MM5中新显式云物理方案的建立和数值模拟

A New Explicit Microphysical Scheme in MM5 and Numerical Simulation

在MM5动力框架内,在其中Reisner2方案基础上采用双变参数方案,增加了云水、雨水、雪和霰的数浓度预报方程。云中凝结核CCN的数浓度采用超几何函数表示;云水向雨水的自动转换过程考虑了云滴谱的特征和发展变化对该过程的影响,而不是采用原方案给定阈值的方法描述该过程;对连续碰并方程不再将粒子落速差作为常量提出积分号外,而是直接作为粒子直径函数在积分号内求解,这样处理可以回避使用粒子群的平均落速带来的误差;增加了霰和雪、霰和冰晶的碰并微物理过程。粒子引入Г分布谱函数,对微物理过程采用了与之相适应的计算公式。通过增加新预报量的计算和输出程序,在MM5的显式方案中新增加了一个可选方案,并称之为新方案。模拟了一次层状云降水过程,在区域二分别采用了新方案和Reisner2方案以作对比。新旧方案降水模拟结果的对比表明新方案在降水范围和强度的预报效果有所改进。模拟结果给出了层状云的合理微观结构和它的一些特征,增强了MM5研究微观云物理的能力;通过单站微物理过程分析,揭示了降水过程的可能形成机制。说明新方案可以为层状云的宏微观结构特征、降水物理过程和人工影响天气研究提供一定依据。

Based on the dynamic frame of MM5 and Reisner 2 explicit cloud scheme, a new double-moment microphysical scheme is developed, in which both the mixing ratios and number concentrations of cloud water, rain water, cloud ice, snow and graupel are predicted. The number concentration of the activated cloud condensation nuclei (CCN) is described with the hypergeometric function. The influences of the cloud droplet spectrum character and the spectrum growth and change are considered in course of the collision and coalescence of cloud droplets to form raindrops, which does not use a cut-off value in the conversion from cloud water into rainwater . To the continuous collection equation, the particles terminal fall velocity difference is not taken outside the integral, but is integrated in the inner integral equation as a function of diameter to avoid the error caused by using particles mass weighted mean terminal fall velocity. In the new scheme, generation rate of graupel due to the collection of snow by graupel and the collection of cloud ice by graupel is included. The new scheme uses the more reasonable gamma law as basis functions for the hydrometeor drop size distribution. Corresponding to the gamma size distribution, microphysical equations in the new scheme are rewritten. By adding the calculation and output program for new prognostic variables, the new scheme is incorporated in the three-dimensional non-hydrostatic model MM5 and becomes a new option in its explicit microphysical scheme, which is called the new scheme in the paper. For comparison, an observed precipitation formed by the stratus cloud system is simulated with the new scheme and the Reisner 2 scheme in domain 2 respectively. By comparing model simulation with an observed precipitation, the new scheme shows some improvement of precipitation location and intensity prediction. The output of the new scheme can give reasonable microstructure of stratus cloud and indicate its some characteristics, which enhance the capability of MM5 to study the cloud microphysical process. The numerical simulation reveals the possible mechanism of the rainfall by single station microphysical processes analysis. These results suggest that the new scheme will provide some valuable information on macro and microstructure characteristic of stratus cloud, physical process of precipitation and weather modification research.