Predicting Nocturnal Boundary Layer Regimes: An Observational Study

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 'quiet' (very stable) boundary layer. A large number of nights (approx. 4500 in total) are analysed using an ensemble averaging technique. From this it appears that indeed two fundamentally different regimes exist: Weakly stable (turbulent) nights rapidly reach a steady state (within 2-3 hours). In contrast, very stable nights reach a steady state much later after a transition period (2-6 hours). During this period turbulence is weak and non-stationary. A new parameter is introduced that appears to separate the regimes clearly. This parameter does not only facilitate a regime division but also opens up opportunities for a theoretical description of the very stable regime.