Large-eddy simulation and parameterization of decaying turbulence in the evening transition of atmospheric boundary layer

This study presents results of numerical experiments with LES (Large-Eddy Simulation) model of the evening transition in the atmospheric boundary layer for sheared and shear-free cases. The turbulent kinetic energy (TKE) balance and its components are analyzed. It is shown that fast and slow decay periods can be distinguished within the transition. Differences in TKE anisotropy between these two periods are demonstrated. During the fast decay period, the majority of the energy within the vertical component is destructed due to thermals inertial movement. This is followed by its redistribution into horizontal components, which leads to the formation of quasi-horizontal turbulence, where TKE dissipation, in comparison to the isotropic state, is significantly slower. It is shown that one-dimensional boundary layer model, where k-epsilon closure is utilized for the parametrization of turbulent diffusion, does not reproduce evening transition dynamics, partially because of gradient approximation of turbulent fluxes. The dissipation rate equation utilized k-epsilon closure shows decreased TKE decay rate during the fast decay period and increased TKE decay rate during the slow decay period. Consequently, possible approaches towards modification of RANS (Reynolds-Averaged Navier-Stokes) closures in order to correctly model transition periods of the atmospheric boundary layer are discussed.