A Lumped Parameter View on Nocturnal Boundary Layer Dynamics

In clear nights a sudden change in turbulence characteristics may occur when the strength of the ambient wind drops below a certain threshold. This threshold is known as the ‘minimum wind speed for sustainable turbulence'. The near-surface inversion strength may rapidly increase resulting in the so-called very stable boundary layer (VSBL). In previous work by the authors, the mechanism behind this regime shift has been explained from a concept called the Maximum Sustainable Heat Flux theory and an expression for the minimum wind speed, Umin, in terms of the net radiative forcing at the surface was given.

The theory, however, made no statement about what happens after the regime shift has been established, i.e. after the VSBL has set in. In other words what happens to the surface energy balance and to the near surface inversion strength when turbulence becomes very weak? As ‘run-away cooling' is not occurring in nature, other thermodynamic processes have to take over.

Here, we aim to answer this question by extending the previous work, through introduction of simple surface feedbacks to the existing concepts. A so-called lumped parameter is introduced which account for the soil/radiative feedbacks in the conceptual model system. For an isolating surface (snow) this parameter has a lower value than over e.g. bare soil, which implies that the soil–feedback is weaker in the first case. As such the VSBL reaches much colder temperatures at equilibrium. Finally, we explore two observational sites with contrasting surface characteristics (Cabauw, Dome C) and interpret the observed characteristics with the new, lumped parameter model.