Tropical severe thunderstorms

Severe thunderstorms in the tropics

The Australian definition of a "severe thunderstorm" is one which produces any of:

• Large hail - 2 cm diameter or greater
• Damaging wind - 90 km/h or greater
• Tornadoes
• Heavy rainfall conducive to flash flooding

The main severe weather types produced by thunderstorms in the tropics are damaging wind and heavy rainfall. The wind gusts are "straight line gusts", that is, not associated with the rotating winds within a tornado, but due to the outflow from the downdraught of a thunderstorm as the air hits the ground and spreads out. The name given to an intense thunderstorm downdraught concentrated on a small area is a microburst. Flash flooding can occur with tropical thunderstorms when they are slow-moving (so that the one location receives a prolonged downpour) or when successive storms move over the same spot, like the carriages of a passing train (this is aptly called the train effect).

Waterspouts are occasional visitors to tropical (and extra-tropical) seas. They are mostly weaker non-supercell tornadoes and are not always associated with thunderstorms - often they develop at the base of smaller (cumulus) clouds which are rapidly growing. Waterspouts pose a danger to boaters, and can sometimes cause damage near the coast if they move over land.

Fig 1: A water spout at Batemans Bay, New South Wales, November 18, 2012, 2.15pm. Copyright Commonwealth of Australia, credit: Amy Lay, Bureau of Meteorology

Tornadoes are thought to be extremely rare over land in the tropics, but they can occur. Tropical tornadoes are at the weaker end of the scale, but even so, can leave a trail of damage to trees and buildings. The presence of an intensifying tropical low pressure system or tropical cyclone in the waters around the Northern Territory can give rise to an environment which is conducive to tornado formation at long distances from the centre of the low. Tornadoes have also been observed to develop in the core and feeder bands of tropical cyclones as they cross the coastline, adding to their destructive power.

Gustnadoes are tornado-like features at the edge of thunderstorm outflows and are sometimes seen in tropical thunderstorms. They can cause minor damage, and are evidence of strong outflows which may lead to damaging straight line wind gusts.

A few reports per year are received of hail in northern Australia. Large hail is very rarely observed in the tropics. Some thunderstorms contain smaller hail high up in the cloud, but in the warm tropical atmosphere, the smaller hail generally melts before it can hit the ground.

Fig 2: Thunderstorm over a tropical beach, QLD

Types of tropical severe thunderstorms

There are two particularly common types of thunderstorms in the tropics. These are known as pulse storms and squall lines, and both can sometimes lead to severe weather.

Pulse storms

A pulse storm is another name for a single or multicell thunderstorm with a brief lifecycle - typically under an hour. Pulse storms tend to form in conditions where there is small vertical wind shear - that is, the direction and strength of the wind does not change much with height. When the conditions are right for pulse storms, the clouds will develop in a more vertical fashion.

Pulse storms are characterized by one or more strong updraughts punching up through the atmosphere, followed by downdraughts which, if intense enough, can bring strong wind gusts and, very occasionally, hail. The effects are usually brief and localised, being limited to the area directly underneath the downdraughts and nearby updraught.

Pulse storms usually occur during the afternoon, when the hot ground can provide maximum energy for the storm updraught. If the atmosphere is very moist and the storm is slow moving, the storm can lead to flash flooding. Conversely, dry layers of air over the lowest several kilometres of the atmosphere can enhance the strength of the downdraught at the surface.

Squall lines

A squall line is a long line of thunderstorm cells, sometimes several hundred kilometres in extent, that share common precipitation cores or cloud mass. Squall lines can last for hours or even days, with new storm cells continually forming along the leading edge of the line. They form in conditions with moist air near the ground and larger vertical wind shear, with the winds near the surface being very different from the winds higher up.

As the clouds develop on days suitable for squall lines, they will tend to look "tipped over" as their tops are "pushed" by the winds higher in the atmosphere. A squall line can arrive at your location at any time of day or night, depending on where it initially formed and how quickly the line has been propagating forward. Top End squall lines often originate over Arnhem Land, or further afield over Cape York. The approach of a squall line is heralded by a wind squall (the sudden onset of gusty winds lasting several minutes). This often coincides with the arrival of an ominous dark bank of low cloud called a shelf cloud. localised damaging wind gusts can occur beneath the most intense storm cells in the line.

There is another type of severe thunderstorm known as a supercell. Supercells are among the most violent storms, and can sometimes be accompanied by strong tornadoes and huge hail. Luckily for those who live in the tropics, true supercells are rarely found within tropical regions except in the vicinity of tropical lows and tropical cyclones which supply wind shear for the development of such storms.