Afternoon transition turbulence decay revisited by Doppler Lidar

The structure and intensity of ABL turbulence is determined by the diurnal heating and cooling of the ground as well as synoptic conditions. During the afternoon transition wherein the nature of diurnal forcing changes and the mixed convective boundary layer moves to a residual layer overlying a stably-stratified nocturnal layer, the flow becomes non-stationary and its structure becomes complex. Previous studies based on large eddy simulation carried out with impulsive removal or empirical decrease of the driving heat flux have identified a time scale of turbulent kinetic energy decay which depends on the time scale of heat flux decrease. In addition these simulations showed a decoupled residual layer, which develops above the stable nocturnal layer, characterized by turbulence scales larger than those of the mid-day eddies. Only a few observational studies have been reported on this transition period. In this paper we study the evening decay of convective turbulence using Doppler lidar data that were collected during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) field experiment, conducted in the south of France, in 2011, and during previous field experiments conducted in Wisconsin, USA in 2007. The aim of the presentation will be to document from the observations: (1) turbulence decay characteristics at different altitude: TKE decrease, vertical transport and changes in space/ time scales…and then to make a relevant comparison with LES results (2) relevant decay time scales and key driving turbulent variables during the transition.