Wind Shear and Buoyancy Reversal at the Stratocumulus Top

Abstract:

Turbulent entrainment in the stratocumulus-topped boundary layer and its interaction with other local processes like radiative or evaporative cooling still remains a source of uncertainty in current atmospheric models. This uncertainty is often due to a lack of understanding of those small-scale processes, and not only to the way we parameterize their effect in the different models. In this work, we investigate the interaction at the cloud top between the buoyancy reversal caused by evaporative cooling and the vertical shear of a horizontal wind. We focus on the entrainment zone, a region of the order of a few tens of meters, and, as a tool, we use direct numerical simulation to remove the uncertainty associated with turbulence models. While buoyancy reversal, when acting alone, is too weak to affect significantly the large-scale dynamics of the boundary layer, we find that the enhancement of mixing by local wind shear can render buoyancy reversal comparable to other forcing mechanisms, like long-wave radiative cooling. However, we also find that (i) the velocity increment across the capping inversion needs to be relatively large and typical values of about 1 m s^-1 associated with the large-scale convective motions inside the boundary layer are generally too small, and (ii) there is no indication of a runaway instability that breaks up the cloud in a relatively short time (the so-called 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, and expressed in terms of the velocity and buoyancy increments across the cloud top, and the buoyancy reversal parameters. 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.