Diurnal Variations of Summer Precipitation in the Beijing Area and the Possible Effect of Topography and Urbanization

The present study examined the diurnal variations of summer precipitation in the Beijing area by using subdaily precipitation and wind observations. A combined effect of topography and urbanization on the characteristics of diurnal variations was suggested. It was shown that stations located in the plain area exhibited typical night rain peaks, whereas those in the mountainous area exhibited clear afternoon peaks of precipitation diurnal variations. The precipitation peaks were associated with wind fields around the Beijing area, which were found to be highly modulated by mountain-valley circulation and urban-country circulation. The lower-tropospheric wind exhibited a clear diurnal shift in its direction from north at 0800 LST to south at 2000 LST, which reflected mountain-valley circulation. The transitions from valley to mountain wind and the opposite generally happened after sunset and sunrise, respectively, and both occurred earlier for the stations located closer to mountains. By comparing the diurnal variations of precipitation at stations in a northeast suburb, an urban area, and a southwest suburb, it was revealed that the northeast suburb group had the highest normalized rainfall frequency, but the southwest group had the lowest from late afternoon to late evening. On the contrary, in the early morning from about 0200 to 1000 LST, the southwest group and urban group had the highest normalized rainfall frequency. This pattern might originate from the combined effects of mountain-valley topography and urbanization.