Impacts of Surface Shear on Shallow Convection

Surface wind shear is one of the most important factors organizing convective flow structures (thermal updrafts and subsidence) in the boundary layer and further in the cloud layer. It is well known that the structure of boundary-layer-convection can be determined by the ratio of the friction velocity to the convective velocity scale. In the convective boundary layer, for example, horizontal convective rolls occur when surface friction is dominant while convection cells occur when thermal forcing overwhelms surface friction. Although the organization of boundary-layer-convection affects the vertical transport of heat and moisture and clouds above the boundary layer, the impacts of surface shear on vertical transport and on shallow cumulus clouds are poorly understood. We performed a series of large-eddy simulations of maritime shallow convection with the friction velocity changing from 0.07 to 0.56 m s^–1. Convection cells in the boundary layer change to horizontal convective rolls with increasing friction velocity and more moisture converges on the upward branches of horizontal convective rolls, inducing more buoyant convective updrafts (and less buoyant subsidence). Thus, stronger surface shear induces larger upward mass flux in the boundary layer, transporting more moisture into the cloud layer which leads to higher liquid water content. Besides the impacts on the vertical transport of heat and moisture, surface shear is expected to affect the distribution of clouds (e.g. size, number, and distance between clouds). In addition, surface shear can change en/detrainment of individual clouds or in the bulk sense. More details on the impacts of surface shear on clouds will be presented.