Reinvestigation of potential deformation during a heavy rainfall event in North China

ABSTRACT

In this study, the physical meaning and generation mechanism of potential deformation (PD) are reinvestigated. A main trait of PD is that it contains deformation, which is an important factor to precipitation but not well applied in precipitation diagnosis. This paper shows PD shares similar features to deformation, but contains much more physical information than deformation. It can be understood as a type of deformation of a thermodynamic-coupled vector (u*, v*). For convenient application, squared PD (SPD) is used instead for analysis. By deriving the tendency equation of SPD, it is found that whether SPD is produced or reduced in the atmosphere is associated with the angle between the dilatation axes of PD and geostrophic PD. When the angle is less than $\frac{\pi }{2}$, SPD is generated. The diagnostic results during a heavy rainfall event in North China on 20 July 2016 show that the process of rapid increase in precipitation can be well revealed by SPD. The distribution of SPD becomes more organized and concentrated with increasing precipitation intensity. A diagnostic analysis of the SPD tendency equation shows that concentrated SPD is associated with the generation of SPD in the boundary layer followed by upward transport of the SPD. The concentration of SPD indicates a confluence of precipitation-favorable factors—namely, vertical wind shear and moist baroclinity, which can enhance vertical motions and thus cause an increase in precipitation. These diagnostic results further verify PD as a useful physical parameter for heavy precipitation diagnosis.

Graphical abstract

摘要

以往研究发现，变形场与降水关系密切，但这种相关性在降水诊断中的应用不多。位势变形将大气热力因子与变形场结合，将变形场更好地引入到暴雨诊断分析中。本文研究了位势变形的物理意义及其在暴雨过程中的产生机制。研究发现，位势变形具有与变形相似的特征，但包含了更多的物理信息，可看作一种热力耦合速度矢（u*, v*）的变形。进一步，推导了位势变形的平方（SPD）的倾向方程，发现大气中SPD的产生或减少与位势变形的伸展轴和地转位势变形的伸展轴的夹角有关，当夹角小于π/2时，SPD产生。 将位势变形应用于2016年7月20日一次华北暴雨的诊断分析中，发现SPD对暴雨过程中的降水快速增强有良好反映。随着降水增强，SPD分布更加集中、有组织性。对SPD倾向方程的诊断结果表明，SPD的集中和SPD在边界层产生并向上输送有关。SPD的集中是对降水有利的物理因子（如垂直风切变、湿斜压性）在降水区汇合的表现，这些物理因子有助于上升运动增强，从而引起降水加强。该研究进一步验证了位势变形在暴雨诊断分析中的有用性。

KEYWORDS:

• Deformation
• potential deformation
• heavy precipitation

关键词:

• 变形
• 位势变形
• 暴雨

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This study was supported by the Strategic Pilot Science and Technology Special Program of the Chinese Academy of Sciences [grant number XDA17010105], the National Key Research and Development Project [grant number 2018YFC1507104], the Scientific and Technological Developing Scheme of Jilin Province [grant number 20180201035SF], and the National Natural Science Foundation of China [grant numbers 41875056, 41775140, and 41575065].