| Rapid urban expansion and increasing urban populations have caused remarkable modifications to the local underlying land use type and regional environment. The change in meteorological variables in urban areas may have serious influences on human health, especially in extreme weather events. This paper focused on the development of a next generation mesoscale model, WRF, coupled with urban canopy parameterization scheme. The aim was to quantify the effects of urbanization on local thermal environment over Nanjing, China, during a heat wave. Six scenarios (UCM, BEP, BEM, BEM(noAH), BEM(target_T), and BEM(1993)) were integrated from 26 July to 4 August 2010.(1)The first three cases with different urban canopy scheme were designed to compare the simulation performance of WRF model coupled with urban canopy schemes. The mean bias between observation and simulation of UCM, BEP and BEM scheme were -0.54℃,-0.72℃,0.27℃ respectively, and the RMSE were 1.37℃,1.55℃,1.27℃, respectively. The radiation balance of the simulation is close to practical energy balance of city, for example, the RMSE of downward long-wave radiation were 9.0W·m-2,11.0 W·m-2,7.3 W·m-2.(2) In the day, whether or not the ACs are working does not matter much to the urban atmospheric temperature, but in the night, it will rise by 0.6℃ or above when the ACs are working, especially in the densely populated residential quarters, and by nearly 2℃ at most between 22 O’clock and 23 O’clock. Changing the target temperature will result in a change of the total amount of the indoor-outdoor heat exchange. Once the interior temperature is changed from 25℃ to 27℃, the total heat emission toward outside will decrease by 12.66% and the amount of the energy saved is 214567.9. As for the deviation of the outdoor atmospheric temperature, it remains stable between 8 O’clock and 11 O’clock in the morning and 20 O’clock at night and 8 O’clock next morning and the temperature drops the most between 13 O’clock and 16 O’clock in the afternoon, reaching an average of 1℃. What’s more, such a deviation is subjected to the surrounding environment of the stations, i.e., the more close-packed with buildings the area is, the bigger the deviation is; The fewer buildings there are in the area or closer to some open places like Yangtze river, a park and an airport so on, the smaller the deviation is.(3)The simulation results show that 2m temperature in urban region increases from 1993 to 2010, and the addition of temperature can reach 3K at 24:00. Results show that urbanization weakens the wind and enhances the pollutant trapping and therefore contributes to the overall poor air quality in the region. Urban heat island leads to the convergence of airflow in the lower layer of atmosphere, With more urbanized region, stronger turbulent kinetic energy(TKE), for example the maximum of TKE at noon increased from 2.1 m2·s-2 to 2.7 m2·s-2. Urban sprawl enhances vertical mixing and hinders horizontal downward dispersion which caused pollutants to accumulate in urban areas. |
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