四川暴雨过程中盆地地形作用的数值模拟

利用WRF-Chem模拟了2012年7月20日一次四川盆地暴雨降水过程，并基于控制试验设置填充四川盆地地形的敏感性试验。利用大气动力-热力学和云降水物理学对两试验差异进行诊断分析，与敏感性试验相比，控制试验虽然延迟强降水出现时间，却增强了降水强度。研究表明:偏南气流自南向北经过盆地时，在四川盆地南部形成正涡度扰动中心，延迟水汽、能量到达盆地北部的时间，使强降水出现时间偏晚；地形高度及动力差异使控制试验近地面累积大量水汽、能量，低层能量到达盆地北部迎风坡后受地形抬升与正涡度扰动共同作用激发了强烈的对流；控制试验中，盆地北部大气强烈对流运动及其携带盆地内大量水汽有利于云系的垂直发展，雨滴、雪晶、霰粒子质量浓度明显增大，使降水强度增强至大暴雨量级。

Simulation of Basin Topography Impacts on Rainstorm in Sichuan

Topography, especially the height and shape conditions have significant effects on precipitation. Previous studies focus on effects of mountain topography upon precipitation, while influencing mechanisms of the basin topography are not widely discussed. The Weather Research and Forecasting (WRF) with Chemistry model is used to simulate a heavy rain event which occurs on 20 July 2012, over Sichuan Basin. A sensitive test is designed in which the topography of Sichuan Basin is uplifted, with other conditions the same as the control test. The topography in the sensitivity test shows a trend of slow decline from west to east, eliminating the role of basin topography, but keeping the influence of the Tibetan Plateau around the Basin.From the atmospheric dynamics, thermal and cloud micro-physics standpoints, diagnostic analysis is used to analyze results of these two tests, and differences between two experiments are discussed. Results show that the time of heavy rainstorm in the control test is later than that in the sensitivity test, and the rainfall intensity in control test is strongly enhanced. From the point of atmospheric dynamics, when southwesterly airflow through the basin from south, a stronger positive vorticity center forms in the south of the Basin in the lower layer of troposphere in the control test, and southern wind is weakened. Therefore, the water vapor and energy reach the northern part of the Basin later, leading to the precipitation delaying. At the same time, with the southward wind transport, the positive relative vorticity of the lower layer in the northern part of the Basin is continuously strengthened. Favorable dynamic structure strengthens the vertical motion and thus increases the precipitation intensity. From the thermodynamic view, there is more heat and water vapor in control test due to its lower height. Besides, these variables accumulate subjected to topographic dynamics, and are less likely to diffuse, providing sufficient water vapor for the rainstorm. In addition, the high temperature and high humidity condition makes the low level of the control test accumulates moist static energy. When airflow carrying moist static energy reaches the northwest of the Basin, strong upward is stimulated under the influence of topography and the positive relative vorticity in the lower troposphere. From the opinion of micro-physics, the stronger vertical motion provides advantage for the vertical development of the cloud system, and more water vapor provides greater supersaturation for precipitation particles in the control test. Under these conditions, precipitation particles, especially rain water, snow crystals and graupel, are generated and transformed in large quantities, enhancing the precipitation intensity to heavy rainstorm.