Exploring vertical turbulence structure in neutrally and stably stratified flows using WRF-LES

Following the observation work by Sun et al. 2012 (S12) (Journal of the Atmospheric Sciences, 2012, Vol. 69, 338-351), vertical variation of turbulence in stably stratified and neutral environments are investigated using the large-eddy simulation (LES) methodology of the Weather Research and Forecasting (WRF) model. Firstly, the WRF-LES was validated for its performance in the stable boundary layer against the LES models in the Global Energy and Water Cycle Experiment Atmospheric Boundary Layer Study (GABLS). In order to simulate a variety of stable and neutral atmospheric conditions, the WRF-LES was run with a range of geostrophic winds and surface cooling rates. The simulation results are compared with observations from the Cooperative Atmosphere-Surface Exchange Study 1999 (CASES-99). The obtained results (see figure below) are qualitatively consistent with the observed dependences of turbulence strength (V_TKE) on wind speed (V) and with the observed vertical variation of their relationships under nearly neutral conditions (regime 2) and when the subgrid contribution to turbulent variables is small. The strong thermal coupling from applying the surface skin temperature instead of the aerodynamic temperature in the Monin-Obukhov bulk formula leads to unrealistic strong heat transfer and a stable layer with turbulence characteristics of a nearly neutral layer. Therefore, the weak turbulence regime (regime 1) below a certain wind speed threshold (Vs) in the observed stable atmospheric boundary layer cannot be achieved. In addition, detailed comparisons of the vertical variations of turbulent strength vs. wind speed and the spectra of vertical velocity between the WRF-LES results and field data suggest that the potential temperature inversion layer leads to downward transport of heat, which also impacts the vertical variation of the turbulence.