南京地区城市冠层效应的模拟试验研究

Simulation study of the effects of urban canopy on the local meteorological field in the Nanjing area

利用Weather Research and Forecasting Model(WRF)模式分城市下垫面(wrf-ucm)、城市冠层下垫面(wrf+ucm)和自然下垫面(wrf-no urban)三种情况模拟了南京地区2005年7月17-18日的2 d天气过程.当模拟城市冠层效应时,WRF模式与Urban Canopy Model(UCM)模式耦合.结合同期外场试验观测结果和模式模拟结果,分析城市冠层效应对局地天气的影响.结果发现:考虑冠层效应时,城市地区的2 m温度比不考虑冠层效应时略低一些,但都高于自然下垫面;白天地面感热通量与不考虑冠层时基本相当,而在夜间前者高于后者,两者都明显高于自然下垫面情形;地面潜热通量比不考虑冠层低,均远低于自然下垫面.考虑冠层效应的地面通量模拟结果较为接近观测结果.冠层效应使得城市地区近地层水平风速明显减小;考虑冠层时城市地区气流垂直运动强于自然下垫面,冠层对气流的影响夜间强于白天.

Abstract： Numerical experiments were made to investigate the effects of urban canopy on local meteorological field in Nanjing area, from July 17^th to July 18^th, 2005, by using the Weather Research and Forecasting Model（WRF）. Three types of surface conditions were employed in the simulations： the wrf-no urban case, in which the natural surface was chosen; the wrf ucm case, in which the urban surface was chosen but the Urban Canopy Model（UCM） was not used; the wrf＋ucm case, in which the urban surface was chosen, and the UCM was employed. The results of numerical experiments were compared with the data of the field observations. This study shows that the temperature at 2 m in the wrf＋ucm case is slightly lower than that in the wr^ucm case, and both are higher than that in the wrf-no urban case. Meanwhile, the sensible heat flux of the urban surface with canopy is similar to that of urban surface without canopy in the daytime, while the former is slightly higher than the latter during the night. The values in the two cases are obviously higher than in the case with natural surface in the area. However, the latent heat flux in the wrf＋ucm case is lower than that in the wrf-ucm case, and both are much lower than that in the wrf-no urban case. That is to say, urban ground makes the urban fields drier than the natural ones. On the other hand, the effects of urban canopy influence the air flow in the area, which makes the horizontal wind be reduced over the city, and consequently the vertical motion is enhanced. It seems that this influence is more obvious during the night than in the daytime.