Analysis of boundary layer environment conditions during a warm-sector rainstorm in North China

A regional rainstorm occurred in North China from 12 to 13 August 2020. This process can be divided into two stages, warm-sector rainfall and frontal rainfall. In the warm-sector rainfall stage, there were three types of boundary layer conditions including the low-vortex shear, orographic uplift, and sea-land low-level jet. To understand the mechanism of this warm-sector rainstorm in North China, the evolution characteristics, circulation background, and the three different boundary layer conditions in the warm-sector rainfall stage during this rainstorm are analyzed, using multi-source data including the meteorological observation data, the infrared cloud image of FY-4A satellite, doppler weather radar data, ERA5 reanalysis data, and wind profile radar data. The results are as follows. (1) This rainstorm occurred in the southerly warm and humid airflow outside the subtropical high. The warm-sector rainstorm is caused by mesoscale convective systems under three different kinds of boundary layer conditions. (2) The three mesoscale rainfall centers are the spiral rain band in central and southern North China (region Ⅰ), the band-shaped rain band along the Taihang to Yanshan Mountains lying in the southwest-northeast direction (region Ⅱ), and the block-shaped rain band along the west coast of the Bohai Sea (region Ⅲ). (3) For region Ⅰ, the low-vortex shear system and wind convergence are the key causes triggering the heavy rainfall. The fierce precipitation zone was located in the right front of the moving direction of the vortex, lasting more than 16 hours, which caused the most intense precipitation. For region Ⅱ, the orographic lifting and convergence motions are the major causes of the heavy rain band. The intense rainfall occurred in the piedmont plain and lower areas of the Tainhang Mountain and lasted 6 hours. For region Ⅲ, the boundary layer jet combined with the convergence effects of the sea breeze front are the major causes of the heavy rainfall. It occurred in the convergence zone in front of the easterly jet, which lasted for a short duration, so the rainfall only lasted 4 hours.