华北一次暴雨过程中潜热对中尺度系统和降水影响的数值研究

针对2005年7月22日的发生于华北的暴雨中尺度对流系统,在用中尺度ARPS模式数值模拟和分析云场、动力场以及微物理过程释放的潜热垂直分布和作用特征的基础上,通过改变主要微物理过程潜热做敏感性数值试验,研究和分析了潜热对云系发展演变、云系宏观动力场、水汽场、云场和降水的影响,总结出云暖区潜热的影响途径。结果表明,在对流云团中,5000 m以上微物理过程起加热作用,以下起冷却作用。不同物理过程潜热加热的云层高度不同:高层起加热作用的主要为水汽凝结、云冰初生和雪凝华增长、霰撞冻云水过程;中层起加热/冷却作用的主要为水汽凝结、霰/雹融化过程;低层雨水的蒸发过程起冷却作用。微物理过程潜热通过影响云系和降水发展过程、云系动力场,进而影响水汽场、云场和降水。忽略霰/雹融化潜热,相当于增加云系暖区潜热,促进了低层气旋性环流的形成,增强了低层动力场的辐合,使得低层辐合区增多、增强;中低层水汽通量辐合区增多、面积扩大,明显地促进了对流云系的发展,增大了含水量和覆盖范围,云系的降水量显著增加,强降水区覆盖范围扩大。即使减少20%的凝结潜热,云系的发展也受到极大抑制,没有气旋性环流生成,低层辐合区缩小、强度降低,水汽通量辐合区也同样缩小、强度降低,云系对流发展减弱、含水量降低,因此,降水量大为减小,降水范围也显著缩小。此外,微物理过程潜热还影响到此次中尺度对流系统发展演变过程,改变了云系的形态、影响到系统的移动和系统中对流云团的发展强度和分布情况。

Numerical study of the influence of the latent heat on the mesoscale convective system and precipitation during a torrential rain event in North China.

A mesoscale convective system which produces torrential rainfalls on 22-24 July 2005 in North China is selected as a research case.This rainfall process is reproduced successfully by the ARPS model.On the base of the analyses of the cloud field,the dynamical field and the vertical distribution as well as the function characteristics of the latent heat released by the microphysical processes,the influence of the latent heat on the development,evolvement,dynamical field,water vapor field and rainfall of the cloud system is studied and analyzed via changing the latent heat of the main microphysical processes for conducting the sensitivity experiments.Finally,the impacting passes of the latent heat in the warm zone of the cloud is summarized.The results show that the microphysical processes play a heating up role over the 5000 m height and a cooling one under this height and the impacting levels of the latent heat of different microphysical processes are different: the vapor condensation,initial formation of cloud ice,sublimation of snow and accretion of graupel play the heating role in the upper level,the vapor condensation/graupel melting have a heating/cooling role in the middle level,and the diabatic cooling effect in the low level is mainly produced by rain evaporation.The latent heat has an important role on development processes of the cloud system and precipitation,and on the dynamical field and thus on the water vapor field,cloud field and rainfall.When cooling effect of graupel melting is ignored,which is corresponding to increasing latent heat in the warm zone of the cloud,it is propitious to form the cyclonic circulation in the low level and increase the convergence area and intensity of vapor flux.Therefore,it accelerates the development of the convective cloud system,expands the heavy rain region and increases obviously the precipitation amount.If the vapor condensation latent heat is decreased by 20%,the development of the cloud system is restrained evidently and the cyclonic circulation is not formatted in the low level.As the same time,the convergence area is decreased and the convergence of the vapor flux is lowered and,the water content in the cloud and the rainfall are all decreased.Moreover,the latent heat has an effect on the development and evolvement process of the mesoscale convective system and it changes the conformation of the cloud system,its movement and the intensity and the distribution of the convective cloud clusters.