Numerical simulation of atmospheric boundary layer flow over isolated and vegetated hills using RAMS

Atmospheric boundary layer flows over hills are important in the analysis of wind energy systems, dispersion of pollutants in the atmosphere and many meteorological and engineering applications. The objective of this work is to use the Regional Atmospheric Modeling System (RAMS), a numerical mesoscale model generally used for weather forecast and atmospheric case studies, to simulate the flow over isolated hills, covered with vegetation of uniform and non-uniform roughness length. The ability of the model to simulate this type of flows is tested by comparison with actual microscale data. The flow is assumed to be two-dimensional and quasi-steady, and the atmosphere is dry under statically neutral and non-neutral stability conditions. The numerical grid covers a large physical domain, with constant mesh spacing in the horizontal direction and a telescopic mesh in the vertical direction. All cases studied show that the domain size, the boundary conditions and the turbulence models play an important role in the simulations. The numerical results indicate that the Mellor and Yamada turbulence model performs better than the Smagorinsky model. When compared to the Askervein, Black Mountain, Cooper's Ridge field data and other numerical and analytical results from the literature, the RAMS results predict reasonably well the vertical profiles of the mean velocity and of the absolute and relative speedups.