Abstract
Abstract. Gradient-based turbulence models generally assume that the buoyancy flux ceases to introduce heat into the surface layer of the atmospheric boundary layer in temporal consonance with the gradient of the local virtual potential temperature. Here, we hypothesize that during the evening transition a delay exists between the instant when the buoyancy flux goes to zero and the time when the local gradient of the virtual potential temperature indicates a sign change. This phenomenon is studied using a range of data collected over several intensive observational periods (IOPs) during the Boundary Layer Late Afternoon and Sunset Turbulence field campaign conducted in Lannemezan, France. The focus is mainly on the lower part of the surface layer using a tower 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 explained by considering the convective timescale and the competition of forces associated with the classical Rayleigh–Bénard problem. This combined theory predicts that the last eddy formed while the sensible heat flux changes sign during the evening transition should produce a delay. It appears that this last eddy is decelerated through the action of turbulent momentum and thermal diffusivities, and that the delay is related to the convective turnover timescale. Observations indicate that as horizontal shear becomes more important, the delay time apparently increases to values greater than the convective turnover timescale.
Highlights
The general behavior of the diurnal cycle of the atmospheric boundary layer (ABL) under clear sky fair weather conditions is well-known (Stull, 1988)
The transition occurring after the peak in solar insulation can be divided into two distinct periods: the afternoon transition, when the surface sensible heat flux starts to decrease from its midday maximum, and the evening transition, when the surface sensible heat flux becomes negative (Nadeau et al, 2011)
This paper focuses on the behavior of the buoyancy flux and temperature gradient in the surface layer during the evening transition period by analyzing measurements obtained during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST; Lothon et al, 2014) field campaign
Summary
The general behavior of the diurnal cycle of the atmospheric boundary layer (ABL) under clear sky fair weather conditions is well-known (Stull, 1988). The hypothesis, which was described in the introduction, can be related to the well-known Rayleigh–Bénard (R–B) problem (thermal instability) associated with the heating of a quiescent layer of fluid from below, which results in turbulent free convection (Kundu and Cohen, 2010). The standard R–B problem is based on the idea that there is a layer of fluid heated from below, the upper part of the layer is heavy enough to stifle the convective movements. Both viscosity and thermal diffusivity make it difficult for convection movements to happen. The idea was previously introduced and experimentally studied by Cole and Fernando (1998), who designed a laboratory water tank experiment to observe the decay of temperature and velocity fluctuations in the CBL in response to cooling the surface
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