北京市快速城市化对短时间尺度降水时空特征影响及成因

快速城市化对区域降水过程的影响已成为人类活动对不同时空尺度水循环影响研究的热点。基于北京全区2011-2015年20个自动气象站逐小时降水资料,利用Circular统计法等多种方法,在揭示北京市降水总体特征基础上,进一步从日、场次、小时等多种精细化时间尺度来探究北京市不同类型降水特征。研究表明：① 场次暴雨平均降水量、场次降雨持续时间及降水强度高值中心主要位于北京城区,与郊区降水过程相比,城区总体降水过程历时较长、雨量较大。城市雨岛效应可能是上述降水指标在城区具有高值中心的原因之一。② 北京全区降水日分布不均,不同区域降水类型有较大差异。北京市的暴雨雨型主要以午后型为主,占总雨型的47.53%。山区总体及北部近郊、远郊区、南部近郊分别以午后型、正午型、夜晚型降水为主。而城区的暴雨类型显示出主城区的东西两侧有较大的差异,西侧受山谷风环流及热岛环流影响类型较为复杂,东侧以傍晚型为主。③ 场次暴雨的降水峰值出现时间主要集中在12:00-19:00,且城区降水峰值发生时间较郊区降水峰值发生时间推迟。海拔的升高会使日降水峰值的出现时间的不确定性增大。④ 极端降水量指标及持续干燥、湿润指标均在城区显示出高值区,极端降水频率指标高值区位于城区下风向。城市化可能通过人口膨胀、土地利用类型变更等方式间接提高极端降水发生的风险。

Effects of urbanization on spatio-temporal distribution of precipitations in Beijing and its related causes

Due to rapid urbanization and relevant impacts on precipitation in both space and time, effects of urbanization processes on precipitation changes have been widely discussed in recent years. Based on hourly precipitation dataset from 20 stations covering the period of 2011-2015 across Beijing city, statistical methods such as circular statistical method were applied to examine spatio-temporal characteristics of precipitation with respect to precipitation types, extreme precipitation events and so on. The results indicated that: (1) regions in the central Beijing city are dominated by higher rainstorm precipitation amount, duration and precipitation intensity. Compared with rainstorm events in suburbs, precipitation processes in the central city are characterized by long duration and larger precipitation amount. Urban Rain Island effects could contribute to the above-mentioned rainstorm changes in the central city. (2) Uneven daily distribution of precipitation can be found across Beijing with remarkable regional difference. Rainstorm events mostly occurred after noon, accounting for 47.53% of the total precipitation type. Rainstorms in mountainous regions, northern and southern Beijing occurred mostly after noon, at noon and even at night. However, due to valley wind circulation and heat island circulation, precipitation patterns in central Beijing are relatively complicated and storms in western-central part of the city occurred mainly at nighttime. (3) Peak value of precipitation intensity of rainstorms occurred mainly during 12:00-19:00 and delayed occurrence of peak rainstorm values was observed in central city when compared to that in suburbs. Meanwhile, altitude can further induce uncertainty in occurrence time of peak precipitation amount. (4) The extreme precipitation index and the continuous drought as well as moisture index showed the high value in the urban area. The high value of the frequency of extreme precipitation index was found in the downwind direction of the urban area. Urbanization may indirectly increase the risk of extreme precipitation by population growth and land-use change.