| Summer heat waves caused by urban heat island has aroused widespread concern.Chongqing city is one of the regions with high incidence of heat waves.The particular location,dense population in such a small area and unique architectUral distribution make it a typical high-temperature area of the country.Based on WRF model coupled with BEP(Multilayer Urban Canopy Scheme)and BEM(Building Energy Model)scheme,a simulation was conducted to explore characteristics,causes of Chongqing urban heat island and impact of local circulation on it.Meanwhile a couple of controlled cases had been conducted to investigate the influence of air conditioner system(AC)and high reflective roof on urban air temperature.The results indicated that:(1)Model can well reproduce thermal environment characteristics of Chongqing.The correlation coefficient of 2m air temperature are more than 0.89,mean bias are lower than 1.5?,and air temperature and urban heat intense can be well reproduced.However model tend to overestimate daytime air temperature,which leads to a high uhi.This is partly contributed to single urban landuse category and inaccurate urban parameters.(2)The spatial distribution of air temperature are affected by topography and urban presence.When closer to the city,greater temperature is affected by the urbanization.Higher temperature in lower height.Daytime urban heat island is not obvious and flow field has clear vertical trait with a 1.5? uhi at 14:00.At night due to the prevailing southeast background wind,blocking effect of lofty mountains on the southeast side of city makes the air flow turn around or flow o-ver the mountain,which leads to a somewhat burning effect.The mountains on the west side of city block the outflow,leading to a promotion of urban heat island.Besides,due to the fact that Chongqing is in a valley,heat released from the city is hard to be taken away at night,showing a significant heat-holding effect.Since the weakening of turbulent activity at 20:00,there are the strongest heat island effect,and the simulated uhi is about 3 ?.(3)The air conditioning system will discharge excess heat indoor and heat generated by itself into urban atmosphere to strengthen the intensity of urban heat island.During the day,whether AC is operating has little impact on urban air temperature,the city temperature will increase above 0.6?,in some areas due to turbulent horizontal mixing,temperature difference reaches about 2?.And because of high vertical mixing capacity,temperature difference between the two cases can continue to a high altitude.At night,in densely populated areas,there is a maximum temperature increase of about 0.6 ? at 22:00 and 23:00,and the maximum impact value will gradually decrease with time.Changing the target temperature of AC system will change indoor-outdoor exchange energy.When the indoor temperature rises from 25? to 27 ?,total energy released from AC system was decreased by 12.66%,however the temperature drops little and max temperature deviation varied by site location and surrounding environment.(4)Increasing the roof albedo of buildings leads to an increase of upward shortwave radiation,which reduces the net radiation received by the city and lowers temperature.When the roof albedo is set as 0.8,simulated peak temperature reduced by an average of 0.5?.In some areas,heat is accumulated due to the influence of the topography,and temperature can drop by as much as 2?.There is no shortwave reflection at night,but owing to weakening of the heat island during daytime,2m temperature can be reduced by an average of 0.05 oC.Different roof albedo settings feedback linearly on surface radiation energy,latent heat and 2m temperature. |
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