Morphological Structure of Snowburst in the Winter Monsoon Surges

Abstract

We studied the structure of snow trails from convective snow cloud developed along the west coast of Hokkaido Island, Japan in winter monsoon season. Photographic observations on the snow trails were carried out in company with radar and surface meso-network observations from 20 January to 20 February 1987. Snow trails were categorized into "burst type" and "virga type" from their morrphology. The former type was a peculiar case for snow showers. It had sharp edges and accompanied by a strong downdraft, which was also recognized by surface wind gusts and temperature drops. The downward velocity of downdraft was estimated about 10 m/s from time-series photographs. On the other hand, the latter type, which is a typical case of ordinary snowfall, showed little change in surface wind and temperature; and it caused no significant downdraft.
Burst-type snow trails occurred where the surrounding sub-cloud layer was conditionally unstable and below 0°C temperature. The sub-cloud condition, high humidity and low temperature was quite different from that of thunderstorm or squall line downbursts. The downdraft from precipitating snow cloud in the convective mixing layer was not recognized as a dry microburst nor wet microburst. Therefore, we named this burst-type snow trail "snowburst".