Processes leading to a severe windshear incident at Hobart Airport

Around 0230 UTC 21 January 1997 Qantas flight QF617 encountered extreme turbulence at approximately 500 ft altitude as it approached Hobart Airport. A near-surface southerly wind surge under-cutting a strong synoptic-scale northwesterly airflow generated a vertical wind shear of over 30 kn through a layer some 25 m deep. Mesoscale numerical weather prediction model output is used in conjunction with after-event subjective analyses of observations to diagnose the processes that led to the development of the southerly wind surge. It is shown that a combination of heating over land during the daylight hours, leading to locally lowered pressures, and the rapid movement of a mesoscale high-pressure system around the southern coast of Tasmania led to a local intensification of the onshore pressure gradient and the rapid development of a flow-reversal in the lowest levels south of Hobart Airport. Analysis of archived radiosonde data suggests that this event, while containing ingredients that might be expected to occur relatively routinely over southern Tasmania in summer, was unusual in its intensity.