Cold pools in shear

Gravity currents exist in many forms and develop within many types of flow. The dynamics of long-lived cumulonimbus are thought to be strongly influenced by the interaction of a spreading pool of cold, low-level air, which may act as a gravity current, and the ambient sheared flow in the boundary layer. In interpreting two- and three-dimensional numerical models of such phenomena, the behaviour of the vorticity dynamics in the ambient flow and the spreading cold air is commonly discussed. In order to investigate these vorticity ideas from a gravitycurrent perspective, three-dimensional numerical simulations have been performed in which a cold pool spreads from an isolated source in the presence of a horizontal flow that is sheared with height. It has been found that the shear of the incoming ambient flow does not qualitatively alter the horizontal spread of the cold pool, and that the location of the region of maximum horizontal convergence remains on the along-wind axis: the actual strength of the ambient wind, which is shown analytically to be measured by the wind strength at the head height, has a greater bearing on the cold-pool shape than the shear. Consequently, it must be the details of the moist convection which play a crucial role in determining the cold pool's development in sheared flows. As such, this model is a general indication of the robustness of gravity-current theory, even in the presence of mean shear. The vortex structure on the flanks of the spreading cold air is found to be significantly modified, with a reversal in the dominant sign of vorticity and an increase in its intensity, when such mean shear exists.