一次层状云系结构和降水机制的观测与数值模拟

A Case Study of Stratiform Cloud Structure and Precipitation Mechanism Based on Observation and Simulation

利用机载微物理探测仪器获得了2010年4月20～21日我国中东部地区的一次大范围降雨过程的观测资料,详细分析了云系的微物理结构和降水过程,并利用WRF模式对降水过程进行了数值模拟分析,详细探讨了此次降雨形成的微物理机制。分析表明,降水前期,云体在垂直方向上存在分层结构,云粒子探头(Cloud Droplet Probe,CDP)粒子浓度存在较大起伏。降水中期,CDP粒子主要存在于4.27km以下,其中3.69km处浓度较大,峰值普遍超过100cm-3;降水粒子和尺度较大的云粒子同样在4.27km以下。4.27km、3.69km处粒子形态较丰富,经历了不同尺度的片状、不规则状、针状及辐枝状的变化,3.69km处CDP粒子浓度较少时,降水粒子以针状为主,而CDP粒子浓度充足时则转化为尺寸更大的辐枝状粒子。4.27km、3.69km高度层存在的主要粒子是雪晶,其次是少许冰晶。降水后期,云体从顶部开始趋于消散,表现为3.9km高度以上无明显CDP粒子,仅存在部分云粒子和降水粒子,其形态为不规则状。降水主要形成于3.9km以下的云层,此时冷层仍存在部分针状的冰雪晶。WRF模拟结果表明:雨滴大部分形成于2.9km以下,0℃层下方,2.2～2.9km雪的融化贡献最大,2.2km以下重力碰并为主。雪是冷暖层主要的降水粒子,雪在冰雪晶层大部分高度仍以凝华增长为主,混合层以凝华增长和结淞增长为主,而雪的增长程度可能受上升气流强度、过冷水含量影响。

A wide range of precipitation occurred in mid-eastern region of China from 20 April to 21 April 2010. The purpose is to analyze the microphysical mechanism of the precipitation through airplane observation and simulation by the Weather Research and Forecasting （WRF） model. The airplane observation revealed a stratified structure in the cloud at an early stage of precipitation, and fluctuation of the Cloud Droplet Probe （CDP） droplet concentration was high. In middle stage of precipitation, CDP droplets existed mainly below 4.27 km; the peak at the maximum level of 3.69 km exceeded 100 cm^-3. Large-scale clouds and rain droplets also existed below 4.27 km. At 4.27 km and 3.69 km, several types of particle patterns were observed to include flakes and irregular, needle, and dendritic patterns. At 3.69 km, pattern of acicular and dendritic particles appeared in the areas of the least and most CDP droplet concentration, respectively. At 4.27 km and 3.69 km, the main particles were snow with a small amount of ice. In the late stage of precipitation, the cloud dissipated from the top, which indicates that few irregular clouds and rain droplets with no apparent CDP droplets existed at 3.9 km; rainfall formed mainly below this level. Several ice and snow particles appeared in the cold layer at this level. The simulation of WRF showed that the rain droplets were mainly formed below 2.9 km at the layer of 0 ℃. At heights of 2.2 to 2.9 km, snow melting to rain was the main factor in shaping rain droplets. Below 2.0 km, the rain droplets were formed by accretion droplets of rain. Snow droplets were mainly precipitation particles. Snow formed mainly through sublimation in the ice layer and through sublimation and riming in the mixed layer. The intensity of updraft and the content of supercooled water may have influenced the growth level of snow.