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Large-eddy simulation of katabatic winds. Part 2: Sensitivity study and comparison with analytical models

The effects of the slope angle, surface buoyancy flux, and background stratification on steady-state katabatic winds are studied using large-eddy simulation (LES). The numerical code was described and validated in a companion paper (Part 1). Our numerical results are interpreted in the light of analytical Prandtl model, and we find that our vertical profiles of the downslope velocity, buoyancy, and the momentum and buoyancy fluxes exhibit many of the features from the analytical solution. On the other hand, there are also differences between the analytical and numerical results due to the assumptions in the analytical model. One of the assumptions is that the Prandtl number is constant throughout the boundary layer. However, the simulations show that this number varies with height, and also that the Prandtl number increases with increasing gradient Richardson number. The immediate benefit of LES over analytical models is its capability of resolving turbulent motions. In our study of the turbulence kinetic energy budgets, we find that the wind shear is the largest production term, and that it is mainly balanced by turbulence dissipation. Near the wind maximum, where the shear vanishes, the turbulence transport is the only production term.