Urbanization Impacts on Tropical Cyclone Rainfall Extremes-Inferences From Observations and Convection-Permitting Model Experiments Over South China

How urbanization affects tropical cyclone (TC) rainfall is inferred from station observations over the Great Bay Area (GBA, located on the South China coast) and numerical model experiments. Observations from 41 TCs indicate that surface wind is noticeably weaker in urban compared to rural stations during TC passages, while the urban heat island effect is considerably suppressed. Extreme (99th percentile of) hourly rainfall for these TC events during 2008-2017 is more intense over urban compared to rural stations. For eight selected TC cases, dynamical downscaling was carried out using the convection-resolving Weather Research and Forecasting model, each with three parallel experiments: "Nourban" in which the urban area was replaced by cropland; "AH0" ("AH300") in which the diurnal maximum anthropogenic heat (AH) was set to 0 (300 W/m(2)) in city locations. Both AH0 and AH300 show a significant increase in urban hourly rainfall intensity and probability in all ranges (most obvious for heavy rainfall >40 mm/hr), over the GBA mega-city. Further diagnosis indicates increased moisture flux convergence in urban areas over a 2-day period during the landfall, contributes to increased rainfall, likely induced by the surface roughness. The influence of AH, however, is found to be insignificant. The increase in accumulated rainfall due to urbanization is proportional to the storm residence time over the city, implying greater rainfall exacerbation for slower or larger TCs. Overall, urbanization intensifies extreme TC rainfall over the coastal GBA mega-city mainly due to surface frictional convergence, with stronger intensification for storms residing longer over the city.