Impact Of Water Distribution On The Atmospheric Environment In Urban Area

Water area, as one urban underlying surface type, is one of typical elements in the urban planning. The atmospheric characteristics in urban area are largely determined by the local underlying surface. The growth ratio of wetlands in urban area is propitious to the temperature reduction, humidity increase and change of local flow field, furthermore, which can improve the local atmospheric and micrometeorological environment. The existing observing and analysis focus on the climatic effect of the large lake and river rather than the microclimatic effect of water area in urban. Papers on the effect of urban water area distribution can hardly be found.The aims of this study are to find the relationship between the urban water area and local climate, carry a microclimatic observing experiment which pays close attention to the water area in Beijing, analyze temperature and humidity characteristic in water, commercial and traffic area respectively, analyze the thermal character of water body and its effect on surrounding area based on remote sensing technology. In this paper Urban Scale Model in the system is used to simulate the variability of surface air temperature, humidity, wind velocity, atmospheric diffusion conditions and their effect on urban atmospheric environment on condition of the dispersed or converged water distribution with the proportion of urban water area in urban area being 4%,8%, 12% and 16%, respectively. As a sample, simulate and analyze the difference between the urban atmospheric environments in Beijing before and after carrying out the water distribution. The results show as follows:(1) Water body contributes to the temperature reduction and humidity increase in its surrounding area. The monthly mean temperature of surrounding area of water body is 0.1～0.4℃lower than the traffic and commercial areas. The monthly relative humidity is 10% higher than other areas. Because of the high ratio of impervious ground and low heat capacity, the high temperature duration is longest and humidity is lowest for commercial area in urban.(2)The water area has cool effect in every season in urban area. The mean temperature of building area around water area within 400 meters distance is obviously lower than that of other areas. The difference is 1.2,0.6 and 0.4℃while the distance is 100,100～200 and 200～300 meters respectively. Meanwhile, the mean temperature of building area around greenland within 100 meters distance is 0.4℃lower than that of other areas, and no temperature difference beyond this distance.(3) The increase of water body in urban enhances the temperature reduction, humidity increase and mean wind speed raise to some extent, no matter with a dispersed or a converged water area distribution. It seemed that the impact of dispersed water area distribution has a more signature impact on local micrometeorological environment. While the variation of water area ratio from 4% to 16%, the urban daily mean temperature in summer decrease by 0.37℃, relative humidity increase by 7.26% and wind speed amplifying by 0.16m/s with a dispersed distribution. The dispersed water body distribution can relieve the high temperature in summer, weaken the heat island effect in urban. While the variation of water area ratio from 4% to 16%, the mean heat island intensity reduced by 25%, the strong heat island decreased by 24%. In another word, mean heat island intensity decrease by 0.2℃when the water area doubles. The increase of water body in urban improves the capability of air pollution dispersion, no matter how the water area distribute, but dispersed water body distribution works better. With a dispersed water body distribution, weak wind area in urban decrease by 50% while water area ratio varying from 4% to 8%.The urban dispersion capability increase by 11% while water area ratio varying from 4% to 16%。(4) Analysis of numerical simulation on Beijing wetland planning showed that the increase of urban water body area had an obvious effect of temperature decreasing and humidity and wind speed increasing at a range of 2 kilometers windward and 4 kilometers leeward. Single water body less than 0.25 km2 almost has noinfluence on the environment, while multiple, dense water body can obviously affect the microclimate.