An observational study of the evening transition boundary-layer

Data collected during the evening transition period are described. Well-defined flux profiles are established rapidly after the surface sensible-heat flux changes sign. The sensible-heat flux profile is strongly curved, so that significant cooling only occurs within the lower half of the boundary layer. The turbulent kinetic energy flux is a significant term in the turbulent kinetic-energy balance in the upper half of the boundary layer early in the transition period. Later the magnitude of the turbulent kinetic-energy flux decreases to small values, so that the turbulence in the upper half of the boundary layer begins to decay. The timescale of this decay is of the order of u*/h, and should also be the time-scale for the changes in the boundary-layer depth. Local scaling does not appear to be valid, except in a shallow layer near the surface. This is because the turbulence time-scale is comparable to the adjustment time-scale of the mean now over much of the boundary layer.