Modelling Tropical Cyclones

A tropical cyclone as seen from a satellite (off the northwest coast of Western Australia). The spiral cloudbands and the eye (dark spot in the centre) can clearly be seen. From the mid 60's, satellite photography has improved the detection and tracking of cyclones. Geostationary satellites are able to give pictures of the same area of the earth at hourly intervals, allowing us to follow the movements of tropical cyclones.

On average, 10 tropical cyclones develop in the Australian region each year and 6 cross the coast.

The Life Cycle of a Tropical Cyclone

The life cycle of a tropical cyclone may be divided up into four stages:

1. Formative - clouds start forming over the ocean between 5 and 15 degrees latitude from the equator.
   
   
2. Immature - if the conditions are favourable, clouds collect and move with the winds that start rotating clockwise around a low pressure area. Winds start to increase in strength.
   
   
3. Mature - the cyclone reaches its peak of intensity and destructive power.
   
   
4. Decaying - the power of the cyclone decreases when it moves over land or poleward over colder waters. In this decaying stage the winds often decrease rapidly and the cyclone eye and cloud patterns disappear.

Development

A tropical cyclone will only develop when the surface of the ocean is warm enough. It must be 26.5 degrees Celsius or more through an ocean depth of about 50 metres, in order to supply enough evaporating moisture, which is the initial energy source of the cyclone.

Along with warm water, there must be an existing low pressure area such as the Monsoon Trough. This trough forms between two wind systems. One originates in the South China sea, flows across the equator and becomes the northwest monsoon as it passes through the Indonesian archipelago towards northern Australia. The other wind system is the southern hemisphere trade winds.

Large scale cyclonic spin is generated in this trough where the winds meet (aided by the spin of the earth). The monsoon trough then becomes a zone favorable for the birth of tropical cyclones.

There must be little change in the wind speed between the upper and lower atmosphere, otherwise deep thunderstorms will be sheared (torn apart) before they can become organised. When deep thunderstorms are allowed to develop the cyclonic spin causes the clouds to form in spiral bands (see the animation below).

Tropical cyclones can cause a lot of damage through strong winds, heavy rain and associated flooding. Winds of over 300 kilometres per hour have been recorded in the most intense cyclones. They may also cause storm surges (a large sudden rise in sea level) which result in coastal areas being flooded by the sea. The intense rainfalls from tropical cyclones coming on shore in mountainous areas such as North Queensland can result in land slides.

One of the main roles of the Bureau of Meteorology is to give warnings of dangerous weather such as severe thunderstorms and tropical cyclones, and weather conditions leading to floods or bush fires. If you want to know more about the Bureau's role go to our Service Charter.

Below is a simplified animated model of a tropical cyclone.
It has been cut in half to show the 'eye' and the movement of air.

Move your cursor onto the numbers below to see the animations.
You will need to wait while the animation is downloaded.

(Animation 67K) Warm air rising around the eye of the cyclone produces cloud and rain. Air sinking inside the eye prevents cloud and rain from forming.


(Animation 45K) The air spirals inwards (clockwise in the southern hemisphere) in the lower levels of the cyclone and outwards in an anticlockwise direction in the upper atmosphere.


(Animation 55K) Winds at a high altitude turn in an anticlockwise direction (in the southern hemisphere). Winds at sea level turn in a clockwise direction.

The graph at left (Figure 1) shows typical variations (differences) in air pressure and wind speed across a cyclone.

Look at the dotted line (air pressure). It starts at 1010 hPa (hectopascals) and starts dropping as we move towards the eye of the cyclone. It drops down to 950 hPa then increases as we move out of the eye.

Notice how both air pressure and wind speed drop in the eye of the cyclone.