积层混合云结构特征及降水机理的个例模拟研究

Numerical Simulation of the Structural Characteristics and Precipitation Mechanism of Stratiform Clouds with Embedded Convections

积层混合云是我国一种重要的降水系统,其降水既有对流云又有层状云特征。基于积层混合云的重要性,本文利用中尺度数值模式WRF(Weather Research and Forecasting Model),结合三维粒子运行增长模式对2012年5月29日北京地区的一次积层混合云降过程进行了模拟研究。模拟的降水与雷达回波与实测结果基本一致。在此基础上,重点分析了混合云系中积状云与层状云各自的微物理结构特征与降水的发生机理等。结果表明:降水过程云内存在着明显的"播种—供给"机制,层状云中"播种—供给"机制相对简单。而对流云区中由于降水粒子可以发生上下多次的循环增长,"播种—供给"机制可在云的上下层间双向进行,云中粒子群可以增长得更大。在积层混合云中,在低层,层状云中已有的水凝物粒子进入内嵌的积云块中,而在高层水成物粒子又从积云中落到层云中,积层混合云系充分发挥了积云和层云各自的优势,从而降水效率较高。

Stratiform clouds with embedded convections represent an important precipitation system in China. Precipitation from this system has characteristics of both convective and stratiform cloud. We studied this important process on May 29, 2012 using a mesoscale Weather Research and Forecasting （WRF） model and a three-dimension growth model. Since the resulting simulation of rainfall and radar echo were similar to observation results, we then analyzed the microphysical structural characteristics and precipitation mechanism of stratiform and convective clouds, respectively, in stratiform clouds with embedded convections. Results show that there is a ＂seeder-feeder＂ mechanism in this precipitation system, and this mechanism is simple in stratiform cloud. In convective cloud, however, because of the frequent circular particle movement between the upper and lower levels, this mechanism can occur in both the upper and lower cloud levels and so the particles can grow bigger. At low levels of the cloud system, the particles move into the embedded convective cloud, while at high levels, the particles fall from the convective cloud to the stratiform cloud. Stratiform clouds with embedded convections give full play to the advantages of convective and stratiform cloud, so their precipitation efficiency is high.