Modulation of Surface-Layer Turbulence By Large Scale Motions in the Convective Boundary Layer

One assumption made in Monin-Obukhov similarity theory is that large-scale turbulent motions (that scale on the convective boundary layer depth) do not interact with locally generated turbulence in the atmospheric surface layer. However, a number of studies have suggested that motions on the order of the boundary layer depth are the cause of observed deviations from Monin-Obukhov similarity. In addition, a number of recent studies in neutrally-stratified wall-bounded turbulent shear flows have revealed the existence of so-called large scale motions (LSMs) that populate the logarithmic layer and modulate small-scale turbulent fluctuations near the wall. While properties of these LSMs are well-understood in laboratory flows, the implications of these previous studies for convective boundary layer (CBL) turbulence are not entirely clear, since the morphology of both small-scale and large-scale turbulent structures is known to be strongly affected by buoyancy [e.g. Salesky et al., Bound.-Layer Meteorol. 163:41-68 (2017)].

In this work, inner-outer interactions in the CBL will be investigated using a suite of large eddy simulations spanning weakly to highly convective conditions. Simulation results reveal that, as the atmosphere becomes increasingly unstable, the inclination angle of structures near the ground increases from 12-15° to nearly 90°. Furthermore, the scale separation between the inner and outer peaks in the premultiplied velocity spectra decreases until only a single peak remains (comparable in magnitude to the boundary layer depth). The extent to which large-scale motions modulate small-scale turbulent fluctuations is investigated using the decoupling procedure introduced by Mathis et al. [J. Fluid Mech., vol 628, 311-337 (2009)]. It is found that amplitude modulation by the large-scale streamwise velocity is largest for weakly convective conditions and decreases as the CBL becomes increasingly unstable. However, amplitude modulation by the large-scale vertical velocity is present in both weakly and strongly unstable CBLs. In particular, a positive correlation is found between between large-scale vertical velocity and the envelope of small-scale fluctuations, so that small-scale fluctuations are excited in large-scale updrafts and suppressed in large-scale downdrafts. Connections between these results and changes in the topology of turbulence with increasing instability will be discussed.