Comparison between the Roles of Low-Level Jets in Two Heavy Rainfall Events over South China

Two heavy rainfall events occurred over the Pearl River Delta during 20–22 May 2020: the first was a warm-sector event and the second a frontal event. Based on ERA5 reanalysis data and observations from wind profilers and Doppler weather radars, the structures and roles of low-level jets (LLJs) during these two heavy rainfall events were analyzed. The results show that: (1) South China was affected by a low-level vortex and a low-level shear line during the two processes. The two heavy rainfall events were both associated with a synoptic-system-related low-level jet (SLLJ) and a boundary layer jet (BLJ). The coupling of the convergence at the exit of the BLJ and the divergence at the entrance of the SLLJ produced strong lifting for the warm-sector heavy rainfall, and the strong convergence between the LLJs and northerly winds as the cold front moved southwards was the main lifting reason for the frontal heavy rainfall. (2) The BLJ was the main transport of water vapor during the two processes. The coupling of the BLJ and SLLJ caused the water vapor convergence to be concentrated in the boundary layer during the first process, whereas the strong convergence between the LLJs and northerly winds led to the lower and middle troposphere having strong water vapor convergence during the second process. (3) During the period of these two heavy rainfall events, the lower and middle troposphere remained unstable. Further analysis shows that the differences in the intensity, location, and direction between the BLJ and SLLJ resulted in the pseudo-equivalent potential temperature advection in the boundary layer being significantly larger than in the lower and middle troposphere, which compensated for the energy loss caused by heavy rainfall and maintained the convective instability. These findings add to our knowledge on the roles of LLJs in the pre-summer rainfall over South China.