The Impact of Moist Physics on the Sensitive Area Identification for Heavy Rainfall Associated Weather Systems

Abstract

The impact of moist physics on the sensitive areas identified by conditional nonlinear optimal perturbation (CNOP) is examined based on four typical heavy rainfall cases in northern China through performing numerical experiments with and without moist physics. Results show that the CNOP with moist physics identifies sensitive areas corresponding to both the lower- (850–700 hPa) and upper-level (300–100 hPa) weather systems, while the CNOP without moist physics fails to capture the sensitive areas at lower levels. The reasons for the CNOP peaking at different levels can be explained in both algorithm and physics aspects. Firstly, the gradient of the cost function with respect to initial perturbations peaks at the upper level without moist physics which results in the upper-level peak of the CNOP, while it peaks at both the upper and lower levels with moist physics which results in both the upper- and lower-level peaks of the CNOP. Secondly, the upper-level sensitive area is associated with high baroclinicity, and these dynamic features can be captured by both CNOPs with and without moist physics. The lower-level sensitive area is associated with moist processes, and this thermodynamic feature can be captured only by the CNOP with moist physics. This result demonstrates the important contribution of the initial error of lower-level systems that are related to water vapor transportation to the forecast error of heavy rainfall associated weather systems, which could be an important reference for heavy rainfall observation targeting.

摘要

本文基于中国北方四个典型暴雨个例,通过包含湿物理过程的湿过程试验和不包含湿物理过程的干过程试验,研究了湿物理过程对条件非线性最优扰动(CNOP)寻找的暴雨天气系统敏感区的影响。结果显示,湿过程CNOP找到的敏感区对应低层(850-700 hPa)和高层(300-100 hPa)的天气系统,而干过程CNOP只找到对应高层天气系统的敏感区。垂直分布显示,干、湿过程CNOP的大值集中在不同层次的原因可以从算法和物理意义两方面解释。算法方面,干过程CNOP的目标函数相对于初始扰动的梯度大值区集中在高层,使得CNOP的大值集中在高层,而湿过程CNOP的梯度大值集中在高层和低层,使得CNOP的大值区集中在高层和低层。物理意义方面,高层敏感区对应斜压性大值区,能够被干、湿过程的CNOP捕捉到,而低层敏感区对应低层湿区,只能够被湿过程CNOP捕捉到。该研究显示了与水汽输送相关的低层天气系统的初始误差对暴雨天气系统预报误差的重要贡献,这对暴雨天气系统的目标观测有重要参考价值。