中文摘要：

利用MM5中增加的双参数显式云物理方案模拟了西北地区一次层状云降水过程,模拟结果显示对小雨的预报评分较高,对中雨以上评分低而且位置有一定偏差,即对层状云降水模拟效果较好。模式中增加了雷达反射率的计算,与延安站雷达RHI回波相比较,模拟的回波结构基本符合层状云回波特征,存在0℃层亮带。采用三层模型解释模拟的层状云降水形成机制和过程：第一层为冰晶区,无过冷水;第二层存在过冷水,为各种冰相粒子增长区,第一层和第二层的分界不固定;第三层和第二层的分界在0℃,为暖云。第一层对第二层播种冰晶,第二层为第一层播种下的冰晶供给过冷水,使冰晶快速增长;第二层对第三层播种雪和霰,使其在第三层融化成雨,第三层同时消耗云水。模拟给出了三层模型层状云场的空间结构,延安站不同时刻微物理量垂直分布和各种水凝物粒子的生成源项分析揭示了三层模型降水形成机制和主要微物理过程。三层模型可以完整和全面地解释层状云降水形成机制和过程。

英文摘要：

A stratiform precipitation in Northwest China is simulated by using the PSU/NCAR mesoscale model（MM5） with a new double-moment explicit microphysics scheme.The simulation results show that the threat score of 24-h precipitation forecasting at the categories 0.1-9.9 mm is high while the threat scores of threshold greater than 10 mm are low and the predicted precipitation locations have bias error compared with observation.The results indicate that the model has certain prediction ability for stratiform precipitation.The output result of new added radar reflectivity calculation in the model agrees with the radar echo on the Range-Height Indicator（RHI） display at Yan＇an station,which has the characteristic of stratiform radar echo and 0℃ isotherm bright band exists.To explain the physical mechanism and processes of the simulated stratiform precipitation formation,a three-layer model is proposed.The first layer contains ice crystals and has no supercooled liquid water.The second layer contains supercooled liquid water and is the region for different ice phase particles growth. The boundary deck between the first and second layers is not fixed.Below the 0℃ isotherm is the third layer which contains warm cloud water.The first layer precipitating ice crystals to the second layer clouds is called ＂seeder＂ zone and the second layer supplying supercooled liquid water is called ＂feeder＂ zone,in which ice crystals can grow and multiply very rapidly.Once the snow and graupel which produced in the second layer fall below the freezing-level,melting occurs,and precipitation is formed in the third layer.At the same time,rain water continuously grows at the cloud water droplet＇s expense for the warm cloud processes.The second layer is called ＂seeder＂ zone with respect to the third layer.The output of the model could provide the stratiform cloud field spatial structure of three-layer model,the microphysical values profile and the different hydrometeor particles sources analysis reveal the main precipitat