Spatiotemporal Evolution Characteristics and Causes of Short-Duration Heavy Rainfall in the Presummer Rainy Season over the Pearl River Delta Region Before and After the South China Sea Summer Monsoon Onset in the Past 20 Years

Compared to daily-scale heavy rainfall, short-duration heavy rainfall (≥20 mm h⁻¹) is responsible for mountainous region torrents, mudslides, and urban waterlogging. Using automatic meteorological stations and ERA5 reanalysis data, this study focuses on the temporal and spatial characteristics of short-duration heavy rainfall over the Pearl River Delta region before and after the onset of the South China Sea summer monsoon and explores the possible causes of the differences of short-duration heavy rainfall before and after the monsoon onset. Results show that (1) compared to that before the monsoon onset, the precipitation over the Pearl River Delta region after the monsoon onset increased, owing to the short-duration heavy rainfall frequently occurred. Regional mean intensity and extremes of the short-duration heavy rainfall were comparable during the stages before and after the monsoon onset, but the frequency of short-duration heavy rainfall increased by 70% after the monsoon onset. (2) Short-duration heavy rainfall mainly occurred in the northeastern and southwestern parts of the Pearl River Delta region. The frequency of short-duration heavy rainfall in the two abovementioned areas significantly increased after the monsoon onset, and exhibited a double peak (i.e., in the morning and afternoon) versus a single peak (i.e., in the afternoon) before the monsoon onset. (3) The occurrence of short-duration heavy rainfall varied over the Pearl River Delta region. The mean intensity and extremes of short-duration heavy rainfall after the monsoon onset were higher insouthwestern parts of the Pearl River Delta region than in inland areas. The frequency peaked in the afternoon before the monsoon onset while in the morning after the monsoon onset along the coast. However, the frequency consistently peaked in the afternoon in inland areas. (4) After the monsoon onset, the low-level moisture content during the short-duration heavy rainfall period increased by 16% compared to the climatology. Abundant water vapor was transported to the coast supported by the nocturnal acceleration of the monsoonal flow, confronting the land breeze, which enhanced the low-level convergence. (5) The frequency of short-duration heavy rainfall was closely related to the magnitude of the low-level water vapor flux after the monsoon onset. The increased short-duration heavy rainfall in the midnight and early morning in the coastal areas was associated with the dynamic forcing in the middle and low levels. However, the short-duration heavy rainfall in inland areas was attributed to thermal conditions and unstable atmosphere. These results help to understand the characteristics and formation mechanism of the short-duration heavy rainfall over the Pearl River Delta region.