Impacts of surface heterogeneity on dry planetary boundary layers in an urban-rural setting

Abstract

Understanding the impacts of land use and land‐cover change such as urbanization is essential in many disciplines. This study investigates the impacts of urban‐rural contrasts in terms of momentum roughness length (z 0) and aerodynamic surface temperature (TSK) on dry planetary boundary layers (PBLs) using large‐eddy simulations (LES) with the Weather Research and Forecasting (WRF) model. In addition, the impacts of small‐scale heterogeneities within urban areas are also examined. The original WRF‐LES is modified in order to use prescribed TSK as surface boundary conditions. Numerical simulations are then conducted to examine turbulence characteristics and mesoscale circulations resulting from large‐scale urban‐rural contrasts as well as small‐scale heterogeneities in urban areas. The results indicate that (1) the urban‐rural contrasts in z 0 and TSK have significant but different impacts on surface heat fluxes, mesoscale circulations, and the wind and potential temperature profiles. Compared to the case where the whole domain is homogeneous, increases in z 0 and/or TSK in urban areas in the center of domain induce stronger sensible heat fluxes, stronger urban circulations, and weaker inversions at the top of the PBL. (2) When the patch size that characterizes the urban heterogeneity scale is comparable to the size of the whole urban area, the simulated results are strongly dependent on both the heterogeneity scale and the specified surface temperature values. As the patch size decreases, the simulated results become more similar to those over a homogeneous urban surface.