Large‐eddy simulation of very stable boundary layers. Part II: Length scales and anisotropy in stratified atmospheric turbulence

Abstract

AbstractIn very stable boundary layers (SBLs), turbulence is expected to be weak, anisotropic, and non‐stationary. Moreover, viscous effects likely become relevant under very weak‐wind conditions. This study investigates the stratification regimes based on the relative strength of the forces that control the flow (i.e., buoyancy‐ or viscosity‐dominated flow) and the degree of turbulence anisotropy in large‐eddy simulations of the very SBL with weak winds. The simulations explored here correspond to an Ekman‐layer‐type boundary layer with geostrophic winds equal to and 2 ms, and surface cooling rates of 1 and 3 Khr, at high latitude. According to the buoyancy Reynolds number and the horizontal Froude number , these SBLs are in strongly stable conditions (i.e., and ). Moreover, the vertical profiles of indicate that the turbulent flows are in an energetic state near the surface but gradually transition to a viscosity‐affected stratified state in the upper part of the SBL as viscous effects become important. For the most stable simulation, the results show that even the small scales are somewhat affected by buoyancy. The invariant analysis of the Reynolds stress anisotropy tensor shows that the flow is in an anisotropic state and is governed by the streamwise component of the turbulent flux (one‐component turbulence). Additionally, based on the two‐point spatial correlation function, the shape of the large coherent turbulent structures is nearly isotropic in the vertical direction and anisotropic in the horizontal direction, and their size increases with height. The evaluation of the turbulent kinetic energy budget shows that the most stable simulation is in a non‐stationary state, which impacts the turbulence mixing efficiency measured by the flux Richardson number.