Composite hodographs and inertial oscillations in the nocturnal boundary layer.

In this work the dynamic behaviour of the wind in the nocturnal boundary layer is studied, with a particular focus on systematic behaviour of the near-surface wind. Recently, an extension of the well-known Blackadar model for frictionless inertial oscillations above the nocturnal boundary layer was proposed by Van de Wiel et al. (2010), which accounts for frictional effects within the nocturnal boundary layer. It appears that the nocturnal wind velocity profile tends to perform an inertial oscillation around an equilibrium wind profile, rather than around the geostrophic wind vector (as in the Blackadar model).

In the present study we propose the concept of ‘composite hodographs’ to evaluate the ideas and assumptions of the aforementioned analytical model. Composite hodographs are constructed based on a large observational dataset from the Cabauw observatory. For comparison and deeper analysis, this method is also applied to single-column model simulations that represent the same dataset. From this, it is shown that winds in the middle and upper part of the nocturnal boundary layer closely follow the dynamics predicted by the model by Van de Wiel et al. (2010). In contrast, the near-surface wind shows more complex behaviour that can be described by two different stages: 1) a decelerating phase where the wind decreases rapidly in magnitude due to enlarge stress divergence in the transition period near sunset (an aspect not included the analytical model), followed by stage 2): a regular type of inertial oscillation, but with relatively small amplitude as compared to the oscillations in the middle and upper parts of the nocturnal boundary layer.