双参数云物理方案中谱形参数作用分析和数值模拟试验

Analysis and Numerical Simulation of the Role of Spectral Shape Parameter for Two-Moment Bulk Microphysics Scheme

云物理参数化方案中,各种粒子通常采用Г谱分布函数n(D)=N0Dαexp(-λD)。双参数方案中谱参数截距N0和斜率λ通过预报给出,谱形参数α设为固定值。为了讨论谱形参数的作用,理论上分析了谱形参数取值对沉降过程和微物理源汇项的影响。利用MM5模式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,模拟针对雨滴谱的谱形参数rα不同取值分别做了试验。模拟结果基本符合理论分析,谱形参数rα变化时模拟出的地面降水和云的宏微观特征有差异。

In bulk microphysics scheme, the size distribution of each hydrometeor category is often described by a gamma distribution law n（D）=N0D^αexp（-λD）. Two-moment scheme generally treat intercept No and slope λ as prognostic parameter while holding spectral shape parameter α constant. The role of the spectral shape parameter α is investigated by examining its effects on sedimentation and microphysical source/sink terms. A stratiform precipitation in Northwest China is simulated by using the PSU/NCAR mesoscale model （MM5） with a new double-moment explicit microphysics scheme. Various rain water spectral shape parameters, αr, is introduced for the model simulation experiments. It is found that the model simulation results are generally in agreement with the theory analysis and there are differences between surface precipitation and cloud macro-and micro characteristics for various rain water spectral shape parameter αr.