Abstract/Summary

This study builds a parameterisation strategy for the vertical profile of the horizontal mean wind speed over urban canopies with building height variability (BHV). An intermediate layer (IL) is introduced between the layers deep inside and far above the urban canopies, where exponential (EL) and logarithmic layers (LL) are assumed, respectively. Based on the momentum flux budget, in the IL we propose a linear velocity profile as a simple estimation. Input parameters reflect the BHV geometry (namely the standard deviation and average building height, the highest and lowest building height, and the frontal and plan area indices). Physical parameters such as the bulk drag coefficient and the correction factor for eddy diffusivity in the IL are parametrised using a database containing large eddy simulations (LES) of flows through various random height block arrays covering a wide range of geometries. Our new fully analytical model provides wind speed profiles spanning the top of the surface layer to the ground. There is good agreement to a LES database with realistic urban cases for both mean wind speed and the momentum flux. Analysis of a correction strategy for thermal stratification reveals qualitative consistency with observations in the literature under weakly stable and unstable conditions. This model provides in-canopy wind information, data that are essential for many applications such as estimating thermal sensation at the pedestrian level and evaluating energy consumption in urban agglomerations under changing climate.