The decay of convective turbulence during evening transition period

The decay of convective turbulence during evening transition was studied using VTMX and PAFEX-1 field data as well as a laboratory experiment. The aim was to document the decay characteristics of turbulence in the evening atmospheric boundary layer upon slow subsidence of solar radiation. Data taken from two VTMX sites as well as the PAFEX-1 site were analyzed in the frameworks of a simple theoretical model and past numerical simulations, based on which the relevant decay time scales were identified. The observed decay was found to be different from those of previous numerical predictions, in part due to the presence of a rich variety of small-scale phenomena that are not represented by existing sub-grid parameterizations. The laboratory experiments consisted of heating with a steady heat flux either a linearly stratified or a non-stratified fluid to generate a convective layer, and then subjecting it to gradual cooling. The presence of stratification on the top of the convective layer (as in the case of the atmospheric boundary layer) changed the decay behavior substantially, depending on the bulk Richardson number of initial convection. Surprisingly, the decay dynamics is dominated by the horizontal turbulent velocities rather than their vertical counterpart. The stratification that develops near the ground (heated surface) during cooling and strong buoyancy effects in the proximity of the upper entraining interface suppress vertical motions in the convective layer, leading to horizontal layering and a "jet"-like flow structure. The dissipation in these layers leads to rapid decay of turbulent kinetic energy, after about 3-4 convective eddy turnover time scales. In this case, too, the laboratory measurements differ from previous LES results, pointing to the unsuitability of sub-grid models used in simulations.