Boundary-Layer Transition across a Stratocumulus Cloud Edge in a Coastal Zone

Abstract

We examine the lateral transition from a stratocumulus-covered boundary layer to a clear-sky convective boundary layer during onshore flow in a coastal environment, using both mobile sodar observations and a numerical model. During four observation periods, the vertically averaged wind speed increases by roughly a factor of 2 within 5 km of the cloud edge, and the boundary-layer-averaged wind direction backs 40°?60°. The numerical predictions, driven by horizontal heat flux differences between cloudy- and clear-sky regions, agree quantitatively with both the observed wind speedup near the cloud edge and the observed boundary-layer growth, but the wind direction backing is underpredicted. In both observations and predictions, the surface wind speed maximum moves inland with time, whereas the boundary-layer-averaged wind speed maximum remains at the cloud edge. At the cloud edge, a predicted subsidence maximum coincides with an observed dip in boundary-layer depth. In the clear-sky region, concomitant rising motion?not entrainment?is primarily responsible for the rapid boundary-layer growth with distance. An energy balance approach, which neglects this upward motion, greatly underpredicts boundary-layer growth. The sodar indicates regions of strong wind shear under the clouds, but shear of that magnitude is not predicted by the model. Significant wind and boundary-layer changes, primarily due to baroclinicity induced by cloud shading, occur at the quasi-stationary stratocumulus cloud edge; these changes, which we term a cloud breeze, can transcend the influence of the land-water interface in coastal regions.