夏季青藏高原低涡结构的动力学研究

应用卫星云图资料分析了两例夏季青藏高原低涡发展过程及其结构演变,揭示出高原低涡结构特征的若干观测事实。在此基础上借鉴研究类热带气旋低涡的方法,将暖性青藏高原低涡视为受加热和摩擦强迫作用,且满足热成风平衡的轴对称涡旋系统,通过求解柱坐标系中的线性化涡旋模式,得出边界层动力作用下低涡的流函数解,重点讨论了地面热源强迫和边界层动力"抽吸泵"对高原低涡流场结构的作用。研究认为,由于边界层加热和摩擦的共同作用,高原低涡的温度场呈暖心结构。热源强迫的边界层低涡的散度场存在一个动力变性高度,该高度的位置与边界层顶高度有关。通过边界层动力抽吸作用,当边界层顶有气旋性涡度时,能引起边界层低涡的水平辐合运动和随高度增强的上升运动,并可加强低涡的切向流场;如果低涡的中心区域为"内冷外热"型加热分布,则热源强迫的低涡中心区域下层为辐散气流和随时间减弱的切向流场,上层为辐合气流和随时间增强的切向流场,并伴有下沉运动,从而有利于形成涡眼(或空心)结构,在卫星云图上表现为低涡中心为少云(或无云)区,即这类高原低涡具有与台风类似的眼结构,因而可视为类热带气旋涡旋的新例证。最后通过高原低涡的简化模型对低涡所含的波动进行了分析和讨论,结果表明:高原低涡中既含有涡旋Rossby波,又含有惯性重力波,即低涡波动呈现涡旋Rossby-惯性重力混合波特征。

Dynamics Study of Low Vortex Structure over Tibetan Plateau in Summer

By using the satellite cloud imagery, development and structure evolvement of two cases of low vortex over the Tibetan Plateau in summer are analyzed, and several observed facts of structure characters of plateau vortex are revealed. Based on this, and by using the method of TCLV （Tropical Cyclone-like Vortices） researching for reference, the vortex over the Tibetan Plateau is assumed to be axisymmetrical and thermal-wind balanced system forced by diabatic heating and friction, and by solving as an initial-value problem of linearized vortex equation set in cylindrical coordinate, the flow function solutions of the vortex in the boundary layer under motion are found, and mainly discuss the effect of surface heat forcing and dynamic pumping of the atmospheric boundary layer for the vortex structure. This research indicates that the temperature field of the vortex is same as warm-core structure caused by the effect of boundary layer heating and friction. There is a dynamical height in the divergence field of the heating forced boundary vortex, whose situation is related to the height of the top of boundary layer. While there are cyclonic vorticity in the top of boundary layer, it can cause horizontal convergence motion and height enhanced ascent motion of the vortex in boundary layer, and tangential flow field of vortex can be enhanced through the effect of dynamic pumping in the boundary layer. If the heating distribution of the vortex center is internal cold and external warm, the lower layer of the heating forced vortex center is divergence airflow and time decreased tangential flow field, and the higher layer is convergence airflow and time increased tangential flow field which accompany sinking motion. This is helpful to form vortex eye （or hollow-core） structure, which indicated in the satellite cloud imagery is partly cloudy （or cloudless） area in the vortex center. That is to say this plateau vortex has the same eye structure as the typhoon, therefore it can be considered as a new evidence of TCLV. Lastly, analysis and discussion for the wave in the plateau vortex is performed through the simplified model of the vortex, the results showed the plateau vortex not only contain vortex Rossby wave, but also contain inertial gravitational wave, i.e. the wave in the plateau vortex exhibits vortex Rossby-inertial gravitational mixture wave.