LAUSR

A statistical analysis of the spatiotemporal characteristics of hourly heavy rainfall (HHR) events and rainstorm days, defined as greater than 20 mm h−1 and 50 mm day−1, respectively, is performed using observations at 5-min intervals from 155 automated weather stations (AWSs) over the Beijing metropolitan region (BMR) during the warm seasons of 2007–2014. Results show pronounced variability in the frequencies and rainfall amounts of both HHR events and rainstorm days across the BMR of less than 150 km width from the west to east. That is, higher- (lower) frequency HHR events with more (less) accumulated rainfall amounts take place in eastern Haidian and over the BMR's northeast mountains (west and northwest mountains). Many extreme rainfall and even record-breaking events, in terms of the frequency, duration, rainfall amount and intensity, are found to occur in the regions of high-frequency HHR events and rainstorm days. The frequency of rainstorm days with HHR events accounts for more than 50% of that of total rainstorm days over the BMR's plains and near the northeast mountains. Results also show that HHR events occur most frequently in late July, and typically peak during 1600–2000 LST. A comparison of the averaged surface meteorological variables and upper-air sounding at 0800 LST between HHR and non-HHR days reveals that the occurrences of the HHR events coincide with the distribution of surface warmer and more humid air as well as organized convergence in the presence of an unstable environment with surface south- to southeasterly winds, low-level south to southwesterly flows and a deep layer of higher moisture content. These results suggest likely the positive influences of urban environment and mountain-plain circulations on the generation of the HHR events, given favourable larger-scale conditions.