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Characteristics and Formation

3. What is the life-cycle of a tropical cyclone?

Tropical cyclones have a distinct life cycle. For cyclones that reach at least severe (category 3 or higher having wind gusts of at least 170 km/h) the life-cycle may be divided into four stages. For non-severe cyclones, their development is constrained by one or more of a number of factors such as being located in an unfavourable atmospheric environment, movement over cooler water or making landfall.

1. The formative stage

On satellite images the disturbance appears as an unusually active, but poorly organised, area of convection (thunderstorms). The circulation centre is usually ill-defined but sometimes curved cumulus cloud bands spiralling towards an active area of thunderstorms indicate the location of the centre. Initially the amount of convection near the centre is dependent upon the normal diurnal cycle of tropical convection, increasing overnight and subsiding during the day. As development occurs the convection persists throughout the day. The strongest surface winds may be well removed from the centre, tend to occur in disorganised squalls and are often confined to one quadrant, for example the northwesterly monsoon winds to the north of the centre. Apart from local squalls the maximum wind is usually less than gale force. When formative stage tropical cyclones move inland they produce little or no damage on landfall but are often associated with heavy rain and sometimes flooding over northern Australia.

2. The immature stage

In this stage the area of convection persists and becomes more organised. Intensification occurs simultaneously. The minimum surface pressure rapidly drops below 1000 hPa and convection becomes organised into long bands spiralling inwards. Gale-force winds develop with the strengthening pressure gradient, and the maximum winds (which now may be storm-force or more) are concentrated in a tight band close to the centre. The circulation centre is well defined and subsequently an eye may begin to form. In satellite images several well organised curved bands of active convection may be seen spiralling in towards a central dense mass of clouds covering the focal point of the banding, or surrounding the centre. The eye (if it exists) may be masked by a canopy of cirrus cloud, which itself may contain curved striations associated with the outflow at the top of the tropical cyclone. The immature tropical cyclone can cause devastating wind and storm surge effects upon landfall, although damage is usually confined to a relatively small area. In this stage of development very rapid intensification can occur and the associated structural changes observed when the cyclone is under radar surveillance can sometimes be confusing.

3. The mature stage

During this stage the tropical cyclone acquires a quasi-steady state with only random fluctuations in central pressure and maximum wind speed. However, the cyclonic circulation and extent of the gales increase markedly. Asymmetries in the wind field may also become more pronounced. In satellite images the cloud field is highly organised and becomes more symmetrical. The more intense cyclones are characterised by a round central dense overcast containing a well-centred, distinct round eye. The surrounding convective bands are tightly coiled and quasi-circular. Typically a cyclone spends just a day or so at maximum intensity until it begins to weaken, unless the cyclone remains in a highly favourable environment.

4. The decay stage

The warm core is destroyed during this stage, the central pressure rises, and the belt of maximum wind expands away from near the centre. Decay may occur very rapidly if the system moves into an unfavourable atmospheric or geographic environment, but sometimes only the tropical characteristics are modified while the cyclonic circulation moves on to higher latitudes.

In satellite images the decaying stage is characterised by the weakening of organised convection near the centre and disappearance of major curved convective bands. The low-level circulation centre may still be very well defined by narrow bands of low clouds. Those cyclones that cross the coast and weaken over land may continue to produce heavy rain a considerable distance inland.

An abbreviated life cycle of tropical cyclone Paul, April 2000.
1. Formative stage: 11 April.
2. Immature stage: 14 April.
3. Mature stage (920 hPa): 15 April.
4. Decay stage: 20 April.