Impacts of urban land cover and anthropogenic heat (AH) on extreme local rainfall over the coastal PRD megacity region during boreal summer are investigated by conducting numerical experiments using the Weather Research and Forecasting (WRF) model coupled with a single-layer urban canopy model (SLUCM). To examine the relative importance of land cover change vs the presence of AH, three numerical experiments corresponding to different levels of urbanization in the PRD area were designed: one with cropland covering the whole region, one with urban land cover but zero AH, and one with urban land cover and a strong diurnal maximum of 300Wm-2 of AH in the model simulations. Results show that the increase of accumulated rainfall in the urban area is much more sensitive to the intensity of AH than the mere change of surface properties. Urbanization with intense AH can enhance both the intensity and frequency of extreme rainfall, which can be attributed to higher surface temperature (of about 3.5 to 4oC), higher convective available potential energy (CAPE), and lower convective inhibition (CIN), thus creating an environment more conducive to strong convection over the urban areas. Moreover, enhanced rainfall is supported by moisture supply from the South China Sea and increased water vapor flux convergence over the PRD city area. The amount of moisture flux converging over the coastal megacity area was found to depend on the direction of prevailing background wind.

This study compares the impacts of global warming and intense anthropogenic heat (AH) on extreme hourly precipitation over the Pearl River Delta (PRD) megacity, located in coastal South China. Using the cloud-resolving Weather Research and Forecasting (WRF) model coupled with the single-layer urban canopy model (SLUCM), three downscaling experiments were carried out: the first (second) having zero (300W/m2 as diurnal maximum) AH values prescribed over PRD urban grids, under the same current climate conditions. The third experiment with AH=300W/m2 under future projected climate representative concentration pathway (RCP) 8.5. Boundary conditions were derived from PRD extreme rainfall episodes, identified from the Geophysical Fluid Dynamics Laboratory Earth System Model (GFDL-ESM2M) historical and RCP8.5 runs. Global warming forcing leads to ~20 to more than 100% increase in the probability of hourly precipitation with the magnitude of 20-100mm/hr over urban locations. The enhancements from intense AH forcing were similar. However, two types of forcings have distinct signatures in modulating the thermodynamic environment. Warming due to AH is limited to the lowest 1km above ground, while global warming warms up the whole troposphere. Intense AH results in enhanced convective available potential energy (CAPE) and reduced convective inhibition (CIN) within the megacity, allowing convection to be triggered more easily and with more vigor. On the other hand, global warming enhances both CAPE and CIN, over both urban and rural areas. Our results highlight the different physical mechanisms of AH and global warming in exacerbating extreme urban rainfall, despite their having similar impacts on the rainfall intensity.