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2 Acta Geophysica

2 2009

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

Axelsen S.L., van Dop H.

Acta Geophysica

2009

Vol. 57, no. 4

837-856

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.

Large-eddy simulation of katabatic winds. Part 1: Comparison with observations

Axelsen S.L., van Dop H.

Acta Geophysica

2009

Vol. 57, no. 4

803-836

Steady-state quasi-one-dimensional large-eddy simulations of slope winds over simple terrain are presented. The model results of up-slope flow are compared to previous simulations by Schumann (1990), and good agreement is found. Modelled downslope winds are compared to meteorological observations from two glaciers. The vertical profiles of velocity and buoyancy agree with the observations, whereas larger discrepancies are found between the modelled and the observed momentum and buoyancy flux profiles. Despite some discrepancies, the model captures the main characteristics of the observed downslope winds fairly well. The numerical model is used in a companion paper (Part II) to study how some external input parameters affect katabatic winds.