Convective Boundary Layers subject to non-stationary Surface Heat Fluxes

More often than not atmospheric boundary layers are in a state of transition, continually adapting to changing external/boundary conditions. In this study we systematically investigate the response of dry convective boundary layers to non-stationary surface heat fluxes. Not only is this relevant during sunset and sunrise, but also when for example clouds modulate incoming solar radiation. Because the timescale of the associated change in surface heat fluxes may differ from case to case, we consider in the present study the generic situation of oscillatory surface heat fluxes with different frequencies and amplitudes, and study the response of the boundary layer in terms of transfer functions. To this end we use both a Mixed Layer Model (MLM) and a Large Eddy Simulation (LES) model, where the latter is used to evaluate the predictive quality of the mixed layer model. The mixed layer model performs generally quite well for slow changes in the surface heat flux, and provides analytical understanding of the transfer characteristics of the boundary layer such as amplitude and phase-lag. For rapidly changing surface fluxes, i.e. changes within a time frame comparable to the large eddy turn-over time, it proves important to account for the time it takes for the information to travel from the surface to higher levels of the boundary layer such as the inversion zone. As a follow-up to the study of Sorbjan (1997) who showed that the conventional convective velocity scale is inadequate as scaling quantity during the decay phase, we address the issue of defining, in (generic) transitional situations, a velocity scale that is solely based on the surface heat flux and its history.

Z. Sorbjan 1997 Decay of convective turbulence revisited. BLM 82 p501-515