What is a Tropical cyclone?
Tropical cyclone (TC) is a storm system characterized by a low-pressure centre and numerous thunderstorms that produce strong winds and heavy rain. A Meteorologists’ definition of TC is an atmospheric system characterized by the rapid inward circulation of air masses about a low-pressure centre, usually accompanied by stormy, often destructive weather.
How is a TC formed?
Cyclones happen or they usually form over seasonal period, the summer period in the tropics that is from November to April the following year. They form in the tropics, between 3– 5 South or the North of the equator. There are identified regions/areas in which cyclone systems breed and generally they follow a regular direction for their tracking paths. Above the equator the cyclone systems move towards the north-easterly direction, while below the equator they move on a south-easterly direction.
Fig.1 Identified world cyclone breeding areas.
In western North Atlantic, central and eastern North Pacific, Caribbean Sea and Gulf of
Mexico, this weather phenomenon is called a “hurricane”
In the western North Pacific, it is called a “typhoon” e.g. China and Japan
In the western South Pacific and south-east Indian Ocean, it is called a “Tropical
Cyclone” E.g. Vanuatu, Fiji and Australia
TCs are named alphabetically according to the lists of tropical cyclone names (A-Z). Names are provided to provide ease of communication between tropical cyclone forecasters and the general public regarding forecasts, watches and warnings. Naming reduces confusion if there are two TC at the same time in an area.
Fig. 2 The life cycle of a Tropical Cyclone
a. The formative stage
1. The cloud arrangements are spiral like but scattered
b. The immature stage
2. The cloud arrangements is good but without the ‘eye’
c. The mature stage
3. The cloud arrangements is good with the ‘eye’
d. The Decay stage
4. The cloud arrangements weaken and dissipates
The physical structure of a cyclone in a cross section looks like this,
Fig. 3 Cross-section of a cyclone cloud.
Some fundamental conditions for the formation of TC;
• An area of dense cloud or active and prolong convection during the cyclone season
• Region of low pressure system (999hPa or less)
• Formed in the tropics (3-5 south or north of the equator)
• Warm sea surface temperature (27C or greater) to a depth of 50m
• High (70% or more) humidity/moisture in lower to mid troposphere
• Wind speed equal or greater than 34 knots
• Low wind shear for TC circulation
• Rapid cooling with height for heat of condensation which powers TC
How a TC moves and factors that determines the direction/track of a TC.
1. Steering winds- TC over earth surface are steered by large scale winds in the atmosphere
2. Coriolis acceleration causes TC to turn towards the poles in the absence of steering currents
3. Interaction with the mid-latitude westerly’s
Grading of TC
• Category 1: Damaging Gusts – Gale Wind (34knts to 47knts)
o Negligible house damage
o Damage to some garden crops, trees and local thatch house
• Category 2: Destructive Gusts – Storm force winds (48knts to 63knts)
o Minor house damage
o Significant damage signs, trees and local thatch house
o Heavy damage to garden crops
o Risk of power failure
• Category 3: Very destructive Gusts – Hurricane winds (more than 64knts)
o Some roof and structural damage
o Power failures likely
• Category 4: Very destructive Gusts – Hurricane winds
o Significant roofing loss and structural damage
o Dangerous airborne debris
o Wide spread power failures
• Category 5: Very destructive Gusts – Hurricane winds
o Extremely dangerous with widespread destruction
Impacts of TC
• Flooding in low lying areas
• Combination of storm surge and wind waves
• Damage to infrastructure
• Damage to garden crops, etc
• Coastal erosion
Vanuatu TC area is 12S to 23S and 160E to 175E