Buoyancy reversal refers to the formation of negatively buoyant parcels of fluid within an otherwise stable stratification as a consequence of the local cooling caused by the evaporation of the droplets at the cloud interface. This process leads to convective instability, as heavier parcels of fluid lie on top of lighter ones. This instability promotes turbulence, and hence entrainment and further evaporation of droplets, a feedback process that could eventually lead to a rapid dessication of the cloud -- the so-called cloud-top entrainment instability. Recent work has demonstrated that, in contrast to previous postulates, buoyancy reversal caused by evaporative cooling is not a sufficient condition to break up the cloud: turbulence is indeed enhanced, but very mildly. The reason is that buoyancy reversal alone leads to a meta-stable layer as the mixing rate, or entrainment rate, is diffusively limited, so that the eventual breakup of the cloud by buoyancy reversal alone occurs on time scales that are much too long to be relevant to the stratocumulus-top boundary layer. To become relevant, evaporative cooling requires the interaction with other local mechanisms of turbulence generation, like wind shear or radiative cooling. In this work, we study wind shear effects, without radiative cooling.
There are at least two reasons to study shear effects. First, shear is ubiquitous, as local shear associated with large-scale eddies will also be evident even in the absence of a mean wind. Second, shear alone cannot sustain a continuous deepening of the layer, as shear generated turbulence will locally thicken the entrainment zone, but in the absence of other sources, the turbulence will eventually decay once a critical entrainment-zone thickness is reached. The latter makes it interesting to combine shear with the convective destabilization of the cloud-top layer through buoyancy reversal, as neither process acting alone is efficient in supporting significant mixing at the cloud top. In contrast, by generating convective eddies which locally thin the entrainment zone, buoyancy reversal might help enhance shear, which in turn locally enhances the mixing which sustains the buoyancy reversal, raising the possibility that the processes are self-reinforcing.
Results show that the enhancement by local wind shear can render buoyancy reversal comparable to other forcing mechanisms. However, we also find that (i) the velocity jump across the capping inversion, Δu, needs to be relatively large and typical values of about 1 m s-1 associated with the convective motions inside the boundary layer are generally too small, and (ii) there is no indication of cloud-top entrainment instability. To obtain these results, parametrizations of the mean entrainment velocity and the relevant time scales are derived from the study of the cloud-top vertical structure. Two overlapping layers can be identified: a background shear layer with a thickness (1/3) (Δu)2/(Δb), where Δb is the buoyancy increment across the capping inversion, and a turbulence layer dominated by free convection inside the cloud and by shear production inside the relatively thin overlap region. As turbulence intensifies, the turbulence layer encroaches into the background shear layer and defines thereby the entrainment velocity. Particularized to the first research flight of the Second Dynamics and Chemistry of Marine Stratocumulus (DYCOMS-II) field campaign, the analysis predicts an entrainment velocity of about 3 mm s-1 after 5-10 minutes, a velocity comparable to the measurements and thus indicative of the relevance of mean shear in that case.
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