Numerical simulation of urban land surface effects on summer convective rainfall under different UHI intensity in Beijing

To investigate the effect of urban land surface on rainfall under different urban heat island intensity (UHII) conditions in Beijing, numerical simulation and sensitivity experiments on two individual summer convective rainfall events are performed, using the Weather Research and Forecasting/Noah/urban canopy model (UCM) model system. It is found that the cloud ice and cloud graupel formation processes in the microphysics parameterization scheme play a pivotal role in increasing the model's ability to simulate convective rainfall. When UHI is weak prior to the start of rainfall, urban land surface primarily affects rainfall through its dynamic action. Rainfall systems develop a tendency to bifurcate in the windward periphery of the urban area and move around it along both sides. Consequently, precipitation in the central urban area and its downstream area decreases, whereas precipitation in the suburban areas at the sides of the periphery of the urban area increases. When UHI is strong before rainfall begins, the thermal effect of the urban land surface is the main factor that affects rainfall. The lower atmosphere over urban area is more unstable, and horizontal convergence is enhanced, increasing the intensity of the convective system after it moves to the urban area. As a result, precipitation increases in the urban area. This study demonstrates that UHII can be used as an important factor for distinguishing the effect of urban land surface on rainfall.