During the early evening transition, net radiation and sensible heat flux at the surface change sign. Residual turbulent activity in the newly forming stable surface layer promotes continuing evaporation. The resulting water vapor flux convergence leads to a readily identifiable jump in specific humidity, among other scalars, in the surface layer. This jump can be used to identify the time at which the surface layer becomes decoupled from the boundary layer using data from ordinary surface weather stations. Such information could be of great use in pollutant concentration prediction schemes. McNider et. al. (1995) show that the coupling is highly sensitive to external conditions. It leads to the fact that current surface layer parameterizations can exhibit non-monotonic behavior at these times.
In this work, we use data collected by a network of 26 surface stations that operated for two months during the fall of 1982 in the region of Albany, NY. Humidity jumps and corresponding behaviors in the other variables can be identified in many of the days. The high sensitivity to initial conditions determines that the decoupling transition happens at different times at each station. We also look at the same problem using the analytical two-layer model used by McNider, which allows the transition to occur as a response to the external forcing. Finally, we present results from a large eddy simulation in which a convective boundary layer is generated, followed by the suppression of the surface heat flux. The objective is to get an understanding of the conditions that control the amount of scalar surface layer flux convergence, and how factors like topography or large scale wind direction affect the occurrence of the jumps.