Areal differences in diurnal variations in summer precipitation over Beijing metropolitan region

Using hourly rain-gauge measurements for the period 2004-2007, differences in diurnal variation in summer (June-August) precipitation are investigated in four distinct areas of Beijing: the urban area (UA), suburban area (SA), north mountainous area (NMA), and south mountainous area (SMA), which are distinguished empirically based on underlying surface conditions and verified with a statistical rotated empirical orthogonal function. The diurnal cycles and spatial patterns in seasonal mean precipitation amount, intensity, and frequency in the four areas are compared. Results show that the four areas have distinct diurnal variation patterns in precipitation amounts, with a single peak observed in UA and NMA in the late afternoon, which are 80 % and 121 % higher than their daily average, respectively, and two peaks in SA during the late afternoon and early morning with magnitudes exceeding the daily mean by 76 % and 29 %, respectively. There are also two peaks in SMA: a weaker nocturnal diurnal peak and an afternoon peak. The minimum amounts of rainfall observed in the forenoon in UA, SA, and SMA are 53 %, 47 %, and 57 % lower than the daily mean in each area, respectively, and that observed in the early morning in NMA is 50 % lower than the daily mean. The diurnal variations in precipitation intensities resemble those for precipitation amount in all four areas, but more intense precipitation is observed in SA (2.4 mm/h) than in UA (2.2 mm/h). The lowest frequency for the whole day is observed in UA, whereas the highest frequency occurs in the mountainous areas in the daytime, especially in the late afternoon in SMA. Diurnal variations in surface air temperature and divergence fields in the four areas are further investigated to interpret the physical mechanisms that underlie the spatial and temporal differences in summer diurnal precipitation, and the results indicate the possible dominance of the local circulation arising from mountain-valley wind and the differences in underlying surface heating between the urban, suburban, and mountainous areas of Beijing.