On the parameterization of surface roughness at regional scales

The goal of this presentation is to address issues surrounding the parameterization of surface roughness at regional scales, using a local scale model and an upscaling approach. For that end, a suite of large eddy simulations of neutral atmospheric boundary layer flow over surfaces with varying roughness height is performed. The LES use the scale-dependent Lagrangian dynamic subgrid model and rough wall modeling based on the logarithmic law with spatially varying local roughness height. Several patterns of roughness distribution are studied, including random tilings of rectangular patches with a wide distribution of length-scales. First, we show that the 1D structure function of the roughness height may be used to characterize the distribution by means of a unique well-defined integral length scale. Then, we propose a simple upscaling parameterization for regional scale blending height and equivalent roughness height, based on the local scale large-eddy simulation results. In general, good agreement is found between the LES results and the simple parameterization.