Tropical cyclones - Extreme Events: A Physical Reconstruction and Risk Assessment - page 77

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Tropical cyclones
Formation of tropical cyclones
Tropical cyclones form over tropical seas and oceans where the sea sur-
face temperature is at least 27 ◦ C. They are low pressure systems that develop a
warm core or eye structure which is an area of subsiding air in the centre of the
system. Around and towards the eye, air spirals inwards and upwards from the
outer parts of the cyclone with the area of maximum uplift occurring adjacent
to theeye.Asthe air converges inward, it is deflected to the left (clockwise)
in the southern hemisphere and to the right (anticlockwise) in the northern
hemisphere due to the Coriolis effect. The wind velocity increases towards the
eye, approximately doubling as the distance from the eye is halved. This does
not mean that larger diameter cyclones necessarily have stronger winds as the
wind velocity is also dependent on the pressure gradient across the system with
lower central pressures usually having a stronger pressure gradient.
Tropical cyclones can be likened to a thermodynamic heat engine where
energy, due to evaporation from the ocean surface, is lost via thermal radiation
after the air rises and diverges between 12 and 15 km altitude (Holland and
McBride, 1997). If the air is allowed to continue to rise and diverge at the top of
the troposphere (lower layer of the Earth's atmosphere) then air will continue
to be drawn into the centre of the system and the tropical cyclone will intensify.
There is a limit to this intensification. Energy in this system is also lost to the
ocean surface by frictional coupling or by wind. The frictional losses by wind
increase with the cube of wind speed, while energy gained from evaporation
increases in a linear fashion with wind speed. Hence, as wind speed increases
with the intensification of the tropical cyclone the energy loss increases relative
to energy gain (Holland, 1997). Therefore, there is a maximum limit to which
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