Abstract
In this paper, the impact of urban surface roughness lengthz0parameterization scheme on the atmospheric environment simulation over Beijing has been investigated through two sets of numerical experiments using the Weather Research and Forecasting model coupled with the Urban Canopy Model. For the control experiment (CTL), the urban surfacez0parameterization scheme used in UCM is the model default one. For another experiment (EXP), a newly developed urban surfacez0parameterization scheme is adopted, which takes into account the comprehensive effects of urban morphology. The comparison of the two sets of simulation results shows that all the roughness parameters computed from the EXP run are larger than those in the CTL run. The increased roughness parameters in the EXP run result in strengthened drag and blocking effects exerted by buildings, which lead to enhanced friction velocity, weakened wind speed in daytime, and boosted turbulent kinetic energy after sunset. Thermal variables (sensible heat flux and temperature) are much less sensitive toz0variations. In contrast with the CTL run, the EXP run reasonably simulates the observed nocturnal low-level jet. Besides, the EXP run-simulated land surface-atmosphere momentum and heat exchanges are also in better agreement with the observation.
Highlights
With the intensified human activities and urbanization processes, the impacts of urbanization on atmospheric environment at the local to regional scale have drawn considerable attention during recent years [1, 2]
The aerodynamic roughness length (z0), which gives a measure of the capacity of the surface elements in absorbing momentum, is one of the fundamental parameters in atmospheric models to link up the turbulent exchange process with surface morphology
There are two mechanisms for z0 parameterization scheme affecting the simulation of urban atmospheric environment
Summary
With the intensified human activities and urbanization processes, the impacts of urbanization on atmospheric environment (e.g., urban heat islands and urban breezes) at the local to regional scale have drawn considerable attention during recent years [1, 2]. Many a mesoscale modeling study has been performed to investigate the urban issues and much progress has been made in understanding the physical processes such as the turbulent exchanges over urban surfaces [3, 4]. Partly owing to the complexity of urban landscapes, the turbulent exchange between urban surface and atmosphere has not yet been fully understood and properly represented in most mesoscale atmospheric models. To better describe the turbulent exchange process over urban surfaces, the effects of surface morphology (i.e., urban form) must be considered. The aerodynamic roughness length (z0), which gives a measure of the capacity of the surface elements in absorbing momentum, is one of the fundamental parameters in atmospheric models to link up the turbulent exchange process with surface morphology. Z0 is a crucial parameter for the calculation of aerodynamic resistance to momentum, heat, and moisture transfers, which in turn affects the land surface-atmosphere interactions [9]
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