Large eddy simulation of katabatic flow

During the winter period, the atmosphere above glaciers and ice caps is mostly stably stratified, and the energy balance in the escarpment region is mainly determined by the long wave radiative cooling and heating due to turbulent heat fluxes. The surface cooling will extract heat from the adjacent air, which in turn becomes more dense. Close to the surface, air will become negatively buoyant with respect to air further downslope, causing relatively cold air to flow down the slope. These winds are called katabatic (or glacier) winds.

Numerical mesoscale models typically use resolutions that are too coarse to fully resolve the shallow katabatic boundary layer flow. Therefore, operational models often rely on boundary layer parameterizations based on output from numerical large eddy simulation (LES) which have historically provided much insight into the convective boundary layer. Studying the stable boundary layer using LES poses new problems because buoyancy reduces turbulence length scales close to the surface that require a finer resolution of the numerical grid. Nonetheless, several previous studies have shown that it is possible to successfully apply LES to simulate a weakly stable boundary layer. In this study, we use LES to reproduce basic characteristics of katabatic flows. The boundary condition imposing periodicity of the potential temperature in the along-slope direction is modified to impose periodicity of temperature deficit.

In katabatic wind studies, 1-D numerical models can provide satisfactory results on the mean wind and (potential) temperature, but contain no information on the statistics on the 3-D turbulent boundary layer. The strength of LES is that it is able to calculate among other terms the shear and buoyancy production of turbulence kinetic energy (TKE) and its dissipation.

In our work we look at the effects of changing the three flow control parameters: the surface heat flux, the background stratification and the slope angle. Changes in mean statistics such as wind and temperature will be discussed, as well as profiles of heat and momentum fluxes, and the terms in the TKE budget.