南京复杂下垫面条件下的三维城市热环境模拟

Numerical study of the three-dimensional thermal environment over a complex underlying surface in Nanjing

运用WRF模式,选取考虑城市冠层结构(UCM算例)及不考虑城市冠层(NOUCM算例)两种城市下垫面参数化方案,对南京2010年夏季晴天小风典型天气条件下的城市热环境以及不同下垫面的边界层特征进行了模拟研究。结果表明:1)UCM方案模拟结果与实际情况较为吻合。其中2 m气温的模拟有较大的改进,模拟结果明显高于NOUCM方案,与观测更为吻合,同时更好地模拟出了冠层建筑物对于近地层风速的拖曳,10 m风速的模拟有非常明显地提高。2)UCM方案较好地模拟出了城市的三维热岛分布。由于建筑物地表对辐射的截留,白天14时(北京时间,下同)热岛较强,地面2 m高度处热岛范围较大,热岛面积大约为120 km2,强度为2℃。同时建筑物的存在使得城市湍流动能更大,向上的垂直扩散增加,距地面20 m时,依然能看出明显的热岛效应,热岛强度为1.5℃。距地面55 m处,UCM模拟所得的热岛范围缩小,热岛强度为1.1℃。UCM模拟所得的白天地表热量的扩散影响可达143 m,02:00 2 m处热岛最强为2℃,热岛影响也可达70 m以上。3)不同下垫面呈现出了不同的边界层特征,城市冠层结构对周边下垫面边界层结构存在程度不等的影响,14:00城市区域的湍流混合更强,城市边界层高度升高100 m左右,02:00,城市冠层结构的存在,导致近郊庄稼下垫面及紫金山植被下垫面的稳定逆温层结明显减弱。

Mass and energy exchanges in the urban canopy directly affect urban atmospheric thermodynamic and dy- namic processes, and thus affect the structure of the urban boundary layer.Urban canopy parameterization is an in- dispensable physical process in the numerical simulation of atmospheric motion. In the present work, simulations were conducted for the urban thermal environment of Nanjing under typical summer weather conditions （sunny days with weak wind）, for the period 1-3 August 2010, using the WRF mod- el.By selecting a parameterization that considered the urban canopy structure（ UCM experiment）, and one that did not（NOUCM experiment）, the boundary layer characteristics over different underlying surfaces were examined. The results showed that： （ 1 ） The UCM simulation results agreed with the observational data very well.The simulation of 2-m temper- ature was improved considerably compared to that of NOUCM. Moreover, the drag of buildings to near ground wind speed was better reflected;the simulation of 10-m wind speed was substantially improved.According to observational data analyses,the heat island intensity reached its maximum at 2100 BST, and was about 4 ℃.The UCM scheme simulated a value of around 3 ℃, while the NOUCM scheme simulation yielded a value of around 1.5℃. （2） The UCM experiment simulated the 3D heat island distribution favorably.Owing to the retention of radi- ation, the heat island at 1400 BST was relatively strong, and it covered a wide range at about 2 m above the ground （ - 120 km2） ,with an intensity of 2 ℃ .The heat island intensity in the NOUCM scheme simulation was 1 ℃ .Meanwhile,due to the existence of urban structures, the turbulent kinetic energy was greater and the upward vertical diffusion increased.At 20 m above the ground, an obvious heat island effect could still be found, and the intensity produced using the UCM scheme was 1.5 ℃ .The heat island range in the NOUCM experiment was substantially narrower, and the intensity was 0. 8 ℃ .At around 55 m above the ground, the heat island range simulated using UCM narrowed, and the intensity was 1.1 ℃; meanwhile, the intensity of the NOUCM scheme was only 0.4 ℃ .Based on the UCM simulation,the ground heat diffusion effect during the day reached 143 m,the heat island at 02：00 BST and 2 m above the ground reached a maximum of 2 ℃, and there was a detectable heat island impact above 70 m.Based on the NOUCM simulation,however,the range of the simulated heat island shrank rapidly with height, and a heat island effect could not be found with an obvious inversion phenomenon. （ 3 ） Different underlying surfaces produce distinct boundary layer features, with cities having distinct impacts on surrounding underlying boundary structures.During the day, turbulent mixing was enhanced at 14：00 BST in urban areas, and the urban boundary layer height increased to about 100 m.At night, the stable inversion stratifica- tion of the suburbs cropped the underlying surface, and the Purple Mountain vegetation underlying surface decreased significantly at 02：00 BST because of the presence of the urban canopy structure.For water, near-surface potential temperature and air temperature were mainly affected by the water.Cities have a limited impact on nearsurface water;however,they appear to gradually influence surface water after a certain height.