北京城市热岛效应对冬夏季降水的影响研究

The Effect of Urban Heat Island on Winter and Summer Precipitation in Beijing Region

利用北京地区20个气象观测站最近30年（1975-2004年）冬季（12～2月）、夏季（6～8月）平均气温、降水量和降水日数资料，研究了城市热岛效应的年代际变化及其对降水的影响。结果表明：（1）最近30年来，北京城区与北部山区之间的温度梯度在明显加大，其中，冬季温度梯度的平均增幅为0．6℃／10a，夏季约为0．2℃／10a。（2）在北京城区南北两侧，冬季和夏季的降水日数、降水量的相对变化趋势明显不同：相对区域平均而言，在城区及南部近郊区，冬季降水日数和降水量都在明显增加；夏季，城区北侧的降水日数呈加速增长趋势，尽管南部平原郊区的相对降水日数变化不大，但降水量在相对减少。（3）城市热岛效应对不同季节降水分布的影响，可能是城乡温度梯度与盛行风相互作用的结果，就北京地区而言，地形的存在，强化了城区与北部郊区之间的温度梯度：冬季盛行北风气流，在北部郊区，热岛效应强迫产生的边界层下沉运动有可能造成局地降水天气过程相对减少，城区及其南侧则相反；夏季盛行南风气流，随着城市热岛效应的增强，发生在北部近郊区的弱降水天气过程趋于增多。

The inter-decadal evolutions of urban heat island （UHI） and the seasonal precipitation distribution in Beijing area have been investigated by using winter （December - February） and summer （June - August） mean temperature, rain or snow days and precipitation data from the recent 30 years （1975 - 2004） at 20 meteorological stations, and finally, a possible mechanism how UHI effects the seasonal distribution of precipitation has been discussed by a simplified mesoscale dynamical principle. The results indicate that （1） the temperature gradient between Beijing urban areas and northern mountain region has been becoming stronger and stronger in the recent 30 years, specially in winter, its average rising speed is about 0. 6℃ per 10 years, and in summer, it is about 0. 2℃ per 10 years; （2） the relatively evolutional tendencies for the days and amount of precipitation in winter and summer are evidently different on the southern and northern sides of Beijing urban areas： Comparing with regional average, the days and amount of precipitation in urban areas and southern suburb are obviously increasing in winter, however, rain days in northern suburb are becoming more and more in summer, although rain days in southern suburb do not show evidently change, the amount of precipitation is obviously decreasing; （;3） a simplified mesoscale theory shows that the different effect of UHI on seasonal precipitation distribution is possibly related to the interaction between seasonal prevailing wind and the temperature gradient of urban areas and suburb. In Beijing region, topography makes the temperature gradient between urban areas and northern suburb strengthen, therefore, in winter when the prevailing flow is north wind, the descending motion in boundary layer forced by UHI is disadvantageous for local precipitation weather in northern suburb and the days and amount of precipitation decrease, and in urban areas and southern suburb, the situation is opposed to in northern. However, in summer when the prevailing flow is southern wind, the strengthening UHI is advantageous for occurring weak precipitation in northern suburb.