Abstract

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 ... Show moreUsually 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 °C (1.4 °C) 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. Show less