for removal of the population to other places.

14.  How does IMD keep liaison with State officials?

Area Cyclone Warning Centres (ACWCs) and Cyclone Warning Centres (CWCs) maintain liaison with the concerned state Governments in state and district levels on cyclone related activities. The cyclone warning bulletins are  communicated to the Chief Secretary, Revenue Secretary, Special Relief Commissioner, State control room, State Disaster Management Authority and concerned district collectors every three hourly. In addition, the Chief Secretary is personally briefed by Director, ACWC/CWC regularly. Before the cyclone season, ACWC/CWC organizes the precyclone preparedness meeting under the chairmanship of Chief Secretary where all the high state Govt. officials from various departments participate.

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