Mechanism of Urbanization Impact on a Summer Cold-Frontal Rainfall Process in the Greater Beijing Metropolitan Area

Abstract

he WRF?Noah?urban canopy model (UCM) coupled modeling system is used to investigate the influence of urbanization on a typical frontal rainfall event that occurred in June 2008 over the greater Beijing metropolitan area (GBMA). The comparison between results of two experiments with and without the presence of urban areas suggests that more precipitation can be induced over and in the area downwind of Beijing because of the urbanization effects while less rainfall is found in the upwind region of central urban areas. The urbanization effects delay the onset of precipitation over Beijing but keep it sustained for a longer period with stronger rainfall intensity. The underlying urban surface not only retards the cold-frontal movement but also favors the development of a wet and unstable atmosphere. This explains why the precipitation can be sustained for a longer time with higher rainfall intensity over urban areas. From analyses of a density-current speed equation, it is found that the abrupt pressure jump during the frontal passage has a significant influence on the cold-frontal movement. A method derived from the surface pressure tendency equation is used to explore the mechanism of urban effects on the cold-frontal system. The cold advection is less intense in the experiment with urban land surface than in the experiment with urban areas replaced by cropland. Hence, the weaker-than-normal cold advection and subsequent smaller pressure jump during the frontal passage over the GBMA are the major mechanisms behind the urbanization effects, which lead to the retardation of the cold-frontal movement and late onset of precipitation over the GBMA.