Observation of the Degree of Glaciation in Middle-Level 5tratiform Clouds

Abstract

Middle-level stratiform clouds associated with cyclones were observed in spring 1985 simultaneously by a 19.35 GHz microwave radiometer and an 8.6mm-radar. The vertically integrated liquid-water content was estimated by analysing the data of the microwave radiometer together with upper-air sounding data, and the vertically integrated ice content was estimated from radar-echo intensity. Structure and precipitation formation in middle-level stratiform clouds were studied, especially their degree of glaciation which is defined by the ratio of vertically integrated ice content to the total amount of condensed water (the sum of vertically integrated liquid-water and ice).
Analysed cases are divided into two types of clouds (type-A and type-B). Type-A clouds showed a low degree of glaciation, a large amount of integrated liquid-water, and a comparatively thin and weak radar-echo layer only at middle levels. The degree of glaciation in three cases was slightly less than 10% and an average and the total amount of condensed water was about 55mg/cm² . Type-B clouds had a deep and intensive radar-echo layer, whose tops were far above middle levels. The degree of glaciation in three cases was about 65% on average and the total amount of condensed water was about 40mg/cm². The degree of glaciation was significantly different between the two types, though total condensed-water amounts were comparable.
The structure and precipitation formation processes of the two types of clouds can be summarized as follows. The type-A cloud is a one-layer middle-level cloud. Ice particles are initiated near its top where the air temperature is not low enough to allow a sufficient number of ice-nuclei to be activated. The glaciation process is not efficient though the supercooled water content is large due to strong updraft in it. On the other hand, the type-B cloud is composed of an upper-level cloud and a middle-level cloud which seem to have formed independently. Ice particles supplied by the upper-level cloud promote the glaciation in the middle-level cloud. By comparing glaciation proceses in two types of clouds we showed quantitatively that only a middle-level stratiform cloud could not produce solid precipitation efficiently, even though it contains a large amount of super-cooled water, unless an upper-level cloud supplies sufficient ice particles to it.