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Gradient-based turbulence models generally assume that the buoyancy fluxceases to introduce heat into the surface layer of the atmospheric boundarylayer in temporal consonance with the gradient of the local virtual potentialtemperature. Here, we hypothesize that during the evening transition a delayexists between the instant when the buoyancy flux goes to zero and the timewhen the local gradient of the virtual potential temperature indicates a signchange. This phenomenon is studied using a range of data collected overseveral intensive observational periods (IOPs) during the Boundary Layer LateAfternoon and Sunset Turbulence field campaign conducted in Lannemezan,France. The focus is mainly on the lower part of the surface layer using atower instrumented with high-speed temperature and velocity sensors.  The results from this work confirm and quantify a flux-gradient delay.Specifically, the observed values of the delay are ~ 30–80 min. The existence of the delay and its duration can be explainedby considering the convective timescale and the competition of forcesassociated with the classical Rayleigh–Bénard problem. This combinedtheory predicts that the last eddy formed while the sensible heat fluxchanges sign during the evening transition should produce a delay. It appearsthat this last eddy is decelerated through the action of turbulent momentumand thermal diffusivities, and that the delay is related to the convectiveturnover timescale. Observations indicate that as horizontal shear becomesmore important, the delay time apparently increases to values greater thanthe convective turnover timescale

Countergradient heat flux observations during the evening transition period