北京城市化对夏季大气边界层结构及降水的月平均影响

Month-averaged impacts of urbanization on atmospheric boundary layer structure and precipitation in summer in Beijing area

为克服针对一次或几次天气过程研究城市化对边界层结构及降水影响的局限性,尝试研究北京城市化对夏季大气边界层结构及降水的月平均影响,本文首先总结了2006年8月份的主要天气过程,分析了气象站观测的10 m高度风速、2 m高度气温、2 m高度比湿和24 h降水的月平均分布特征,然后利用WRF/Noah/UCM模拟系统,进行了该月30个个例的高分辨率数值模拟及检验分析,并通过多组不同城市化情景的敏感性试验对比分析了城市化对夏季大气边界层结构及降水的月平均影响.研究表明:本文所用对高分辨率数值模拟结果进行月平均的方法可以较合理地模拟出城市化对大气边界层结构及降水的影响,并再现观测到的各站风频差异.8月份,北京城市化对气温的影响高度白天约为800 m,近地面气温升高1℃以上;夜间约为200 m,对近地面气温的影响达到最大(1.4℃以上).白天,城市化使城市及下风向的一些区域风速略有减小;夜间,城市及周边区域200 m以下风速明显减小,且在100 m左右高度处风速减小最明显,减小达0.8 m/s以上.城市化白天使700 m以下比湿减小,近地面处减小达1.2g/kg以上,夜间使近地面空气比湿略有减小.城市化对城市区域平均降水量的影响随城市发展的不同阶段而不同.初步模拟分析表明,北京城市化已使上风向区域以及城区三环以内降水量减少,海淀和昌平降水明显增加.

Usually the studies on the impact of urbanization on boundary layer structure and precipitation are based on one or several synoptic cases. In order to overcome the limitations of case-based study, this paper focuses on the month-averaged impacts of urbanization. Firstly the synoptic analysis of August 2006 is carried out, and the month-averaged distribution characteristics of wind speed at 10 m height, air temperature and humidity at 2 m height and 24-hour precipitation collected in situ are analyzed. Secondly the high-resolution numerical simulations are conducted for 30 cases by using WRF/Noah/UCM modeling system, and the model outputs are evaluated against the observation. Thirdly the sensitivity tests of different urbanization scenarios are carried out. Results show that, the method of month-averaging of high-resolution numerical simulation results can realistically capture the urbanization impacts on atmospheric boundary layer structure and precipitation and the differences of wind-rose between stations. In August, the impact of urbanization in Beijing on air temperature can reach 800 m （200 m） height and more than 1 ℃ （1.4 ℃） in daytime （nighttime）. In daytime the urbanization slightly decreases wind speed in and downwind urban area, while in nighttime the wind speed below 200 m in and around urban area obviously decreases （the reducing can reach 0.8 m/s）. Urbanization decreases specific humidity below 700 m in daytime （the reducing near surface can reach 1.2 g/kg）, while slightly decreases the near surface specific humidity at nighttime. The impact of urbanization on the mean rainfall in urban area varies with the developing stage of urbanization. The numerical simulation shows that current urbanization in Beijing decreases the rainfall upwind urban area and within the third circle, while markedly increases the rainfall in Haidian and Changping.