一次热对流降水成因的分析和模拟

夏季受西太平洋副热带高压控制的中国大陆地区常发生热对流降水.文中首先利用TRMM卫星观测资料、地面自动站地表温度观测资料和NCEP资料分析了热对流降水的特征及其产生的背景条件;利用区域平均的水汽方程诊断了地表蒸发、水汽平流和水汽通量辐合项的贡献,分析了降水的水汽来源;进一步利用AREM区域数值预报模式,设计了4个敏感性试验,研究了陆气感热、潜热通量对降水的贡献.结果表明,在副热带高压控制的地区,白天强烈的非均匀地表感热加热可导致低层热空气块突破环境下沉气流而上升,周围空气补充形成辐合运动.低层空气的辐合上升既可引起水汽的汇聚,又可把低层的水汽输送到高层.同时非均匀陆面特征造成的非均匀感热和潜热通量的共同作用增强了大气的位势不稳定度,触发了对流,但感热通量对热对流降水的贡献比潜热通量略大.热对流降水的水汽除部分来自地表蒸发外,另一部分来自低层的水平通量辐合.定量计算表明二者对大气可降水量的贡献基本相当,而平流水汽的贡献很小.

A case study on triggering of thermal convective precipitation

Thermal Convective Precipitation (TCP) usually occurs when the West Pacific Subtropical High controls China Mainland in summer. Under a background of prevailing subsidence, how can the convection start and where does the water vapor come from? The two issues are studied by selecting a typical TCP event taking place on 2 August 2003. Firstly, the characteristics of the TCP are illustrated by TRMM (Tropical Rainfall Measuring Mission) observations, automatic weather station observations and NCEP (National Center for Environmental Prediction) data. Then the surface evaporation, water vapor advection, and the divergence of the water vapor fluxes are calculated by the regionally-averaged water vapor budget equation, and the water vapor sources are analyzed. Furthermore, using the regional eta-coordinate numerical model AREM, the contributions of sensible heat and latent heat fluxes are investigated by 4 different sensitivity experiments. The results show that in the regions controlled by the Subtropical High, the surface temperature rises rapidly after sunrise. When receiving enough sensible heat, the air goes up and leads to the convergence in the lower atmosphere. Then the water vapor assembled from the surroundings and ground surface is transported to the upper levels, and the favorable environment for TCP forms. The simulation data diagnoses indicate that about half precipitable water comes from the convergence of water vapor horizontal flux, the other half comes from the surface evaporation, while little is from the advection. The further model sensitivity experiments prove that both sensible and latent heating are essential for the onset of TCP. The sensible heat flux triggers thermodynamic ascending motion, the latent heat flux provides water vapor, but the contribution to the TCP from the later is a little smaller than that from the former.