Urbanization-induced urban heat island and aerosol effects on climate extremes in the Yangtze River Delta region of China
Urbanization-induced urban heat island and aerosol effects on climate extremes in the Yangtze River Delta region of China
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DOI：
10.5194/acp-17-5439-2017
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发表时间：
2016-12
影响因子：
6.3
通讯作者：
Shi Zhong;Y. Qian;Chun Zhao;R. Leung;Hailong Wang;Ben Yang;Jiwen Fan;Huiping Yan;Xiuqun Yang-Xi
中科院分区：
文献类型：
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作者：
Shi Zhong;Y. Qian;Chun Zhao;R. Leung;Hailong Wang;Ben Yang;Jiwen Fan;Huiping Yan;Xiuqun Yang-Xi
抽象。利用WRF-Chem模式和一个单层城市冠层模式（UCM），在对流允许的尺度下对长江三角洲地区进行了5 a的积分，研究了城市化引起的土地覆盖和污染物排放变化对区域气候的单独和联合影响。考虑城市化效应的模拟较好地再现了长江三角洲地区的气温和降水特征。长江三角洲的城市化导致了城市热岛效应，夏季使地表温度升高0.53 °C，长江三角洲主要特大城市的年热浪日数以3.7 d/年的速度增加，并伴随着热应力的增强。在冬季，城市商业区的近地面气温上升约0.7 °C，但周边地区则下降。气溶胶的辐射效应通过减少地表的净短波辐射来冷却地表空气。与城市热岛效应相比，气溶胶对太阳辐射和温度的影响范围更大，尤其是在长三角城市群的下风向。研究还表明，城市热岛通过加强午后城市上空的辐合和上升气流，增加了夏季极端降水的发生频率，有利于深对流的发展。相反，气溶胶的辐射强迫导致地表冷却和高层大气加热，这增强了大气稳定性并抑制了对流。城市热岛和气溶胶对降水的共同影响取决于天气条件。进一步分析了两种典型但不同天气形势下的两次降水过程。结果表明，城市土地覆盖和气溶胶对降水的影响不仅取决于它们对局地辐合的影响，而且还受大尺度天气系统的调制。当天气强迫较强，风场较强，空间辐合较大时，热岛效应和气溶胶效应相对较弱。当天气强迫较弱时，热岛效应和气溶胶对局地辐合的影响占主导地位。这表明天气强迫在调节城市化引起的土地覆盖和气溶胶对单个降雨事件的影响中起着重要作用。因此，由于城市化的影响，降水变化可能会相互抵消，在不同的天气条件下，导致在较长的时间尺度上的平均降水变化不大。
Abstract. The WRF-Chem model coupled with a single-layer urban canopy model (UCM) is integrated for 5 years at convection-permitting scale to investigate the individual and combined impacts of urbanization-induced changes in land cover and pollutant emissions on regional climate in the Yangtze River Delta (YRD) region in eastern China. Simulations with the urbanization effects reasonably reproduced the observed features of temperature and precipitation in the YRD region. Urbanization over the YRD induces an urban heat island (UHI) effect, which increases the surface temperature by 0.53 °C in summer and increases the annual heat wave days at a rate of 3.7 d yr−1 in the major megacities in the YRD, accompanied by intensified heat stress. In winter, the near-surface air temperature increases by approximately 0.7 °C over commercial areas in the cities but decreases in the surrounding areas. Radiative effects of aerosols tend to cool the surface air by reducing net shortwave radiation at the surface. Compared to the more localized UHI effect, aerosol effects on solar radiation and temperature influence a much larger area, especially downwind of the city cluster in the YRD. Results also show that the UHI increases the frequency of extreme summer precipitation by strengthening the convergence and updrafts over urbanized areas in the afternoon, which favor the development of deep convection. In contrast, the radiative forcing of aerosols results in a surface cooling and upper-atmospheric heating, which enhances atmospheric stability and suppresses convection. The combined effects of the UHI and aerosols on precipitation depend on synoptic conditions. Two rainfall events under two typical but different synoptic weather patterns are further analyzed. It is shown that the impact of urban land cover and aerosols on precipitation is not only determined by their influence on local convergence but also modulated by large-scale weather systems. For the case with a strong synoptic forcing associated with stronger winds and larger spatial convergence, the UHI and aerosol effects are relatively weak. When the synoptic forcing is weak, however, the UHI and aerosol effects on local convergence dominate. This suggests that synoptic forcing plays a significant role in modulating the urbanization-induced land-cover and aerosol effects on individual rainfall event. Hence precipitation changes due to urbanization effects may offset each other under different synoptic conditions, resulting in little changes in mean precipitation at longer timescales.