Mega-city development impact on hourly extreme rainfall over the South China Greater Bay Area under near-future climate warming

The impacts of near-future urban development and global warming forcing on hourly extreme rainfall over the South China Greater Bay Area (GBA) area have been investigated, by dynami-cally downscaling General Circulation Model (GCM) outputs using the Weather Research and Forecasting Model (WRF) at convection-permitting resolution, coupled with an Urban Canopy Model (UCM). Three downscaling experiments corresponding to different urban land cover (1999 and projected 2030) and climate (1951-to-2000 and 2001-to-2050 GCM simulations) were designed. Effects of near-future climate change and 1999-to-2030 urban development on GBA extreme precipitation were then examined, using boundary conditions derived from GBA extreme rainfall events in the Geophysical Fluid Dynamics Laboratory Earth System Model (GFDL-ESM2M) historical and RCP8.5 simulations. Results show that climate change and rapid urban development forcing have comparable positive effects on the intensity as well as heavy hourly rainfall probability over the GBA urban area. Global warming tends to increase heavy rainfall probability (from 40 to 60 mm/h) by about 1.3 to 1.8 times, but at the same time suppress the probability of light rainfall (from 1 to 10 mm/h) by about 20%. Urban development increases urban rainfall probability within the whole range of intensity, with frequency for very heavy rainfall (> 90 mm/h) almost doubled. It is worth mentioning that impacts due to rapid urban development can be as important as global warming forcing in the near future in exacerbating hourly extreme rainfall over the GBA coastal megacity.