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

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.