Influences of Horizontal Convective Rolls and Complex Terrain on the Structure of Mesoscale Convective Systems With Multiple Parallel Rainbands

Abstract

This study investigates the formation mechanism of the multiple parallel rainbands’ (MPRB) organizational mode in a mesoscale convective system (MCS) that occurred on 12 August 2017, in the Beibu Gulf—a region with the highest frequency of MPRBs in China. The analysis is conducted using the Weather Research and Forecasting (WRF) model through 1-km high-resolution simulation and terrain modification experiments. The results indicated that the MPRB formed through backbuilding processes, primarily over horizontal convective rolls (HCRs), elevated by low mountains along the coastline. The results showed that mountains from 200 m in valleys to 600 m at peaks did not disrupt HCR formation. Instead, mountains of these heights helped elevate HCRs, thereby facilitating convection initiation. However, after increasing the mountain heights above valley heights by 50%, the higher terrain and larger distance between valleys and peaks decreased boundary layer wind and 0−1 km vertical wind shear, preventing HCR formation and thereby suppressing the development of the MPRB. In contrast, lowering the terrain height reduced the HCR height, thereby weakening or even eliminating MPRB even though it enhanced the organization of HCRs. HCR formation in this case was attributed to convective instability with high vertical wind shear and rich moisture. Coastal mountains functioned as barriers, blocking the flow of moisture from the southern sea, which was also essential for HCR formation and convection initiation.