多层城市冠层模式的建立及数值试验研究

为在城市气象数值模拟中更好地体现由城市发展引起的下垫面土地利用改变及人为活动对大气过程的影响,建立了基于建筑物三维分布的多层城市冠层模式,冠层内动力方程组考虑了建筑物冠层拖曳力的作用及雷诺应力的影响,通过引入建筑物宽度、间距以及垂直分布密度指数等建筑物形态特征参数,以更好地体现城市复杂地表对大气温度、湿度及动量方程的影响.同时,该模式分屋顶、4个侧壤及地面分别考虑辐射及能量平衡求解表面温度,计算各表面与大气的通量交换,并考虑辐射阴影效应、冠层内部各个面之间的可视因子、以及与冠层内建筑物密度指数、可视因子等相关的多重反射辐射导致的辐射截陷作用.模式的离线检验结果表明:(1)冠层模式计算风廓线与风洞实验测量数据吻合良好;(2)离线冠层模式能够模拟实际小区的风速、温度垂直廓线,并能够较好地体现小区内气温日变化.冠层模式与区域边界层模式耦合检验结果表明:(1)耦合模拟的近地面(2 m处)气温及地表温度的结果明显优于传统的水泥平板方案,尤其是在夜间,水泥平板方案与实测气温最大偏差4 K左右,耦合模拟方案为1-2 K;(2)耦合模拟方案考虑了建筑物对冠层之上的拖曳力影响以及建筑物形态结构对雷诺应力的影响,风速(10 m处)计算结果与观测值相差约在1 m/s,水泥平板方案偏差3 m/s左右.

Numerical study on development of a multil ayer urban canopy model

Based on the assumption of 3-dimension building geometry, a multiplayer urban canopy model were developed to research the relationship between atmospheric movement and the urban developing. The canopy model, which consists of a set of dynamic e-quations with a drag force, and there are three major parameters: building width, distance between buildings, and vertical floor density distribution, which is the distribution of a ratio of the number of the buildings that are taller than some level to all the buildings in the urban area under consideration. In addition, an irradiative process in the canopy is introduced, in which the model geometry is assumed to be an infinitely extended regular array of uniform buildings in the horizontal, each of which is composed of six faces (roof, floor, and four vertical walls). Both of the drag force of the buildings and the irradiative process, including the radiation shadowing and trapping effect, depend on the floor density distribution in the vertical. The off-line validation of the canopy model shows that the simulation is agreed with wind tunnel test data and the observation data in sub-domain very well. As a new urban land-surface scheme, the canopy model was then coupled with Regional Boundary Layer Model of Nanjing University(NJU-RBLM), and on-line cases were performed for 4 August 2004 in Beijing city. The simulation result compared with observations from a dense operational observational network. The comparison result shows that the coupled scheme (MUCM case) correctly reproduces the temperature (within 1 - 2 K difference from the observations) and wind speed(about 1 m/s difference from the observations), and is better than the soil vegetation modified scheme (SLAB case), especially for temperature simulation in night time. Besides, the comparison of ground surface temperature between simulated and observed results by Landsat satellite shows that the new urban land-surface process scheme improved the simulation performance at urban underplaying remarkably for taking care of the more reasonable radiation process in three-dimension buildings structure.