While we experience many types of thunderstorms in Australia, some more intense thunderstorms are referred to as severe thunderstorms. Severe thunderstorms can produce damaging wind gusts, large hail, tornadoes and heavy rain which may cause flash flooding and these phenomena can all cause significant damage.
Thunderstorms which produce any of the following are classified as severe in Australia:
Most thunderstorms do not reach the level of intensity needed to produce these dangerous phenomena. The Bureau of Meteorology only issues severe thunderstorm warnings for thunderstorms that are expected to produce any the above severe phenomena.
A thunderstorm is associated with a very tall cloud mass called a cumulonimbus cloud. A good proportion of thunderstorms develop when warm, humid air near the ground is forced upwards due to converging surface winds and rises rapidly in an unstable atmosphere. Thunderstorms can become severe when the atmosphere is particularly unstable and/or additional energy is drawn in from surrounding winds.
Fig 1: Supercell thunderstorm
To produce a thunderstorm the atmosphere needs the right ingredients. These are:
Fig 2: Cumulus clouds before the storm
Every thunderstorm cloud has several features including a core region, a spreading anvil top, and an inflow-outflow region.
Core: The core is the part of the storm where sustained strong updraughts of relatively warm and moist air produce rain and hail.
Anvil: The anvil is a flat, often fibrous cloud sheet, above and usually downwind of the updraught. It is mainly made up out of ice crystals.
Inflow-outflow region: The inflow is a region of usually warmer and more humid air that supplies a thunderstorm with its energy source. Outflows are produced when rain-cooled air first descends before streaming away from the storm as a cooler air flow.
Fig 3: Diagram illustrating some features that may be produced by a severe thunderstorm.
There are three general thunderstorm types: the single-cell, the multicell and the supercell. Each has a distinct structure, circulation pattern, and set of characteristics.
A single-cell thunderstorm is one whose life cycle is limited to the growth and collapse of a single updraught pulse. The cloud forms, grows to maturity, produces a heavy downpour, then decays as descending air cuts off the original warm inflow. Such thunderstorms are most likely to occur in mid-summer and usually last no more than an hour. They rarely produce severe weather. In nature it is rare to encounter a pure single-cell storm as almost all single-cell thunderstorms have a degree of a multicell character (see below).
The multicell thunderstorm type is the most common and consists of successive, separate updraught pulses that help maintain the system's overall strength, structure and appearance. The pulses may be very close together, causing the thunderstorm's characteristics to be quite uniform over time, or they may be widely spaced, resulting in a thunderstorm that cycles repeatedly through stronger and weaker phases. The updraught pulses can be seen from a distance by watching the spacing and growth rate of individual cumulus towers along the flanking line. Multicell thunderstorms can produce any of the phenomena mentioned above, but rarely cause tornadoes to form.
The supercell is a very strong long-lived thunderstorm type in which the system can maintain an almost steady state for many hours. A highly organised cloud-scale circulation with a continuous large updraught and magnified size and impact make this a fascinating but dangerous cloud complex. Supercells account for most of the severe thunderstorm events we experience.
Fig 4: Supercell storm over Broken Bay Pearl Beach NSW Australia
Severe thunderstorms can occur at any time of the year, although they are less common during the winter months in the south or the dry season in the north of Australia. Most strike between September and March when the supply of solar energy is greatest, but severe winter storms linked to cold fronts are not uncommon in the south-west of Western Australia and south-east South Australia.