边界层动力“抽吸泵”对青藏高原低涡的作用

Effect of Dynamic Pumping in the Boundary Layer on the Tibetan Plateau Vortices

考虑热带气旋类青藏高原低涡为受加热和摩擦强迫并满足热成风平衡的轴对称涡旋系统,通过求解线性化柱坐标系中涡旋模式的初值问题,分析了高原边界层动力“抽吸泵”对高原低涡结构及发展的作用,得出了特定热力、动力作用下高原低涡水平流场和垂直流场的典型结构特征,并讨论了低涡发展与边界层抽吸作用的关系。结果表明:高原上较强的边界层动力“抽吸泵”作用对高原低涡的流场结构及发展有重要影响。当边界层动力“抽吸泵”表现为“抽”的效应时,有利于边界层中对流活动的发展;反之表现为“吸”的效应时,有利于边界层以上的高原低涡的加强。

The plateau vortex in this study is one type of vortex generated over main part of the Tibetan Plateau. It is mainly on 500 hPa isobaric surface, its horizontal scale is from 400 to 500 km, its life circle is from one to three days, and temperature structure of most plateau vortex is warm. The vortex over the Tibetan Plateau is advantageous to a severe weather process. It usually induce rain storm or thunderstorm on large scale in the upper reaches of the Yangtze River （especially in Sichuan basin） even in the east of China, when it develops and moves eastward out of the Tibetan Plateau. The genesis and movement of the plateau vortex are intensely influenced by the environmental conditions of the Tibetan Plateau, for example, the structure of warming core of the plateau vortex is closely related to the powerful surface heating of the Tibetan Plateau. The surface sensible heating play an important role in the genesis of vortex, and the contribution of the latent heat of convective condensation is mainly in the intensification of vortex. The researches based on the atmospheric experiments on the Tibetan Plateau show that there is a powerful dynamic pumping （i. e. Ekman Pumping） in the deeper boundary layer of plateau atmosphere, and this pumping is very important to the formation of convective activities and embryonic vortices. Thus some of the vortices over the Tibetan Plateau in the initial stage are very similar to the vortices like TCLV （Tropical Cyclone-Like Vortices）. The vortices over the Tibetan Plateau are assumed to be axisyrnmetrical and thermal-wind balanced systems forced by diabatic heating and friction, and are solved as an initial-value problem of linearized vortex equation set in cylindrical coordinates, the role of dynamic pumping of the boundary layer in the structure and intensification of plateau vortices is analyzed, the horizontal and vertical flow fields of vortex are drawn, the relationship between growth of vortex and dynamic pumping of the boundary layer is discussed in detail. The results show that due to deeper boundary layer and stronger vertical motion than in plain, dynamic pumping of the boundary layer over the Tibetan Plateau is more powerful than in plain and plays an important role in the genesis and intensification of vortices. Moreover, dynamic pumping of the boundary layer is closely related to the distribution of horizontal divergence field of vortex over the Tibetan Plateau. The relationship between dynamic pumping of the boundary layer and convective activities or vortices over the Tibetan Plateau is as follows： when pumping is “up”, it is beneficial to the development of convective activities in the boundary layer of the Tibetan Plateau; when pumping is “down”, it is beneficial to the development of vortices above the boundary layer of the Tibetan Plateau.