Conditions for Cyclone Formation
A number of conditions have to be met before a cyclone can form. A cyclone cannot exist without these conditions, however a cyclone will not always form just because the conditions are favorable.
1. Warm ocean waters greater than or equal to 26.5oC with a depth of at least 50 m. These warm waters are the energy source for Tropical Cyclones.
2. An atmosphere that cools quickly with height so that it is unstable and enhances vertical motion and moist convection.
3. Low values of vertical wind shear. This occurs when the winds at all vertical levels through the troposphere are blowing at the same speed and in the same direction. This enables the energy to remain concentrated.
4. A minimum distance of 500 km from the Equator so that there is significant Coriolis Force. (a force caused by the rotation of the earth which is negligble at the equator) It is the coriolis force that initially makes the cyclone spiral, and maintains the low pressure of the disturbance
5. An existing tropical disturbance at the surface such as an easterly wave, organized thunderstorms, or large convective system (mesoscale thunderstorm.) (AOML, 2002.) (Environment Canada, 2002)
The following figure shows the areas where cyclones form worldwide.
How a cyclone works
As seen on the figure above the conditions for cyclone formation usually occur between the latitudes 5o and 20o North and South. (Tapper, N. & Hurry, L. 1993.) The oceans in these areas are warmer in these areas due to the more direct solar radiation that falls upon them, which also increases rates of evaporation, so there is a large mass of warm humid air. As the warm air rises it develops a twisting motion due to the coriolis force. As the air within the spiral ascends, it cools and condensation occurs. The latent heat of condensation is then released and heats the air within system, causing the air to raise even further. As the air continues to rise more warm moist air is pulled into the system from the surrounding ocean to replace it. As it rises it further contributes to the process of condensation and release of latent heat. (Tapper, N. & Hurry, L. 1993.) This vertical motion of air is known as convection. The updraught of air leads to low level convergence and the inwards motion enhances cyclonic swirl. As the air moves into a tighter rotation it accelerates, until a balance is reached between the gradient force inwards and the centrifugal force upwards. (Linacre, E. & Hobbs, J. 1977.) The inward flow of air is balanced by an outflow of air spiraling away from the top of the eye, so that surface convergence is approximately equal to upper air divergence. (Tapper, N. & Hurry, L. 1993) As a tropical cyclone moves over land it rapidly weakens as there is no longer a supply of heat and moisture to fuel it, convection cannot continue and the storm rapidly fills. (AOML, 2002)
Structure of a cyclone
In the centre of a cyclone there is a roughly circular area of light winds and fair weather known as the ?geye?h. The eye is the region of lowest surface pressure and highest temperatures aloft, and is composed of air that is slowly sinking. The eye is surrounded by the eyewall which is an area of deep convection, which has a net upward flow as a result of many updraughts and downdraughts. It also has the highest surface winds of any area in the cyclone. Convection in the outer regions of a tropical cyclone is organized into long, narrow rainbands. The rainbands are orientated in the same direction as the horizontal wind, spiraling in towards the centre of the cyclone. A direct circulation develops in which warm moist air converges at the surface, ascends through these bands, diverges aloft, and descends on both sides of the bands. (AOML, 2002.) The following diagram of the cross-section of a typical tropical cyclone, shows the structure, air flows within the system as well as mass flows in and out of the system.
Energy of a Tropical Cyclone
To consider the quantitative interconversions of energy, cyclones can be thought of as a simple heat engine; the input from the warm, humid air over the ocean, the releasing of this heat used in latent heat of condensation, condensing the water vapour into water droplets in the thunderstorms of the eyewall and rainbands, generic levaquin uk then giving off a cold exhaust to the upper levels of the troposphere.
The two main processes that involve energy in a cyclone are the amount of energy released by the condensation of water droplets, and the amount of kinetic energy generated to maintain the strong swirling winds of the cyclone. The energetics of both these are considered below:
Energy released through cloud/rain formation:
Consider an average cyclone of radius 665 km that produces 1.5 cm/day of rain.
Convert this to a volume: (665?~1000?~100)2 ?~ ?? ?~ 1.5 = 2.1?~1016 cm3/day
Using a cubic cm of water weighs 1 gram, and the latent heat of condensation, The energy required to condense this volume of water is equal to 5.2 ?~ 1019 Joules/day or 6.0 ?~ 1014 Watts. This is an enormous amount of energy, equivilent to 200 times the world wide electrical generating capacity!
Energy used in wind generation
The amount of kinetic energy generated by wind is equal to the amount dissipated by friction. The dissipation rate per unit area is equal to the air density times the drag coefficient times the wind speed cubed. One could either integrate a typical wind profile over a range of radii from the cyclone0’s centre to outer radius, Or more simply assume an average wind speed for the inner core of the cyclone. Using the second method and using wind speed equal to 40 m/s on a scale of 60 km radius for the inner core, the rate at which wind is dissipated is equal to 1.5 ?~ 1012 Watts.
While still being an impressive amount of energy, this is 400 times smaller than the energy released through the latent heat of condensation. Most of this heat is used to cause the rising motion of in the thunderstorms, and only a small amount drives the storm’s horizontal winds. (AOML, 2002.)
Impacts on Surrounding Environment
Storm surge As a cyclone approaches the coastline, a phenomenon known as storm surge can result from the high speed winds. The storm surge is a raised dome of water typically 60 to 80 kilometres across that can be raised as high as 6 to 10 or more meters above the normal sea level.(Bureau of Meteorology, 1992) It is mostly caused by the winds pushing the ocean surface ahead of the storm, and a small amount (<15%) is due to the partial vacuum associated with the low pressure at the centre of the storm. The extent of the storm surge is dependent on wind strength, coastal topography, angle of incidence of landfall and other factors. (AOML, 2002.) The most destructive conditions occur when a storm surge is superimposed upon a spring tide, and coastal communities can become inundated. (Bureau of Meteorology, 1992)
Rainfall Excessive amounts of rainfall can be produced during a cyclone. This is not surprising considering the amount of energy associated with cyclones and that this energy comes from the latent heat of condensation. Rainfall has the greatest impact when a cyclone makes landfall. Often large amounts of rain occur in short periods of time causing flooding, and in steep topography flash flooding and mudslides can occur. (Environment Canada,2002)
Wind Tropical cyclones by definition have wind speeds exceeding gale force or 63 km/h. Severe tropical cyclones (which are known as hurricanes and typhoons in other areas of the world) have sustained winds that exceed 118 km/hours, however gusts of wind are often up to 50% increase on these speeds. (Bureau of Meteorology, 1992) Winds of these speed are extremely destructive, and cause the most threat to humans as a cyclone moves over land. Gusts are stronger over land because the turbulence increases and acts to bring faster winds down to the surface in short bursts. (AOML, 2002.) Houses and buildings are usually damaged as winds reach 90 km/h, and debris can be seen flying around. The natural environment is also damaged by high speed wind.
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