Numerical Simulation and Structural Analysis of a Squall Line That Occurred over the Beijing-Tianjin-Hebei Region of China on 31 July 2013

A high-resolution simulation of a squall line that occurred over the Beijing-Tianjin-Hebei region of China on 31 July 2013 was carried out using version 3. 5 of the WRF (Weather Research and Forecasting) model. The simulation reproduced many of the features of the squall line quite well, and thus facilitated further analysis of the structures and evolution of the squall line. The simulated vertical sounding of the squall line was consistent with similar studies performed by other researchers in the international science community;for instance, the near saturated sounding within the stratiform region, the "onion shaped" sounding within the wake low, and the mid-level low relative humidity sounding linked to the rear inflow behind the wake low. The study showed that the pressure field of the squall line appeared to be "low-high-low" in its mature stage. Namely meso-highs were located in the northern and middle parts and slightly lagged the convective region. A weak wake low existed at the back of the middle part of the stratiform region and there was a pre-squall low in the front of the southern part of the squall line. In its mature stage, a low-level inflow approached the squall line from the front, ascended in the convective region, and then split into three branches:a leading overturning updraft, an ascending front-to-rear updraft, and a descending front-to-rear downdraft. Besides, a strong rear-to-front inflow descended from the mid-level and diverged as it approached the ground. The characteristics of cloud-to-ground (CG) lightning activity in the squall line were also analyzed. The diagnosis revealed that the mixing ratio of ice-phase particles in the "charging zone" might be linked to the CG lightning. The results presented here are significant in revealing some of the mechanisms involved in squall lines occurring over North China, and lay the foundations for further studies on the development of such phenomena.