Numerical study of urban impacts on summer convective rainfall in Beijing

Three summer rainfall events in Beijing, which happened on 23 Jun, 26 Jul and 14 Aug 2011, were investigated using Weather Research and Forecasting (WRF) modeling system coupled with Noah land surface model and single layer urban canopy model (SLUCM). Numerical study and sensitivity test were conducted on each rainfall event using WRF/Noah/SLUCM modeling system and initial data assimilated with observations in order to examine the behavior of the modeling system and study the impacts of urban underlying surface in Beijing on localized convective rainfall. In addition, primary microphysics schemes, surface layer schemes and planetary boundary layer schemes in the modeling system were compared and the schemes producing better performances were chosen.

The results show that: The WRF/Noah/SLUCM modeling system has better behavior on simulating the chosen rainfall events when using microphysics schemes including cloud ice and graupel. The model results are quite close to observations. The urban underlying surface of Beijing can strengthen the convection cells over urban area and accelerate the movement of convection cells from suburb to urban through thermal effects. On the other hand, larger surface roughness in the city can block the approaching of convection cells and squall line into central urban area, extend the stay time of storms in up-wind urban area, and weaken the convection cells over urban underlying surface through dynamic effects. In these three localized convective rainfall events, the effects of Beijing urban underlying surface to rainfall systems result in concentrated precipitation to up-wind urban area, that is, increasing precipitation amount, strengthening precipitation intensity, and extending precipitation time of this area. Furthermore, precipitation over the central urban area was strengthened when the intensity of UHI (Urban Heat Island) was strong, and precipitation was weakened by dynamic effects of urban underlying surface when UHI was not distinct.