Rainfall response to urban expansion in Beijing and its local climate drivers

Urban areas can substantially modify local hydroclimate, enhancing precipitation over and downwind of cities. Yet, the urban expansion effects on rainfall remain insufficiently understood, and a quantitative relationship between urban growth and rainfall intensification remains to be established. Using the WRF model with eight urban size scenarios for Beijing, a numerical modeling framework was implemented to investigate how changes in urban extent influence rainfall during two representative summers, one relatively wet and one relatively dry. Results show that the rainfall response exhibits an approximately linear dependence on the degree of urban expansion, with the largest impacts occurring over the city center and downwind regions. In general, rainfall increases with urban area enlargement, particularly during nighttime in relatively wet summers due to higher humidity and a more pronounced urban heat island effect. In relatively dry summers, limited moisture supply leads to smaller changes in total rainfall. Changes in hourly rainfall intensity demonstrate a contrasting pattern. Heavy rainfall intensities further intensify in response to urban expansion, while light rainfall is suppressed or remains largely unchanged. Daytime and nighttime rainfall intensity respond to urban expansion in opposite ways, with daytime intensity generally weakening and nighttime intensity strengthening as Beijing expands. These contrasting diurnal behaviors ultimately lead to a reduction in rainfall intensity during relatively dry summers and a slight increase during relatively wet summers. Overall, the results highlight the dependence of urban rainfall modification on city size and background climatic conditions.