A study on the structure and precipitation processes of precipitating stratiform clouds associated with a westerly trough

Affected by a westerly trough and cold front, moderate precipitation occurred over Northern Hebei Province of China from 30 Apr to 1 May 2009. Based on airborne cloud particle probe observations, Doppler radar measurements and surface precipitation data, the cloud structure and precipitation processes were studied comprehensively with the Weather Research and Forecasting (WRF) model. By further investigating the microphysical characteristics, we would like to provide useful insight for future artificial seeding in that region.

The cloud development can be characterized by cirrus clouds at the initialization phase, upper cirrus, middle altostratus and lower stratocumulus clouds at the mature phase, and altostratus and stratocumulus clouds at the dissipating phase. For the initialization phase ahead the cold frontal line, the cirrus cloud was characterized by cloud base at 6.0 km, liquid water content of <0.003 g m^-3 and rainfall of only 0.1 mm. According to information from synoptic settings and rain gauges, the cold frontal line passed Northern Hebei primarily during 02 Beijing Local Time (LT) and 07 LT on 1 May with total rainfall of 7-9 mm. During that stage, the cloud with the radar reflectivity >35 dBZ extended around 100 km in the east-west direction and 150 km in the north-south direction. In addition, scattered radar reflectivity of about 50 dBZ could also be seen, suggesting the existence of embedded convective cells. The weakening of particle growth can be revealed from the aircraft observation during the dissipating phase. During that stage, LWC has the magnitude of only 0.001 g m^-3 and large particle concentration was less than 5 L^-1, suggesting slow growth of ice particles at upper levels. In addition, comparison of particle characteristics at 4.2 km and 3.6 km indicated large particle concentrations of up to 10² L^-1 and particle habits of predominantly dendrites for both levels before 10 LT, while primarily irregulars after 10 LT. However, it is worth noting that particle concentrations at 3.6 km decreased more evidently, with large particle concentration decreased to only 10¹ L^-1 and LWC to less than 0.001 g m^-3. That indicated rapid dissipation of lower stratocumulus clouds.

WRF model results showed gentle updraft of less than 0.2 m s^-1 before and after the frontal line, but higher values of up to 1.0 m s^-1 during the mature stage, indicating the existence of convective regions in stratiform clouds. The maximum cloud water content was 0.2 g m^-3, located at the 0°æ layer (2.7-3.0 km). The cloud water top height was 7.2 km, so the thickness of the mixed phase layer was no more than 4.0 km. Snow was the primary precipitation particles in the cold layer, with the maximum content of 1.0 g m^-3 and concentration of around 20 L^-1. Cloud structure and precipitation mechanisms in Zhangbei station were also studied. For the mature stage, the cloud was characterized by the maximum updraft of 0.4 m s^-1, the ice top height at 12.5 km and cloud water top height at 5.5 km, suggesting thickness of 7.0 km, 2.5 km and 1.0 km for the ice layer, mixed phase layer and warm layer, respectively. Model results suggested that depositional growth accounted for 50 to 80 % of total snow growth for the levels above 6.0 km, while riming replaced depositional growth as the main mode below 6.0 km. Besides, due to thin warm layer, up to 70-90 % of raindrop mass growth was associated with melted ice particles, while only 10 % was from gravitational coalescence. In addition, three stations including Zhangbei, Zhangjiakou and Chongli were selected to analyze contribution of various layers to the total surface precipitation. In the initial stage, the ice layer, the mixed phase layer and warm layer produced precipitation mass of about 25-35 %, 55-65 % and 10 %, respectively. In the mature phase, the contribution of the ice layer decreased to 10 %, while the mixed phase layer and warm layer increased to 65-70 % and 20-25 %. Differently, in the dissipating phase, the contribution of the warm layer decreased again to 5-15 % due to dissipation of the lower cloud.