The science behind Cyclone Debbie

TROPICAL cyclone Debbie made landfall in Queensland as a category 4 cyclone with winds of more than 150kmh.

Cyclone Debbie from space. Picture: Contributed

TROPICAL cyclone Debbie made landfall in Queensland as a category 4 cyclone with winds of more than 150kmh.

The cyclone crossed the coast near Airlie Beach Tuesday afternoon. Reports of wind gusts in excess of 250km per hour and rainfall of hundreds of millimetres have been made in some areas along the central Queensland coast.

Anomalously high moisture, warm ocean temperatures, and low environmental pressures seem to have created the conditions that allowed TC Debbie to form and grow in intensity.

Perfect storm

Tropical cyclones are low pressure systems that form over warm tropical oceans. The warmth and moisture of the oceans are what gives a cyclone its energy. The low pressure, which meteorologists measure in "hectopascals”, draws in the surrounding warm, moist air, which then rises into deep thunderstorm clouds. As the air is pulled into the centre of low pressure, Earth's rotation causes it to spin cyclonically and it continues to intensify.

TC Debbie formed at the eastern end of an active monsoon trough extending from the Indian Ocean across the top of Australia and into the Coral Sea. On March 22, a large region of active thunderstorms began to organise into a weather disturbance off the eastern tip of Papua New Guinea.

Over the following two days the thunderstorms organised about a circulation centre as sea level pressures began to drop and moist air converged into the area. By late on March 24 a tropical depression, a forerunner of a cyclone, had formed and begun to drift south, making a long S-shaped track.

Tropical Cyclone Debbie was named on March 25. It then came under the influence of the subtropical ridge, a zone of stable high pressure that gives much of Australia's fine weather during the summer. This drove Debbie west-southwest towards the Queensland coast while it gradually intensified.

Because of the relatively high amounts of moisture in the atmosphere, and relatively warm ocean waters, Debbie intensified to category 4 by 10 pm on March 27, with the strongest wind gusts reaching 225-280km per hour. On Tuesday afternoon Debbie was a strong category 4 cyclone with a central pressure of 943 hectopascals and surface sustained winds of 185 kilometres per hour. The Bureau of Meteorology downgraded TC Debbie to a category 3 at 4:00 pm EST.

To put Debbie in context, there has been only one cyclone since 1980 to have made landfall in Queensland with a lower central pressure. That was Yasi in 2011.

THE CONVERSATION

Originally published as The science behind Cyclone Debbie