重庆雾的天气成因

在对重庆雾进行天气学分类基础上，利用合成方法分析了几次典型辐射雾和雨雾的环流形势、流场特征以及雾的垂直环流结构，揭示出两类雾的形成原因。分析表明，辐射雾产生在经向环流背景下，中亚地区高压脊为主要影响系统；经向垂直剖面表现为青藏高原东侧的对流层中层有一发展完好的顺时针旋转的垂直经向环流，其前部的下沉气流顺着地形坡地下滑时在四川盆地强迫出一个逆时针旋转的垂直环流，使重庆附近低层的下沉气流得到加强；夜间辐射降温明显，并有贴地逆温存在，大气层结较稳定。雨雾发生在亚洲地区经向环流相对较弱的背景下，青藏高原低压槽和地面冷锋为主要影响系统；垂直环流结构表现为高空低压槽发展东移时在四川盆地诱生一逆时针旋转的垂直环流，其东侧的上升支正好与地面弱冷空气入侵四川盆地时形成的顺时针旋转的垂直环流的上升支相耦合，使重庆附近上升气流加强，地面附近暖湿气流较活跃，明显增湿过程对雨雾形成起到重要作用。

Cause Analysis of Fog Formation in Chongqing

Based the classification of fogs in Chongqing, the circulation patterns, streamline field features and the vertical circulation structures during several typical radiation and water fog events were analyzed by using of a composite method. The formation causes of the two types of fogs were studied. Radiation fogs may form under the background of longitudinal circulation with the main influencing system being the high-pressure ridge over Middle Asia. In the longitudinal vertical circulation cross-section, there existed a well-developed and deasil vertical longitudinal circulation in the middle troposphere on the east side of the Tibet Plateau. The descending currents in the front forced an anticlockwises vertical circulation over the Sichuan basin along the topographical sloping fields, which strengthened the low-level downdraft around Chongqing. The nighttime radiation cooling was obvious and there occurred temperature inversion around the ground with the air stratification being stable. Water fogs may form under the condition of relatively weak longitudinal circulation over the Asia region. The Tibetan Plateau low-pressure trough and the surface cold front were the main influencing systems. The vertical circulation cross-section shows that the eastward moving upper low-pressure trough led to an anticlockwises vertical circulation. The updraft on the east side of the circulation was coupled with the updraft of the deasil longitudinal vertical circulation, which was produced by the weak surface cold air over the Sichuan basin. Thus the updraft around Chongqing was strengthened; surface warm and moist air was relatively active; and the obvious humidity increasing process played an important role in water fog forming.