Developing Urban Surface Parameterizations for Momentum and Scalars Using Large-Eddy Simulations

Understanding of the physical processes modulating transport of both momentum and scalars in urban environments is essential to develop accurate parameterizations of surface-air exchanges that can be used in weather and climate models. Unlike momentum, which is dominated by form drag and has been studied extensively, scalar transport (of heat or water vapor for example) occurs through the viscous exchanges at solid-fluid interface (and is not well-understood). This implies that the dynamics of scalar exchange remain Reynolds number dependent. Large-eddy simulations at high Reynolds number are carried out to compare momentum and scalar transport over rough urban elements. The dissimilarity between momentum and scalars is first demonstrated. Then, the geometric characteristics of an idealized urban surface are varied to examine their impact on turbulence and transport characteristics. The results show that the bulk surface resistances to momentum and scalar transport are strongly dependent on surface geometry. Momentum and scalar roughness lengths are found to respond differently to the change of surface characteristics. Based on the LES results, models for momentum and scalar exchanges are developed for surface parameterizations of urban environments.