An analytic model shows that low-level shear decreases the mean ascent associated with a downshear propagating density current but, if a critical level exists, deep lifting is provided by an overturning updraft. On the other hand, shear can increase the ascent by an upshear propagating current. This aspect is quantified in terms of the Froude number, the ratio of ambient low-level shear to the shear in the density current, and the ratio of the surface inflow ahead and behind the current.
These principles are borne out in numerical simulations. (i) When the surface flow and low-level shear point in the same direction, the effects of shear and surface flow counteract one another. (ii) When they point in the opposite direction, however, both conspire to lower the head height on the downwind side but raise it on the upwind side.
The implications of these results to convection initiation by sea breezes and cold air outflows in MCTEX are discussed. In particular, the commonly held view that sea breezes are more intense in off-shore flow holds only is shear and surface flow have the opposite sign or if the flow is unsheared. Also, the role of density currents in long-lived multicellular squall lines is revisited.