北京冬季城市边界层结构形成机制的初步数值研究

SIMULATION STUDY OF FORMATION MECHANISM OF WINTER URBAN BOUNDARY LAYER STRUCTURE OVER BEIJING AREA

中文关键词:  城市边界层，城市冠层参数化方案,模拟试验

英文关键词:Urban boundary layer, Urban canopy parameterization, Simulation experiments

中文摘要:

      利用耦合了城市冠层参数化方案的MM5模式对2001年冬季北京地区一次典型的城市边界层过程进行成功模拟的基础上，对北京城市化作用、周边地形以及城市化进程发展对城市边界层结构的影响等问题进行了一系列的数值模拟试验。城市化作用的因子分离试验发现，城市化的总体作用即城市下垫面结构对大气热力及动力的综合影响导致了北京冬季城市边界层结构主要特征的形成。此外，揭示了城市结构的不同影响因子——动力因子、热力因子和热动力因子间的相互作用在北京冬季城市边界层结构形成和演变过程中的不同作用。在夜间，城市结构的动力因子对于城市边界层主要特征如市区悬浮逆温、近地层中小的风速及较强的湍流动能等的形成起着主导作用；在白天，城市结构的热力因子则成为影响市区混合层强度以及湍流运动特征等边界层结构的主导因素；热、动力因子间的相互作用对城市边界层结构的形成和演变也有着重要作用，但其影响特征比较复杂。北京周边地形作用的敏感性试验的结果表明，北京周边的特殊地形条件对城市边界层热力结构特征如悬浮逆温层及城市热岛等的结构及分布特征的形成也有着明显的影响，使其具有特殊的局地化特征，同时，它也是北京地区近地层主要气流特征的强迫源。不同城市化程度的敏感性试验结果揭示，随着北京城市建筑高度和密度的增加，市区风速将减小、湍流动能将加强，夜间城市悬浮逆温层底的高度会有所提高，城市热岛的强度也将加强，并可能在白天出现比较明显的城市热岛效应。

英文摘要:

      Based on successful simulation of a typical winter urban boundary layer (UBL) process over Beijing area during the BECAPEX in 2001 by the use of MM5 coupled with urban canopy parameterization, a series of simulation experiments were conducted to investigate the effects of urban influence, terrain surrounding and different degree of urbanization on urban boundary layer structures over Beijing area. The results of factor separation experiments of urban influence indicate that the impact of total effect of urban influence is the comprehensive effect of urban infrastructure on thermal and dynamic structures of atmosphere, which lead to the formation of main UBL features over Beijing area. Meanwhile, the relative importance of thermal, mechanical factors of urban infrastructure and interaction between thermal and mechanical factors on the formation and evolution of UBL over the Beijing area were also explored. The results show that, during night, mechanical factors are responsible for main characteristic of nocturnal urban boundary layer such as elevated inversion layer over downtown area, smaller wind speed and stronger turbulent kinetic energy and its behavior with peak at the top of canopy layer. During daytime, thermal factors play dominant role on the structure of UBL, such as the intensity of mixed layer and temperature in the lower atmosphere of urban area. The interaction between mechanical and thermal factors plays important role on the formation and evolution of UBL, but its specific characteristic of mechanisms are complex. The results of terrain surrounding experiment show that terrain surrounding Beijing area not only determines the characteristic of prevailing airflow over Beijing area, but also has obvious effect on thermal structure of UBL, such as the distribution of elevated inversion and urban heat island, and makes them with special localization feature. The results of different extent urbanization experiment show that with the increase in the density and height of buildings in Beijing area, wind speed would decrease and turbulent kinetic energy increase. Meanwhile, the bottom of nocturnal elevated inversion would increase in downtown area, and the intensity of urban heat island would strengthen, and even probably is obvious in the daytime.