Large‐eddy simulation of very stable boundary layers. Part I: Modeling methodology

Abstract

AbstractThe large‐eddy simulation of very stable boundary layers (SBLs) remains challenging due to the large anisotropy of turbulent motions resulting from buoyancy suppression effects, and to the general reduction of its energetic scales. Here, we develop a novel large‐eddy simulation model setup to enable the modeling of the very SBL, avoiding the use of the Monin–Obukhov similarity theory at its lower boundary condition, as it breaks down in these conditions in agreement with previous studies. The new approach requires very fine grids in the vertical direction, in the order of a few centimeters. A series of Ekman‐layer‐type boundary layers under weak‐wind conditions ( and 2 ms), and strong surface cooling rates (1 and 3 Khr) at high latitude is studied using isotropic grids of 0.05 and 0.10 m resolution. The low‐order statistics show some sensitivity to the grid resolution; for example, both the SBL height and the low‐level jet position decrease with increasing grid resolution. In general, some differences are spotted regarding the shape of the wind speed and temperature vertical profiles, compared with weakly to moderately stratified conditions, suggesting that the structure and characteristics of the SBL evolve with increasing stratification. Moreover, the wind turning is significant from the surface, where the surface wind angle relative to the geostrophic wind is about 50. Lastly, two regimes within very stable conditions can be qualitatively identified primarily based on the gradient Richardson number () profiles: In the less stable regime, increases almost linearly with height and a quasi‐steady state is achieved; however, in the most stable regime, sharply increases near the surface and remains constant throughout the SBL while increasing with time up to . Thus, a quasi‐steady state is not achieved; and although turbulence does not collapse, it remains weak.