A comparative study of the microstructure and precipitation mechanisms for two stratiform clouds in China

The microphysics and precipitation mechanisms of two precipitating stratiform clouds on 1 July 2004 (case 040701) and 21 June 2005 (case 050621) were studied by using aircraft and Doppler radar measurements, and precipitation data. Observations showed that the two clouds were characterized by predominantly altostratus and stratocumulus clouds and showing evident radar bright bands at the mature stages. The cloud in case 040701 exhibited liquid water content (LWC) of less than 0.1 g m− 3, the presence of a dry layer and average precipitation rates of 2.0–3.0 mm h− 1. In contrast, the cloud in case 050621 developed more actively, showing relatively high LWC of 0.5 g m− 3, embedded convective cells with radar reflectivity of up to 40 dBZ and higher precipitation rates of 4.0–6.0 mm h− 1. On the whole, for the mixed-phase layer, ice particle concentrations increased with decreasing height until 0.5–1.0 km above the 0 °C layer. Ice particles mainly formed and grew at levels 4.0–5.5 km with depositional growth dominating, mixed with some aggregation. Ice particle habits in the two cases were characterized by dendrites at − 1 °C, needles at − 3 °C and again dendrites and irregulars at − 6–−9 °C with few spheres coexisting. Aggregation of dendrites was present at temperatures lower than − 6 °C in both cases, but it was not the dominant mode. For case 050621, a combination of needles was present at around − 3 °C. In comparison, aggregation at the melting layer was not dominant, having little influence in increasing average diameters. Gravitational coalescence at the warm layer was not dominant in case 040701 due to the existence of the dry layer, but rather important in case 050621.