Friday, 11 October 2013

[] What is a super cyclone? How can it be tamed


Source: India Meteorological Department, Govt of India and Agencies
NEW DELHI: Super cyclone Phailin, which is half the size of India and moving at speeds of 220 kmph towards India's east coast, has spread panic among the residents of these regions. Thousands of people are fleeing their homes to escape the fury of the cyclone, which is about 500 km off the coast, and major evacuation drill is being carried out in the states of Odisha and Andhra Pradesh.
Authorities say that the super cyclone can cause a major loss of life and property once it hits land.
Satellite images showed that Cyclone Phailin, in the Bay of Bengal off Indian coast, is likely to make landfall on Saturday evening.
The Indian Meteorological Department described Phailin as a "very severe cyclonic storm" with wind speeds of 210-220 km per hour (130-135 mph). The IMD said it would hit Kalingapatnam and the major port of Paradip in Odisha state. The US Navy's Joint Typhoon Warning Centre predicted gusts of up to 305 kmph.
So what is a super cyclone? What kind of energy can it release? Can it be tamed? Here's what you must know about cyclones:
What is a tropical cyclone?
A tropical cyclone is a rotational low-pressure system that occurs in the tropics when the central pressure falls by 5 to 6 hPa (hectopascals) compared to the surrounding pressure. A cyclone's wind speeds speed reach a maximum of 60-62 kmph. It is a vast violent whirl of 150 to 800 km, spiraling around a center 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'. Heary Piddington, who worked as a rapporteur in Kolkata during British rule over India, coined the word cyclone to describe the storm. 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.
What does 'maximum sustained wind' mean ?
India Meteorological Department (IMD) uses 3-minute averaging for sustained wind. The maximum sustained wind mentioned in the bulletins used by the IMD is the highest 3-minute surface wind occurring within the circulation of the system.
These surface winds are observed (or, more often, estimated) at the standard meteorological height of 10 m (33 ft) in an unobstructed exposure (i.e., not blocked by buildings or trees).
The National Hurricane Centre uses a 1-minute averaging time for reporting sustained winds. Some countries also use 10-minute averaging time for this purpose. While one can utilize a simple ratio to convert peak 10-minute wind to peak 1-minute wind or 3-minute wind, such systematic differences to make inter-basin comparisons of tropical cyclones around the world are problematic.
However, there is no significant difference between the maximum sustained wind reported in different basins with different averaging method.
How are low-pressure systems classified in India? What are the differences between low pressure, depression and cyclone?
The low-pressure systems over the Indian region are classified on the basis of maximum sustained wind speeds associated with the system and the pressure deficit divided by the number of closed isobars associated with the system.
The pressure criteria are used when the system is over land and wind criteria are used when the system is over the sea. The system is called 'low' if there is one closed isobar in the interval of 2 hPa. It is called 'depression' if there are two closed isobars; a 'deep depression' if there are three closed isobars, and a 'cyclonic storm' if there are four or more closed isobars.
Considering wind criteria, the system with wind speeds of 17-27 knots is called a depression and the low-pressure system with maximum sustained 3-minute surface winds between 28-33 knots is called a deep depression. The system with maximum sustained 3-minute surface winds of 34 knots or more is called a cyclonic storm
What are super cyclones, super-typhoons, major hurricanes and intense hurricanes?
When the maximum sustained 3-minute surface winds are more than 119 knots, the low-pressure system is called a 'super cyclone' over the north Indian Ocean. Similarly, "super-typhoon" is a term used by the US 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.
Where do tropical cyclones form?
The tropical cyclones form over ocean basins in lower latitudes of all oceans, except south Atlantic and southeast Pacific. The tropical cyclones develop over the warm water of the Bay of Bengal and the Arabian Sea.
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 2,000 km with an average of 300-600 km. The current super cyclone, Phailin, is half the size of India!
How do 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, and the pressure gradually falls.
Low-level convergence, coupled with upper level divergence, gives rise to a vertical motion taking the moist air upwards. This moisture condenses at higher levels (middle troposphere) and gives out latent heat of condensation. Due to release of heat the area warms up resulting into further fall in pressure. This process continues and a low-pressure system gradually intensifies into a cyclonic storm.
Hence, for tropical cyclogenesis, there are several favourable environmental conditions that must be in place. They are:
Warm ocean waters (of at least 26.5°C) throughout a sufficient depth (unknown how deep, but at least on the order of 50 m). Warm waters are necessary to fuel the heat engine of the tropical cyclone.
An atmosphere which cools fast enough with height, such that it is potentially unstable to moist convection. It is the thunderstorm activity which allows the heat stored in the ocean waters to be liberated for the tropical cyclone development.
Relatively moist layers near the mid-troposphere (5 km). Dry mid-levels are not conducive to allowing the continuing development of widespread thunderstorm activity.
A minimum distance of at least 500 km from the equator. For tropical cyclogenesis to occur, there is a requirement for non-negligible amounts of the Coriolis Force (attributed to earth's rotation) to provide the near gradient wind balance to occur. Without the Coriolis Force, the low pressure of the disturbance cannot be maintained. This is the reason why the narrow corridor of about 300 km on either side of the equator is free from cyclones. Because of this there is no inter-hemispheric migration of tropical cyclones across the equator.
A pre-existing near-surface disturbance with sufficient vorticity (rotation) and convergence. Tropical cyclones cannot be generated spontaneously. To develop, they require a weakly organized system with sizeable spin and low level inflow.
Low values (less than about 10 m/s) of vertical wind shear between the lower (1.5 km) and the upper troposphere (12 km). Vertical wind shear is the magnitude of wind change with height. Large values of vertical wind shear disrupt the incipient tropical cyclone and can prevent genesis, or, if a tropical cyclone has already formed, a large vertical shear can weaken or destroy the tropical cyclone by interfering with the organization of deep convection around the cyclone center.
The above conditions are necessary, but not sufficient as many disturbances that appear to have favourable conditions do not develop. However, these criteria fit well over the north Indian Ocean.
Can we tame a tropical cyclone to reduce its damage potential?
Considering the huge energy potential of the cyclones, all experiments in the US under the project "Storm Fury" to tame them have been futile.
The best solution is not to try to alter or destroy the tropical cyclones, but just learn to co-exist better with them. Since we know that coastal regions are vulnerable to the storms, enforce building codes that can have houses stand up to the force of the tropical cyclones. In this regard the Building Material Technology Promotion Council (BMTPC), Ministry of Urban Affairs has brought out a vulnerability map in consultation with IMD which is very useful for disaster managers.
What are the different methods that can be tried to modify a cyclone?
Seeding with silver iodide.
Placing a substance on the ocean surface.
By nuking them.
By cooling the surface waters with deep ocean water.
By adding a water-absorbing substance.
What are the causes of disaster during cyclone?
The dangers associated with cyclonic storms are generally threefold.
(i) Very heavy rains causing floods: The rainfall associated with a storm varies from storm to storm even with the same intensity. Record rainfall in a cyclonic storm has been as low as trace to as high as 250 cm. It has been found that the intensity of rainfall is about 85 cm/day within a radius of 50 km and about 35 cm/day between 50 and 100 km from the center of the storm. This phenomenal rain can cause flash flood.
(ii) Strong winds: The strong wind speeds associated with a cyclonic storm (60-90 kmph) can result into some damage to kutcha (infirm) houses and tree branches likely to break off. Winds of a severe cyclonic storm (90-120 kmph) can uproot trees, damage solid buildings and disrupt communications. Winds associated with a very severe cyclonic storm and a super cyclonic storm can uproot big trees, cause wide spread damages to houses and installations and total disruption of communications. The maximum wind speeds associated with a very severe cyclonic storm that hit Indian coast in the past 100 years was 260 kmph in October 1999 (Paradip super cyclone).
(iii) Storm Surge: A storm surge (tidal wave) is an abnormal rise of sea level when the cyclone crosses the coast. Sea water inundates the coastal strip causing loss of life, large-scale destruction to property and standing crop. Increased salinity in the soil over affected area makes the land unfit for agricultural use for two or three seasons.
Storm surges depend on the intensity of the cyclone.
The severest destructive feature of a tropical storm is the storm surge popularly called 'tidal waves'. The costal areas are subjected to storm surges and further accentuated if the landfall time coincides with that of the high tides. This is more if the sea bed is shallow.
Storm surges as high as 15 to 20 ft. may occur when all the factors contributing to storm surges are maximum. This storm tide inundates low lying coastal areas and has far reaching consequences apart from flooding. The fertility of land is lost due to inundation by saline water for a few years to come.
What are the largest rainfalls associated with tropical cyclones over the north Indian Ocean?
The rainfall can vary from trace/nil rainfall when the system moves skirting the coast to maximum rainfall up to 50-60 cm per day. In the recent super cyclone which crossed the Orissa coast near Paradip on October 29, 1999, Paradip recorded 24-hr cumulative rainfall of about 52 cm at 0830 IST of October 30, 1999.

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