一次闽南暖区暴雨的多尺度动力过程

DYNAMICAL PROCESSES UNDERLYING A WARM-SECTOR RAINSTORM IN SOUTHERN FUJIAN

关键词:  暖区暴雨  正则传输  正压失稳  斜压失稳

Key words:  warm-sector rainstorm  canonical transfer  barotropic instability  baroclinic instability

摘要:

作为气象研究中的一个难点问题，暖区暴雨的动力学一直为学界所关注。基于多尺度子空间变换（MWT）以及基于MWT的局地多尺度能量学分析和正则传输理论，对2018年5月7日的一次闽南暖区暴雨进行研究以了解其多尺度动力过程。首先将原始物理量场重建到三个尺度子空间：背景流子空间、天气尺度子空间和暴雨子空间。重构场上可以很好地看出背景环流尺度的高低空急流，以及暴雨尺度上的垂直环流。以往的研究普遍认为暖区暴雨的动力过程具有弱斜压性这一特征，而就此次事件而言，正压失稳和斜压失稳都起着很关键的作用，暴雨主要落区内既发生了正压失稳，也发生了斜压失稳。研究表明，对流层不同高度上的动力学存在差异，低层主要表现为正压失稳，天气尺度子空间与背景流子空间向暴雨子空间传输的动能相当；中层主要是混合失稳，除正压失稳外，斜压正则传输也将有效位能从背景流子空间传输到了暴雨子空间，再通过浮力转换将有效位能转为动能，从而维持暴雨在中层的动力过程；高层则与低层相似，但只存在背景流子空间向暴雨子空间的能量传输。

Abstract:

As a key problem in meteorological research, the dynamical processes of warm-sector rainstorms have been of continuing interest to scholars. Using the functional analysis apparatus of multiscale window transform (MWT), and the MWT-based localized energetics analysis and theory of canonical transfer, we have analyzed the warm-sector torrential rain on May 7, 2018 in southern Fujian Province for an understanding of its underlying dynamical processes. First, the atmospheric fields from the ERA5 data are reconstructed onto three scale windows, namely, the background window, the synoptic scale window, and the rainstorm window. From the reconstructed fields the upper-layer and lower-layer jets are well represented on the background window, and the vertical motion is clearly seen on the rainstorm window. Previously it has been believed that the warm-sector rainstorms are characterized by weak baroclinicity, but here baroclinic instability matters as well as barotropic instability. It is found that the dynamical processes differ with height. In the lower layer, barotropic instability dominates, and the barotropic canonical transfer directs to the rainstorm window from both the background and synoptic scale windows. In the middle layer, the instability is mixed; apart from barotropic instability, the baroclinic canonical transfer also brings available potential energy from the background window to the rainstorm window, which is then converted into kinetic energy, maintaining the rainstorm-scale motion in the middle layer. The same as the lower layer, the upper layer sees only the canonical energy transfer from the background window to the rainstorm window.