THE BALD HILLS DOWNBURST: A Thunderstorm Downburst Case Study

Abstract

On November 5, 1977, a weak downburst associated with a multi-cell storm passed over an instrumented tower at Bald Hills, a suburb of Brisbane, Australia. Associated with the thunderstorm was a dome of cold air estimated to be 1200 m to 1800 m deep. Two downdrafts, at least one of which was large enough to be called a downburst, penetrated to the ground near the front edge of this dome. The downburst substantially maintained its vertical velocity down to 100 m above the ground, with one parcel of air having a high vertical velocity and a lateral extent ofabout 180 m penetrating right down to the 58 m level. At the 100 m level the vertical downdraft had a peak value of 7.5 m/s and its average value was continuously greater than 4 m/s for at least 50 seconds. The ground wind (measured at l0 m) changed from a north wind of l0 m/s to a south wind of 13 m/s in approximately 90 seconds. Because of restraint by the surrounding flow, the downdrafts did not spread out as a free wall jet, but as a slower, deeper, submerged jet whose thickness was controlled by downstream conditions. About 1 km ahead of the downburst, and within the cold air flowing away from the storm, was a second gust front which may have been the low level manifestation of the ring vortex which initially surrounds a downburst, and is responsible for the severe horizontal outward flow. Just ahead of this second gust front, and centered at a height of about 50 m, was an indication of a counterrotating vortex which suggests that the boundary layer of the cold outflow may have separated from the ground, causing the initial ring vortex to lift and so reduce the horizontal outflow wind speeds at ground level.