Modeling the decay of the convective boundary layer over heterogeneous terrain

Over the last decade there has been an increasing interest in the late-afternoon transition between the convective and stable regimes of the atmospheric boundary layer. The differences between the two regimes are numerous. On one side, the convective boundary layer is characterized by an unstable stratification, turbulent mixing of momentum and heat and buoyancy-driven eddies. On the other side, the stable boundary layer is characterized by a strong stable stratification that tends to suppress vertical motions generated by mechanical turbulence. One of the key issues of this complex transition period is the forcing time scale associated with the surface heat flux. Unfortunately, very few modeling studies have used realistic decaying time scales for the sensible heat flux. Thus, in the first part of this study, we describe a new sensible heat flux function that better represents the afternoon and early-evening transitions and validate it with eddy covariance measurements over different land surface types. The objectives are to realistically capture the buoyancy forcing time scales observed in nature and the influence of surface properties without modeling the complete surface energy balance. In the second part of the study, we present preliminary results of large-eddy simulation (LES) of atmospheric flow over heterogeneous cooling strips. We focus our attention on the temperature advection between the different stripes as a result of their different cooling rates. Overall, this LES study is one of the first to model the convective decay of turbulence using realistic time scales over heterogeneous terrain.