Doppler Radar Analysis on a Series of Downburst Events

For the first time a downburst event is analyzed in detail with Doppler weather radar in China. A severe multi-cell storm develops near the border between Dingyuan county and Feidong county in Anhui Province, and produces a series of downbursts accompanied by severe hails on 6 June 2003. As for the synoptic background, the axis of the trough tilts eastward, so that the dry and cold air mass from the rear of high-level trough superposes upon the moist and warm air mass in front of the lower level trough, building up the convective instability. The lower troposphere is moist with weak vertical wind shear, while the mid and upper troposphere is dry. This is a situation favorable for wet downburst. This series of downburst consists of three sequential bursts of strong downdrafts, produced by three convective cells developping in the multi-cell storm. From 16:14 to 16:33, the continued descending of reflectivity core of the first cell occurs, and at 16:33, a significant midlevel convergence appears. At 16:39, the first downburst hits the ground, with significant divergence on the lowest elevation (600 m height above ground). The same precursor appears before the second and third consecutive downbursts. Before each burst of the downdraft, the core of the corresponding cell descends, accompanied by the convergence above cloud base. This fact can be used to issue downburst warnings with a lead time of about 5 to 6 minutes. At the same time, a rotation appears on the midlevel in the storm. These facts are in agreement with those discovered by Roberts and Wilson with the statistics of 31 microbursts occurred in Colorado, USA. When the downburst attains its maximum strength, the largest velocity difference along the 0.5° scan over a distance of 6 km is 44 m/s, corresponding to a divergence value of 1.5 ×10^-2s^-1. The downburst series, consisting of three sequential downbursts, lasts 74 minutes, with each lasting 25 minutes on average. During the whole period, the storm moves very slowly, due to the cancellation of the advection and propagation, leading to the occurrence of the three downbursts at nearly the same site. Analysis shows that it is the hail and rain's descends that initiate the downdraft, which is then greatly enhanced by the entrainment of dry air into the downdraft and subsequent evaporative cooling, for the environmental air is relative dry at mid troposphere. Furthermore, the steep decrease rate of environmental temperature from surface to 0 ℃ level is quite large, around 8 ℃/km, keeping the downdraft with negative buoyancy down to the ground.