Urbanization Effects on Rainfall Processes Induced by Landfalling Typhoon Lekima (2019) over the Shanghai Metropolitan Area

Urbanization effects on rainfall induced by landfalling tropical cyclones have rarely been studied. Here high-resolution numerical simulations with the Weather Research and Forecasting/Noah/single-layer urban canopy model system (WRF/SLUCM) are conducted to investigate impacts of urban land cover and building heights on heavy rainfall induced by landfalling Typhoon Lekima (2019) over the megacity Shanghai. The default single urban category in WRF was updated to a new land cover data with three urban categories. Results indicate that WRF/SLUCM captures the typhoon intensity, track, and total rainfall amount quite well. Urbanization has a small positive effect on rainfall amount for this event. However, urbanization has a significant impact on the spatial distribution of the accumulated rainfall with enhancement not confined over the urban area but mainly to the southwest of Shanghai possibly due to the changes of the typhoon tracks. With the impact of Typhoon Lekima, the urban heat island disappears, indicating that the thermal effect of urbanization has limited influence on the rainfall processes. The model performance is very sensitive to the building height. More realistic building height values can noticeably improve simulations of the diurnal patterns of rainfall, urban heat island, and the urban wind speed stilling effect. With the rising of building heights, the surface frictional dynamic effect and vertical uplift is enhanced, but seems not enough to evidently intensify the rainfall. The simulated lower-level large moisture flux convergence corresponds well to rainfall peaks. This study has important scientific significance for the accuracy of rainfall forecasts of landfalling typhoons and disaster mitigation in cities.