Abstract

On 5 November 1977 a weak downburst associated with a multicell 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 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 of about 180 m penetrating down to the 58 m level.

About 1 km ahead of the downburst, and within the density current outflow from the storm, was a second gust front which may have been the low level manifestation of the toroidal vortex that initially surrounds a downburst, and is responsible for the severe horizontal outward flow. Just ahead of this second gust front and cantered at a height of about 50 m, was an indication of a counter-rotating vortex, which suggests that boundary layer separation may have occurred, causing the initial toroidal vortex to lift and so reduce the horizontal outflow wind speeds at ground level.

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