一次自上向下发展的高原涡的多尺度动力学分析

高原涡作为经常给我国带来暴雨等灾害的天气系统,其形成一般认为是通过感热和潜热自下而上激发的,然而,2013年5月下旬发生的一次引发其下游灾害性强降水的高原涡却是由对流层高层天气尺度低涡诱发的。为此,基于新发展的多尺度子空间变换和多尺度能量涡度方法以及ERA5再分析资料对其动力学过程进行了详尽的探讨,先将原始场重构到三个尺度子空间,即背景环流尺度子空间、天气尺度子空间和高频尺度子空间,重构场上首次显示此次过程生成于青藏高原西北侧,其成因为对流层高层基本气流尺度向天气尺度的跨尺度动能正则传输,即正压失稳,并且表现为从高层向下。在发展阶段,其能量最终来源为基本气流向天气尺度的有效位能传输和非绝热加热,然而这些过程只发生于涡旋低层的西侧。进一步分析发现,天气尺度内存在一个能量再分配“路径”:首先,低层西侧获得的有效位能转换为动能,西侧垂直的气压梯度力做功将低层获得动能向高层分配;在高层,水平的气压梯度力做功进而将西侧获得的动能向东侧分配;东侧垂直的气压梯度力做功再将动能向低层分配;至此,低层西侧获得的能量被分配到整个涡旋空间中,使得涡旋能够均匀发展。

On the Multiscale Dynamics of A Top-Down Developing Vortex over Tibet Plateau

Vortices over Tibet Plateau often bring heavy rain and other disasters to China.Generally,they are believed to be generated from bottom to top through sensible heating and latent heating.In this study,however,we find that the May 2013 plateau vortex,which has caused severe precipitation downstream,is actually induced by the synoptic-scale eddies in the upper layer of the troposphere.Using a recently developed functional analysis tools,i.e.the multiscale window transform(MWT),and the MWT-based localized multiscale energy and vorticity analysis(MS-EVA),we reconstruct the ERA5 reanalysis fields into three subspaces of scale,or scale windows,i.e.,the basic flow scale window,synoptic scale(plateau vortex scale)window and high frequency window.By the reconstructions,it is shown for the first time that the vortex is generated on the northwestern side of the Plateau,and the generation is through the barotropic instability of the basic flow,i.e.,the canonical transfer of kinetic energy from the basic flow window to the vortex scale window,which develops downward from top to bottom.Moreover,the ultimate source of energy for the vortex development is the baroclinic transfer from the basic flow,rather than the non-adiabatic heating as commonly believed.Further analysis shows that there exists a“path”of energy redistribution on the vortex scale window:In the lower layer,the vortex receives available potential energy on the western side,which is then converted into kinetic energy,and transported via vertical pressure work into the upper part.In the upper layer,he kinetic energy on the western side is transported horizontally(via horizontal pressure work)to the east,while the vertical pressure work there carries the gained kinetic energy downward to the lower layer.In doing this the energy received in the lower-western part of the vortex is redistributed throughout,making it possible for an unusually long-duration vortex to develop.