WHAT IS TROPICAL CYCLONE?
Tropical cyclone is defined as a non-frontal, synoptic-scale cyclone developing over tropical and sub-tropical waters at any level and having a definitely organized circulation. In other parts of the world, these are referred to as hurricanes, typhoons or simply tropical cyclones depending on the region. In the North Atlantic, Eastern North Pacific and South Pacific Ocean, they are called”hurricanes”. In the bay of Bengal, Arabian Sea and Western South Indian Ocean, the name is”cyclonic”. In the eastern part of the Southern Indian Ocean, it is “willy-willy”, and in the Western North Pacific Ocean, they are called “typhoons”.
CLASSIFICATION OF TROPICAL CYCLONES
Tropical cyclones derive their energy from the latent heat of condensation which made them exist only over the oceans and die out rapidly on land. One of its distinguishing features is its having a central sea-level pressure of 900 mb or lower and surface winds often exceeding 100 knots. They reach their greatest intensity while located over warm tropical waters and they begin to weaken as they move inland. The intensity of tropical cyclones vary, thus , we can classify them based upon their degree of intensity.
The classification of tropical cyclones according to the strength of the associated windsas adopted by PAGASA as of 01 May 2015 are as follows:
TROPICAL DEPRESSION (TD) – a tropical cyclone with maximum sustained winds of up to 61 kilometers per hour (kph) or less than 33 nautical miles per hour (knots) .
TROPICAL STORM (TS) – a tropical cyclone with maximum wind speed of 62 to 88 kph or 34 – 47 knots.
SEVERE TROPICAL STORM (STS), a tropical cyclone with maximum wind speed of 89 to 117 kph or 48 – 63 knots.
TYPHOON (TY) – a tropical cyclone with maximum wind speed of 118 to 220 kph or 64 – 120 knots.
SUPER TYPHOON (STY), a tropical cyclone with maximum wind speed exceeding 220 kph or more than 120 knots.
EFFECTS OF TROPICAL CYCLONES
Tropical cyclone constitutes one of the most destructive natural disasters that affects many countries around the globe and exacts tremendous annual losses in lives and property. Its impact is greatest over the coastal areas, which bear the brunt of the strong surface winds, squalls, induced tornadoes, and flooding from heavy rains, rather than strong winds, that cause the greatest loss in lives and destruction to property in coastal areas.
A squall is defined as an event in which the surface wind increases in magnitude above the mean by factors of 1.2 to 1.6 or higher and is maintained over a time interval of several minutes to one half hour. The spatial scales would be roughly 2 to 10 km. The increase in wind may occur suddenly or gradually. These development near landfall lead to unexpectedly large damage.
Tornadoes are tropical cyclone spawned which are to expected for about half of the storms of tropical storm intensity. These are heavily concentrated in the right front quadrant of the storm (relative to the track) in regions where the air has had a relatively short trajectory over land. These form in conjunction with strong convection.
RAINFALL AND FLOODING
Rainfall associated with tropical cyclones is both beneficial and harmful. Although the rains contribute to the water needs of the areas traversed by the cyclones, the rains are harmful when the amount is so large as to cause flooding.
The storm surge is an abnormal rise of water due to a tropical cyclone and it is an oceanic event responding to meteorological driving forces. Potentially disastrous surges occur along coasts with low-lying terrain that allows inland inundation, or across inland water bodies such as bays, estuaries, lakes and rivers. For riverine situations, the surge is sea water moving up the river. A fresh water flooding moving down a river due to rain generally occurs days after a storm event and is not considered a storm surge. For a typical storm, the surge affects about 160 km of coastline for a period of several hours. Larger storms that are moving slowly may impact considerably longer stretches of coastline.