Shear Capacity as Prognostic for Nocturnal Boundary Layer Regimes

Field observations and theoretical analysis are used to investigate the appearance of different nocturnal boundary layer regimes. Recent theoretical findings predict the appearance of two different regimes: the continuously turbulent (weakly stable) boundary layer and the relatively “quiet” (very stable) boundary layer. A large number of nights (approximately 4500 in total) are analyzed using an ensemble averaging technique. The observations support the existence of these two fundamentally different regimes: weakly stable (turbulent) nights rapidly reach a steady state (within 2–3 h). In contrast, very stable nights reach a steady state much later after a transition period (2–6 h). During this period turbulence is weak and nonstationary. To characterize the regime, a new parameter is introduced: the shear capacity. This parameter compares the actual shear after sunset with the minimum shear needed to sustain continuous turbulence. In turn, the minimum shear is dictated by the heat flux demand at the surface (net radiative cooling), so that the shear capacity combines flow information with knowledge of the boundary condition. It is shown that the shear capacity enables prediction of the flow regimes. The prognostic strength of this nondimensional parameter appears to outperform the traditional ones like the similarity parameter z/L and the gradient Richardson number Ri as a regime indicator.