Numerical Study of Winter Urban Boundary Layer Structure over Beijing Area

Based on the successful simulation of a typical winter urban boundary layer (UBL) process over Beijing area during the Beijing City Air Pollution Experiment (BECAPEX) in 2001 by the use of MM5 coupled with urban canopy parameterization, a series of simulation experiments are performed to investigate the eects of urban in uence, surrounding terrain, and dierent extent of urbanization on urban boundary layer structures over Beijing area. The results of factor separation experiments of urban in uence indicate that the total eect of urban in uence, which is the synthetic eect of urban infrastructure on thermal and dynamic structures of atmosphere, is responsible for the formation of main UBL features over Beijing area. Meanwhile, the relative importance of thermal and mechanical factors of urban infrastructure and interaction between thermal and mechanical factors for the formation and evolution of UBL over the Beijing area are also explored. The results show that, during nighttime, mechanical factors are responsible for main characteristics of nocturnal urban boundary layer such as elevated inversion layer over downtown area, smaller wind speed and stronger turbulent kinetic energy (TKE) and its behavior with peak at the top of canopy layer, whereas in the daytime, thermal factors play dominant role in the structure of UBL, such as the intensity of mixed layer and temperature in the lower atmosphere in urban area. The interactio