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 inthe Beijing area by using subdaily precipitation and wind observations. Acombined effect of topography and urbanization on the characteristics ofdiurnal variations was suggested. It was shown that stations located in theplain area exhibited typical night rain peaks, whereas those in themountainous area exhibited clear afternoon peaks of precipitation diurnalvariations. The precipitation peaks were associated with wind fields aroundthe Beijing area, which were found to be highly modulated by mountain-valleycirculation and urban-country circulation. The lower-tropospheric windexhibited a clear diurnal shift in its direction from north at 0800 LST tosouth at 2000 LST, which reflected mountain-valley circulation. Thetransitions from valley to mountain wind and the opposite generally happenedafter sunset and sunrise, respectively, and both occurred earlier for thestations located closer to mountains. By comparing the diurnal variations ofprecipitation at stations in a northeast suburb, an urban area, and asouthwest suburb, it was revealed that the northeast suburb group had thehighest normalized rainfall frequency, but the southwest group had thelowest from late afternoon to late evening. On the contrary, in the earlymorning from about 0200 to 1000 LST, the southwest group and urban group hadthe highest normalized rainfall frequency. This pattern might originate fromthe combined effects of mountain-valley topography and urbanization.