An examination of several parametrizations of mixing lengths in a stable boundary layer: the GABLS case

Turbulent kinetic energy (TKE) schemes depend heavily on the parametrization of turbulence length scales to describe eddy diffusion coefficients for momentum and scalar mixing. This is especially true for shallow, stably stratified boundary layers. Recently, the GABLS (GEWEX Atmospheric Boundary Layer Study) focused on the representation of the stable atmospheric boundary layer with an intercomparison of single column (1D) models and Large-Eddy Simulation (LES) models for a shear-driven, clear-sky stable case over the Arctic.

The presentation will focus on the examination of several popular parameterizations of mixing lengths to simulate the GABLS case in a single column model. These formulations include: 1) a local Richardson number scheme used in the Canadian GEM operational model, 2) the Bougeault-Lacarrere (1989) scheme, 3) a length scale proposed by Therry and Lacarrere (1983), and 4) a modified version of the Lenderink and Holtslag (2004) formulation. It will be shown that the first two length scales tend to give a too deep stable boundary layer, while the two other schemes provide a much improved simulation when compared to the reference LES results.

Examination of the impact of those formulations on mesoscale forecasts with the Canadian GEM model will also be presented at the Symposium.