Numerical Study of Urban Impact on Boundary Layer Structure: Sensitivity to Wind Speed, Urban Morphology, and Rural Soil Moisture

Abstract

A mesoscale model with a detailed urban surface exchange parameterization is used to study urban influences on boundary layer structure. The parameterization takes into account thermal and mechanical factors, and it is able to reproduce the most important observed urban boundary layer features. A series of simulations is carried out on a 2D idealized domain to analyze the urban boundary layer sensitivity to wind speed, urban morphology, and rural soil moisture. The results show that, during the night, wind speed is correlated with inversion height, inversion depth, and inversion strength and that mean building height and street-canyon height-to-width ratio are correlated with inversion height but are anticorrelated with inversion depth and inversion strength. A reduction in rural soil moisture reduces inversion height and increases inversion strength. During daytime, differences between urban and rural boundary layers are strongly linked with wind speed and rural soil moisture. A factor analysis technique is used to evaluate the relative importance of thermal and mechanical urban factors in terms of their effects on boundary layer structure. The results show that, during the night, thermal factors are more important in the lower part of the urban boundary layer and mechanical factors are dominant in the upper part. Interactions between thermal and mechanical factors act to increase nocturnal boundary layer height. During the day, thermal factors play the most important role in modulating the PBL height evolution above the city. Interactions between thermal and mechanical factors act to reduce the daytime boundary layer height. Mechanical factors become important in the evening, when the turbulent kinetic energy produced by interactions between the airflow and buildings causes a delay in the decrease of PBL height.