Impacts of urban expansion and future green planting on summer precipitation in the Beijing metropolitan area

[1] In this study, an analysis of long-term rainfall data reveals that the rapid urban expansion in Beijing since 1981 is statistically correlated to summer rainfall reduction in the northeast areas of Beijing from 1981 to 2005. This coincides with the period in which the shortage of water in the Beijing area has become a serious factor for sustainable economic development. Meanwhile, an analysis of the aerosol optical depth (AOD) from the Total Ozone Mapping Spectrometer spanning the years from 1980 to 2001 shows that there is no clear secular trend in summer AOD in Beijing. With the particular purpose of further understanding the effects of urban expansion on summer rainfall and the potential measures to mitigate such effects, a mesoscale weather/land-surface/urban–coupled model along with different urban land-use change scenarios are used to conduct numerical simulations for two selected heavy summer rainfall events with different, but representative, summer weather patterns in Beijing. Results show that urban expansion can produce less evaporation, higher surface temperatures, larger sensible heat fluxes, and a deeper boundary layer. This leads to less water vapor, more mixing of water vapor in the boundary layer, and hence less (more) convective available potential energy (convective inhibition energy). The combination of these factors induced by expanding urban surfaces is helpful in reducing precipitation for the Beijing area in general and, in particular, for the Miyun reservoir area (the major source for the local water supply). Increasing green vegetation coverage in the Beijing area would produce more rainfall, and model results show that planting grass seems more effective than planting trees. For the same vegetation, the rainfall difference from simulations using two green-planting layouts (annular and cuneiform) is small.