Areal Differences of Rainfall Variability in the Beijing Metropolitan Region: Potential Signs of the Impact of Air Pollution and Urbanization

This study investigates the differences of decadal rainfall changes in three distinct areas of Beijing: the urban area, suburban area, and mountain area, from daily precipitation dataset of 20 meteorological stations for the period 1978-2014. By using moving t-test and Lepage test, a turning point in the long-term trend of rainfall variation based on rainfall data at all stations around Beijing is identified around 2000, contributed chiefly by extreme rainfall in summer. Prior to 2000, the trends are indistinguishable among the stations. After 2000, urban Beijing experienced a much more significant rapid increase in extreme rainfall events than the surrounding suburban and mountain regions. The similar differences are noted for cloud amount, in agreement with changes in precipitation. It is our hypothesis that the differences in the trend of summer extreme rainfall are caused by enhanced urban heat island due to both the expansion of urban area and the reduction of air pollution. An indirect evidence is higher air temperature in urban area that may fuel the convergence of air to intensify updrafts over there, leading to the deep convection and heavy precipitation.