2003年8月“巴蜀夜雨”过程的模拟和分析研究

Numerical simulation of the mid-night rainfall over Sichuan basinduring August 2003.

中文关键词:  夜雨， 日变化， 四川盆地， 地形

英文关键词:Midnight rainfall, Diurnal cycle, Western Sichuan basin, Topography

中文摘要:

      结合中尺度数值预报模式AREM的数值试验和观测资料分析，对2003年8月川西地区的9次夜雨过程进行了模拟研究和综合分析。结果表明，在一定环流背景下，川西地区特殊地形引起的沿坡地的辐合上升运动和下垫面提供给低层大气的热通量所导致的大气层结不稳定，对川西夜雨的形成和发展有重要影响。白天，随着陆 气通量交换的增加，低层大气的温度和湿度逐步升高，并在午后达到极值。与此同时，低层偏南暖湿气流在盆地西部由于气旋性弯曲而形成的东北风在午后逐渐加强，这支气流在盆地西部被地形阻挡，产生爬升运动。辐合上升将低层高温高湿的大气向上输送，使得大气不稳定层结的厚度以及强度都增加；日落以后，低层大气的相对湿度随着气温的降低而增大，容易饱和而形成凝结，同时大气中积累了相当可观的对流有效位能，低层辐合抬升等因素容易触发不稳定能量释放，造成对流性夜雨天气。强烈的对流辐合运动需要周围大气的入流补偿，促使偏东风气流增强且向高空伸展，这令辐合抬升作用进一步增强。

英文摘要:

      Nine mid-night heavy rainfall events during August 2003 over the western part of Sichuan basin were simulated by using an Advanced Regional Eta-coordinate Model(AREM). The mechanisms responsible for the diurnal variation of the rainfall are addressed by analyzing the model outputs. The results show that mid-night rainfall over western Sichuan basin usually occurs under a specific large-scale background circulation,both the orography forcing induced convergence/ascending motion and the unstable layer resulted from the thermal forcing of ground surface contribute to the formation and development of the midnight rainfall over Sichuan basin. During the daytime, the low-level temperature and humidity gradually increase with gradually intensified land-air heat flux exchanges, and reach their maximum values after the noon. In the mean time, the low-level warm/moist northeast flow associated with a cyclonic curvature over the western basin gradually strengthens, and the low-level convergence appears, when the warm/moist northeast flow encounters eastward facing mountain slopes in the western basin. The low-level convergence transports the warm and moist air upward, which leads to the increasing of both the thickness and intensity of the unstable layer in the lower troposphere. After sunset, the relative humidity in the lower atmosphere gradually increases due to radiation cooling, which makes the air easier to be saturated and the convective available potential energy (CAPE) to accumulate in the lower atmosphere. In the mean time, the topography induced low-level convergence triggers the release of accumulated CAPE, which finally results in the occurrence of midnight convective rainfall.