Urbanization Impacts on South China Greater Bay Area Extreme Rainfall—Sensitivity to Synoptic Systems during the Pre-Monsoon Period

Urbanization’s impact on pre-monsoon extreme rainfall in the Greater Bay Area (GBA), coastal South China (SC), and its relation to different synoptic systems remains understudied. This research investigates urbanization effects on pre-monsoon rainfall using hourly station observations and Weather Research and Forecasting model with the Single Layer Urban Canopy Model (WRF-SLUCM) simulations. Observations show stronger pre-monsoon extreme rainfall in GBA cities than surrounding rural areas, with the urban heat island (UHI) intensifying the urban rainfall intensity and probability. Extreme cases were classified into frontal and shear-line warm-sector types. Enhanced urban rainfall due to UHI was more pronounced under shear-line and warm-sector systems. Four frontal and four shear-line cases were dynamically downscaled using WRF-SLUCM, and four parallel experiments were conducted: “Nourban” (urban areas replaced by cropland), “AH0”, “AH100”, and “AH300” [normal land use, with the diurnal maximum anthropogenic heat (AH) set to 0, 100, and 300 W m−2 in SLUCM, respectively]. In frontal cases, significantly reduced urban rainfall in AH0 is due to decreased (enhanced) surface evaporation (wind divergence) in cities compared to cropland. Strong northerly winds and cold-air intrusion suppress the UHI in AH0 and AH100 during the rainfall process; enhanced urban rainfall occurs only in AH300. In contrast, for shear-line cases, urban friction and UHI promote local convection and wind convergence, increasing urban rainfall significantly in all urban experiments compared to Nourban. Overall, urbanization’s influence on SC’s pre-monsoon extreme rainfall is highly sensitive to the type of synoptic systems, necessitating further investigation of urban rainfall in this season.