Certain qualitative changes in turbulent eddy mixing in an atmospheric boundary layer (ABL) during transitional regimes towards stronger stratification are highlighted using numerical simulations with the mesoscale RANS turbulence model. The flux Richardson number Rif (or the mixing efficiency) in the stably stratified atmospheric boundary layer is investigated as a function of the gradient Richardson number Rig. In particular, the flux Richardson number can behave non-monotonic, which has increased with increasing of the gradient Richardson number, saturates and then decreases after a value of Rig around 1.0. Behavior of turbulent eddy mixing coefficients for momentum and heat in this study is consistent with the representation that the flow can sustain propagating internal waves that can effectively transport momentum, but not heat. This behavior is in good agreement with observational results for stably stratified nocturnal boundary layer flows.
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