This study presents the results of large-eddy simulation (LES) of the evening transition in the atmospheric boundary layer in the case of free convection and in the presence of geostrophic wind. The turbulent kinetic energy (TKE) balance and its components are analyzed. It is shown that within the transition, periods of fast and slow decay can be distinguished. The 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 consumed due to inertial movement of the thermals after the cease of convection. This is followed by the TKE redistribution into large-scale horizontal components, which leads to the formation of quasi-horizontal turbulence, where the TKE dissipation is significantly slower in comparison to the isotropic state. It is shown that one-dimensional boundary-layer model, in which turbulent fluxes are parameterized by means of a two-equation closure, is not able to reproduce evening transition dynamics observed in LES. In particular, the use of the gradient approximation in the one-dimensional model leads to the preservation of the convective distribution of the heat flux along the vertical during the transition period and additional TKE generation in the boundary layer due to the action of buoyancy forces. The use of the phenomenological equation for the dissipation rate leads to decreased TKE decay rate during the fast decay period and increased TKE decay rate during the slow decay period. Possible approaches toward modification of the Reynolds-averaged Navier–Stokes (RANS) closures in order to correctly reproduce transition periods of the atmospheric boundary layer are discussed.

Studying of transitional periods as essential non-stationary processes in the diurnal cycle, has proven to be necessary in order to correctly parameterize them in large-scale climate and weather models. This study focuses on the evening transition (from convective to stably stratified boundary layer) and how it is influenced by external parameters, such as kinematic surface heat flux, geostrophic wind, surface roughness and mesoscale subsidence rate, on its dynamics. The single-column numerical model was used. In particular, the decay of turbulent kinetic energy (TKE), which inevitably takes place during the evening transition in the residual layer, was observed under different model settings and external conditions. The results of some of those experiments were compared with the available LES (Large-Eddy Simulation) data from previous studies. It appears that TKE dynamics in these simulations are very similar to those observed in LES experiments. Different external parameters that were studied influence different characteristics of the TKE decay dynamics (such as rate of decay, time of the decay start etc.). The results of this study show that a better parametrization of turbulent exchange processes is required in order to accurately reproduce evening transition and other non-stationary processes in atmospheric boundary layer.

The transitional periods that take place in the atmospheric boundary layer are challenging to model due to their non-stationary nature. Large-eddy simulation (LES) models have proven to be sufficiently accurate for modeling the evening transitions, and now a possibility to adapt turbulent kinetic energy (TKE) closure models for calculations in single-column models appears to be very plausible. In this study, evening transition modeling is analysed with emphasis on the pattern of a TKE decay which follows the power law. E ( t ) α t −α The effects of different parameters on the results of the simulations are explored, along with the geostrophic wind effect on the model. It is shown that the model presented here behaves in a manner similar to that of a LES model, thus showing that the above adaptation is possible and worth being investigated further.