Convection Initiation of Short-Term Heavy Rainfall Events in Shanghai under the Influence of the Western Pacific Subtropical High during the 2022 Extreme Heatwave

Under the influence of the western Pacific subtropical high (WPSH), downdrafts tend to suppress convection; however, local short-term heavy rainfall (STHR) events in coastal cities still occur frequently, and predicting the convective initiation (CI) of such processes remains a challenge. Based on ERA5 reanalysis data and the Automatic Weather Station (AWS) network, the meteorological conditions of STHR and non-rainfall (NR) events are compared and analyzed using hourly synthesis analysis methods. The findings are as follows: (1) Suppression by the WPSH results in stronger vertical upward motion during STHR events, with -0.04 Pa s-1 ≤ ω ≤ -0.03 Pa s-1, than during NR events, with -0.03 Pa s-1 ≤ ω ≤ -0.015 Pa s-1. (2) STHR events, with over 80 % AWSs ≥ 38 °C, exhibit quicker temperature rises, higher peak temperatures, and more extensive areas of heat coverage compared to NR events, with only 16.5 % AWSs ≥ 38 °C. (3) STHR events, with a convergence of the Q vector divergence of -0.4×10-11 hPa-1 s-3, compared to NR events, with a convergence of the Q vector divergence of -0.2×10-11 hPa-1 s-3, are also accompanied by more intense surface convergence and upward motion, conditions which reveal the significance of local dynamics in triggering such weather events. (4) A weaker suppressive effect of the WPSH, combined with significant meso-scale thermodynamic force near the ground, creates a multiscale resonance effect, which is a key condition for triggering local convection. (5) The lower temperature–dew point spread (T-T_d) at 2m and relative humidity below 850hPa for STHR events indicate that moisture conditions are sufficient, but not necessary, for triggering local convection. This research provides new insights into CI mechanisms under the influence of the WPSH for STHR events, offering important clues for future improvements in forecasting capabilities.