Observations of a tornado at an Automatic Weather Station in northern Tasmania

Abstract

On 15 April 2009, the Automatic Weather Station (AWS) at Scottsdale, in north-eastern Tasmania, recorded a wind gust of 54 ms-1 (194 kmh-1) as an active squall line crossed the state. Investigation of the environment, instrumentation, and damage resulted in the conclusion that this was a genuine wind gust caused by a tornado passing very close to the anemometer. This was the first direct AWS observation of a tornado in Australia, and one of very few such observations in the world. In this report, we document the weather event which produced the tornado, briefly outline the synoptic situation leading to its occurrence, document additional background observations that provide context for the event, and discuss the nature of the observations made by the AWS and the method by which the wind gust observations were verified. The squall line was part of a cold front that crossed northern Tasmania on the morning of 15 April 2009. At 300 hPa, the orientation of the short-wave trough associated with the cold front changed from positively to strongly negatively tilted as it moved over central Victoria and Tasmania. The orientation of the trough and position of a jet streak within it suggested strong upper divergence and strong vertical motion. These contributed to thunderstorm development and resulted in very substantial vertical wind shear through the lower half of the troposphere, which in turn contributed to the organisation of the severe convection. Near the surface, low cloud base and strong vertical windshear in the lowest kilometre provided conditions conducive to tornado development.