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Frequently Asked Questions

on

Tropical Cyclones

Frequently Asked Questions on Tropical Cyclones

1. What is a tropical cyclone?

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.

The word cyclone has been derived from Greek word ‘cyclos’ which means ‘coiling of a snake’. The word cyclone was coined by Heary Piddington who worked as a Rapporteur in Kolkata during British rule. 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.

2. Why do ‘tropical cyclones' winds rotate counter-clockwise (clockwise) in the Northern (Southern) Hemisphere?

The reason is that the earth's rotation sets up an apparent force (called the Coriolis force) that pulls the winds to the right in the Northern Hemisphere (and to the left in the Southern Hemisphere). So, when a low pressure starts to form over north of the equator, the surface winds will flow inward trying to fill in the low and will be deflected to the right and a counter-clockwise rotation will be initiated. The opposite (a deflection to the left and a clockwise rotation) will occur south of the equator.

This Coriolis force is too tiny to effect rotation in, for example, water that is going down the drains of sinks and toilets. The rotation in those will be determined by the geometry of the container and the original motion of the water. Thus, one can find both clockwise and counter-clockwise flowing drains no matter what hemisphere you are located. If you don't believe this, test it out for yourself.

3. What does "maximum sustained wind" mean? How does it relate to gusts in tropical cyclones?

India Meteorological Department (IMD) uses a 3 minutes averaging for the sustained wind. The maximum sustained wind mentioned in the bulletins used by IMD is the highest 3 minutes surface wind occurring within the circulation of the

System Pressure deficient hPa w.r.t T No.

Associated wind speed Knots (Kmph) Low pressure area 1.0 <17(<31) Depression 1.0- 3.0 17-27 (31-49) Deep Depression (DD) 3.0 - 4.5 28-33 (50-61) Cyclonic Storm (CS) 6.1-10.0 34-47 (62-88) Severe Cyclonic Storm (SCS)

Very Severe Cyclonic Storm (VSCS)

Extremely Severe Cyclonic Storm (ESCS)

Super Cyclonic Storm ≥ 80.0 ≥ 120 (≥ 222)

5. Are all cyclonic storms equally dangerous? No, all cyclonic storms are not equally dangerous. More the pressure drop at the central region more will be the severity of the storm. The cyclonic storms are generally categorised according to the maximum wind associated with the storm. If the maximum wind is between 34 - 47 knots (about 62-88 kmph) it is called a Cyclonic storm. Severe Cyclonic storm will have maximum wind speed between 48 - 63 knots (about 89-117 kmph). If the maximum wind is 64-89 knots it will be called a very severe Cyclonic storm. If the maximum wind is 90-119 knots it will be called as Extremely Severe Cyclonic storm and when the wind is 120 knots and above it will be called super cyclonic storm. There is very little association between intensity (either measured by maximum sustained winds or by the lowest central pressure) and size (measured by radius of gale force winds) 6. What is the intensity of a cyclone? Intensity of a cyclone can be derived as the near-surface maximum wind speed around the circulation centre, or as the minimum surface pressure at the tropical cyclone pressure centre. The intensity classification of cyclone as per their intensity given in the table above. 7. What are the super cyclone, super-typhoon, a major hurricane and an Intense hurricane?

When the maximum sustained 3 minutes surface winds are more than 119 knots, the low pressure system is called as “super cyclone” over north Indian

Ocean. Similarly, “Super-typhoon" is a term utilized by the U.S. Joint Typhoon Warning Centre for typhoons that reach maximum sustained 1 minute surface winds of at least 130 knots (65 m/s). This is the equivalent of a strong Saffir-Simpson category 4 or category 5 hurricane in the Atlantic basin or a category 5 severe tropical cyclone in the Australian basin.

8. What is landfall of a cyclone? Landfall is the event of a tropical cyclone coming onto land after being over water. A tropical cyclone is classified as making landfall when the center of the storm moves cross the coast; in strong tropical cyclones this is when the eye moves over land. This is where most of the damage occurs within a mature tropical cyclone as most of the damaging aspects of these systems are concentrated near the eyewall. Such effects include the peaking of the storm surge, the core of strong winds comes on shore, and heavy flooding rains. These coupled with high surf can cause major beach erson. When a tropical cyclone makes landfall, the eye closes in upon itself due to the weakening process, which causes surf to decrease. Maximum sustained wind will naturally decrease as the cyclone moved inland due to frictional differences between water and land with the free atmosphere. A "landfall" should not be confused with a direct hit, as a direct hit is where the core of high winds (or eyewall) comes onshore but the center of the storm may stay offshore. The effects of this are similar to a landfall, as this term is used when the radius of maximum wind within a tropical cyclone moves ashore. These effects are; high surf, heavy rains that may cause flooding, water build up along the coast with minor storm surge, coastal beach erosion, high winds, and possibly severe weather. 9. What is radius of maximimun wind of a Tropical cyclone? The radius of maximum wind (RMW) of a tropical cyclone is defined to be the distance between the center of the cyclone and its band of strongest winds. It is considered an important parameter in atmospheric dynamics and tropical cyclone forecasting. 10. How does the radius of maximum wind determined?

Aircraft

The RMW is traditionally measured by reconnaissance aircraft in the Atlantic basin. It can also be determined on weather map as the distance between the cyclone center and the system's greatest pressure gradient.

Satellite

The distance between the coldest cloud top temperature and the warmest

trajectories over different basins

14. 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.

15. What is the 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. The INSAT imagery of Orissa Super cyclone on 29th^ October, 1999 is shown in the figure below.

16. What is a "CDO"?

"CDO" is an acronym that stands for "central dense overcast". This is the cirrus cloud shield that results from the thunderstorms in the eyewall of a tropical cyclone and its rainbands. Before the tropical cyclone reaches very severe cyclonic storm (64 knots), typically the CDO is uniformly showing the cold cloud tops of the cirrus with no eye apparent. Once the storm reaches the hurricane strength threshold, usually an eye can be seen in either the infrared or visible channels of the satellites. Tropical cyclones that have nearly circular CDOs are indicative of favourable, low vertical shear environments.

17. What is the "eye"? How is it formed and maintained? What is the "eyewall"? What are "spiral bands"?

The "eye" is a roughly circular area of comparatively light winds and fair weather found at the centre of a severe tropical cyclone. Although the winds are calm at the axis of rotation, strong winds may extend well into the eye. There is little or no precipitation and sometimes blue sky or stars can be seen. The eye is the region of lowest surface pressure and warmest temperatures aloft - the eye temperature may be 10°C warmer or more at an altitude of 12 km than the surrounding environment, but only 0-2°C warmer at the surface in the tropical cyclone. Eyes range in size from 8 km to over 200 km across, but most are approximately 30-60 km in diameter.

The eye is surrounded by the "eyewall" , the roughly circular ring of deep convection, which is the area of highest surface winds in the tropical cyclone. The eye is composed of air that is slowly sinking and the eyewall has a net upward flow as a result of many moderate - occasionally strong - updrafts and downdrafts. The eye's warm temperatures are due to compressional warming of the subsiding air.

Some of the most intense tropical cyclones exhibit concentric eyewalls, two or more eyewall structures centered at the circulation centre of the storm. Just as the inner eyewall forms, convection surrounding the eyewall can become organized into distinct rings. Eventually, the inner eye begins to feel the effects of the subsidence resulting from the outer eyewall, and the inner eyewall weakens, to be replaced by the outer eyewall. The pressure rises due to the destruction of the inner eyewall are usually more rapid than the pressure falls due to the intensification of the outer eyewall, and the cyclone itself weakens for a short period of time.

18. How does the cyclone look like in a Radar?

According to Radar imagery, a matured cyclone consists of eye, eye wall, spiral bands, pre-cyclone squall lines and streamers as shown in the above figure.

19. What is the wind structure in a cyclone? The ideal wind and cloud distribution in a cyclone is shown in the following figure.

The band of maximum winds may vary between 10 and 150 Km. In this belt, speed decreases rapidly towards the eye of the cyclone. But it decreases slowly and in an irregular fashion outward from the eye wall.

20. 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 ≥ 26.5^0 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.

The waves move generally toward the west in the lower tropospheric trade wind flow. They are first seen usually in October to April. The waves have a period of about 3 or 4 days and a wavelength of 2000 to 2500 km. One should keep in mind that the "waves" can be more correctly thought of as the convectively active troughs along an extended wave train. Though, these waves are generated frequently, but it appears that the number that is formed has no relationship to how much tropical cyclone activity there is over the north Indian Ocean each year. It is currently completely unknown, how, easterly waves change from year to year in both intensity and location and how these might relate to the activity?

22. Is there any extra-tropical cyclone?

Extra-tropical cyclones are low pressure systems with associated cold fronts, warm fronts, and occluded fronts. The extra-tropical cyclone is a storm system that primarily gets its energy from the horizontal temperature contrasts that exist in the atmosphere. Extra-tropical cyclones are low pressure systems with associated cold fronts, warm fronts, and occluded fronts. Tropical cyclones , in contrast, typically have little to no temperature differences across the storm at the surface and their winds are derived from the release of energy due to cloud/rain formation from the warm moist air of the tropics. Structurally, tropical cyclones have their strongest winds near the earth's surface, while extra-tropical cyclones have their strongest winds near the tropopause - about 12 km up. These differences are due to the tropical cyclone being "warm-core" in the troposphere (below the tropopause) and the extra-tropical cyclone being "warm-core" in the stratosphere (above the tropopause) and "cold-core" in the troposphere. "Warm-core" refers to being relatively warmer than the environment at any level.

Often, a tropical cyclone will transform into an extra-tropical cyclone as it

recurves poleward and to the east. Occasionally, an extra-tropical cyclone will lose

its frontal features, develop convection near the centre of the storm and transform

into a full-fledged tropical cyclone. Such a process is most common in the north

Atlantic and northwest Pacific basins. The transformation of tropical cyclone into an

extra-tropical cyclone (and vice versa) is currently one of the most challenging

forecast problems.

23. What is the annual frequency of Cyclones over the Indian Seas? What is its intra-annual variation?

The average annual frequency of tropical cyclones in the north Indian Ocean (Bay of Bengal and Arabian Sea) is about 5 (about 5-6 % of the Global annual average) and in the globe is about 80. The frequency is more in the Bay of Bengal than in the Arabian Sea, the ratio being is 4:1. The monthly frequency of tropical cyclones in the north Indian Ocean display a bi-modal characteristic with a primary peak in November and secondary peak in May. The months of May-June and October-November are known to produce cyclones of severe intensity. Tropical cyclones developing during the monsoon months (July to September) are generally not so intense. The frequencies of Cyclonic systems over north Indian Ocean during 1891- 2006 are given in the figure below.

24. What are the average, most, and least tropical cyclones occurring in this basin? The most, least and average numbers of cyclonic storms and severe cyclonic storms over the north Indian Ocean is given in the Table below:

0

20

40

60

80

100

120

140

160

180

200

no. of cyclonic disturbances Jan Feb M ar Apr M ay Jun M o nt h Jul Aug Sep Oct Nov Dec

Depression Cyclonic Storms Severe Cyclonic Storms

The frequency of severe cyclonic storms is maximum for Andhra Pradesh while that of cyclone is maximum for Orissa. Considering west coast only, Gujarat is most vulnerable.

27. Which is the most intense tropical cyclone on record? Typhoon Tip in the Northwest Pacific Ocean on 12 October, 1979 was measured to have a central pressure of 870 hPa and estimated surface sustained winds of 165 knots (85 m/s). Typhoon Nancy on 12 September, 1961 is listed in the best track data for the Northwest Pacific region as having an estimated maximum sustained wind of 185 knots (95 m/s) with a central pressure of 888 hPa. However, it is now recognized that the maximum sustained winds estimated for typhoons during the 1940s to 1960s were too strong and that the 95 m/s (and numerous 83 to 93 m/s reports) is somewhat too high. Note that Hurricane Gilbert's 888 hPa lowest pressure (estimated from flight level data) in mid September, 1988 is the most intense [as measured by lowest sea level pressure] for the Atlantic basin, it is almost 20 hPa weaker (higher) than the above Typhoon Tip of the Northwest Pacific Ocean. While the central pressures for the Northwest Pacific typhoons are the lowest globally, the North Atlantic hurricanes have provided sustained wind speeds possibly comparable to the Northwest Pacific. From the best track database, both Hurricane Camille (1969) and Hurricane Allen (1980) have winds that are estimated to be 165 knots (85 m/s). Measurements of such winds are inherently going to be suspect as instruments often are completely destroyed or damaged at these speeds. Orissa super cyclone, 1999, which crossed Orissa coast near Paradip on 29th October, 1999 was the most intense cyclonic storm over north Indian Ocean in the recorded history of the region. The estimated sustained maximum surface wind speed was about 140 knots at the time of landfall and lowest estimated central pressure was 912 hPa. A few cyclones that have originated over the Bay of Bengal have reached the intensity of Super Cyclones and have caused great devastations to life and property. The estimates of maximum sustained winds of these systems are estimated from satellite imageries. The list of very intense Cyclones in the Bay of Bengal since 1990 is given below:

28. Which are the largest and smallest tropical cyclones on record?

Typhoon Tip had gale force winds 34 knots (17 m/s), which extended out for 1100 km in radius in the Northwest Pacific on 12 October, 1979. Tropical Cyclone Tracy had gale force winds that only extended 50 km radius when it struck Darwin, Australia, on 24 December,1974.

Considering north Indian Ocean, Orissa super cyclone of October, 1999 and the cyclone, ‘Ogni’ were the largest and smallest cyclones during 1891-2007.

29. Which tropical cyclone over north Indian Ocean have caused the most deaths and most damage?

The death toll in the infamous Bangladesh Cyclone of 1970 has had several estimates, some wildly speculative, but it seems certain that at least 300,000 people died from the associated storm tide [surge] in the low-lying deltas.

30. Why, there are fewer cyclones over the Arabian Sea as compared to the

Place of landfall Date of landfall

Maximum sustained winds(kmph) - estimated on the basis of satellite imageries Chittagong 13 November, 1970 224 Chirala,Andhra Pradesh 19 November, 1977 260 Rameshwaram 24 November 1978 204 Sriharikota 14 November, 1984 213 Bangla Desh 30 November, 1988 213 Kavali, Andhra Pradesh 9 November, 1989 235 Machlipatnam, AP 9 May ,1990 235 Chittagong 29 April, 1991 235 Teknaf (Myanmar) 2 May, 1994 204 Teknaf 19 May, 1997 235 Paradip, Orissa 29 October, 1999 260 89.80E, Bangladesh 15 November, 2007 220 16.00N, Myanmar 02 May, 2008 200

1994. (It formed in the northeast Pacific, reached hurricane force there, moved across the dateline and was renamed Typhoon John, and then finally recurved back across the dateline and renamed Hurricane John again.) Hurricane Ginger was a tropical cyclone for 28 days in the north Atlantic Ocean back in 1971. It should be noted that prior to the weather satellite era (1961) many tropical cyclones' life cycles could be underestimated. 33. 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. 34. How is cyclone monitored by satellite technique?

The satellite technique can be used to find out the centre and intensity of the system. It can also be used to find out various derived parameters which are useful for monitoring and prediction of the cyclones and associated disastrous weather. Dvorak’s technique based on pattern recognition in the cloud imagery based on satellite observation is used to determine the intensity of cyclonic storm. For this purpose a T. No. where T stands for tropical cyclone is assigned to the system. This scale of T Nos. varies from T 1.0 to T 8.0 at the interval of 0.5. The T 2. corresponds to the intensity of a cyclonic storm. The detailed classification of cyclonic disturbances based on above technique is given below: “T” CLASSIFICATION OF CYCLONIC STORM AND CORRESPONDING WIND SPEED AND PRESSURE DEFECT (  P)

35. What is the utility of Radar in cyclone monitoring? The radar can be utilized to find out the location of the cyclonic storm more accurately when the system comes within radar range. In addition, it can find out convective cloud cluster, wind distribution and rainfall rate etc. 36. What is the present network of Cyclone Detection Radars? A network of conventional Cyclone Detection Radars (CDRs) has been established at Kolkata, Paradip, Visakhapatnam, Machilipatnam, Chennai and Karaikal along the east coast and Goa, Cochin, Mumbai and Bhuj along the west coast. These conventional radars are being phased out and replaced by Doppler Weather Radars (DWRs). DWR have already been installed and made operational at Chennai, Kolkata, Visakhapatnam and Machlipatnam. An indigenously developed DWR Radar by Indian Space Research Organisation (ISRO) has been installed at Sriharikota. It is proposed to replace all the conventional radars by DWRs during the next 3-4 years.

T. Number/ C.I. Number

Classification of Cyclonic Disturbance

Wind speed in Knots

Wind speed In Kmph

 P Wind criteria in Knots

Wind criteria in Kmph

T1.0 L  17  31

T1.5 D 25 46.3 17-27 31-

T2.0 DD 30 55.6 4.5 28-33 50-

T2.5 CS 35 64.9 6.1 34-47 62-

T3.0 45 83.4 10.

T3.5 SCS 55 101.9 15.0 48-63 89-

T4.0 VSCS 65 120.5 20.9 64-89 118-

T4.5 77 142.7 29.

T5.0 ESCS 90 166.8 40.2 90-119 167-

T5.5 102 189.0 51.

T6.0 115 213.1 65.

T6.5 SuCS 127 235.4 80.0 120 AND ABOVE

222 AND

ABOVE

T7.0 140 259.5 97.

T7.5 155 287.3 119.

T8.0 170 315.1 143.