Potential deformation and its application to the diagnosis of heavy precipitation in mesoscale convective systems

Abstract

Many observational studies have shown that deformation, like vertical vorticity and divergence, is closely related to the occurrence and distribution of strong precipitation. In this paper, to involve deformation in precipitation diagnosis, a new parameter called potential deformation (PD) is derived and then applied to precipitation detection within a simulated mesoscale convective system (MCS). It is shown that PD includes both stretching deformation and shearing deformation and shares similar characteristics with deformation insofar as it does not change with the rotating coordinate. Diagnosis of the simulated MCS reveals that PD performs well in tracing the MCS’ precipitation. In terms of their distributional pattern, the large-value areas of PD are similar to the precipitation in the different development stages of the MCS. A detailed analysis of the physical processes contained within the PD shows that it can reflect the three-dimensional moisture variation, vertical wind shear and wind deformation within the MCS. These structures are usually a comprehensive reflection of the characteristics of the surface cold pool, rear inflow jet, downward cold air flow and upward warm moist flow within the precipitating convective cells. For this reason, the PD shows much stronger anomalies in the precipitating atmosphere than the non-precipitating atmosphere, which implies considerable potential for its application in detecting heavy precipitation within MCSs.

摘要

许多研究表明, 变形和涡度, 散度一样与强降水的发生发展及分布密切相关. 为了将变形引入到降水的诊断中, 本文提出了一个新的参数—位势形变. 位势形变包含位势伸缩形变和位势切变形变, 且可证明其和形变一样不随坐标的旋转而改变. 将位势形变应用到一次MCS降水过程的诊断分析发现, 位势形变对MCS降水有良好指示意义, 在MCS降水发展的不同阶段, 位势形变的大值区均随着降水的移动而移动. 对包含在位势形变中的物理过程的分析发现位势形变包含三维水汽梯度, 垂直风切变和变形, 这些物理信息能够综合反映MCS中降水单体内的冷池, 尾部入流, 下沉冷气流与上升暖气流的对峙等结构. 因此, 位势形变能够显著区分降水大气与非降水大气, 这体现了位势形变在MCS降水追踪方面的潜力.