Effects of urbanization on hourly rainfall in relation to tropical cyclones- Inference from observations and convection-permitting model experiments over the South China Great Bay Area

How urbanization affects tropical cyclone (TC) rainfall is inferred from hourly station observations over the Great Bay Area (GBA) and convection-permitting atmospheric model experiments. Based on 41 TC cases from 2008 to 2017, our analysis reveals stronger TC extreme rainfall (defined as 99th percentile of hourly rainfall) over urban compared to rural stations. During TC passages, surface wind at GBA urban locations is noticeably weaker; at the same time the urban heat island (UHI) effect is suppressed ~10 hours before the peak of TC rainfall. Using the Weather Research and Forecasting (WRF) model at convection-permitting scale of 3km × 3km, coupled with a single-layer urban canopy module (SLUCM) and with ERA-Interim data as boundary conditions, downscaling was done for four different TCs affecting GBA. Three parallel experiments were conducted for each storm: In the first one, urban land use was replaced by cropland; in the second and third experiment, normal land use was adopted but with the anthropogenic heat (AH), respectively, set to zero and the extreme diurnal maximum of 300W/m2 for all urban grids. Hourly TC precipitation intensity and probability in all ranges (most obvious for hourly rainfall > 50mm) were increased over the GBA megacity in the second and third experiments with normal land use, compared to the first experiment, especially for slow moving TCs. This can be attributed to stronger friction over the urban area, leading to enhanced low-level moisture flux convergence. However, for a very large TC (namely TC Mangkhut), extreme AH (thus UHI effect) was found to also lead to higher heavy rainfall probability over the city area after the TC passing by GBA. Overall, urban landuse can intensify extreme TC rainfall over the coastal GBA megacity mainly due to frictional convergence of moisture. We speculate that these effects tend to be stronger for slower storms. For some cases, local convection induced by the UHI effect can still play a role since water vapor is abundant.