Convection Initiation by Density Currents: Role of Convergence, Shear, and Dynamical Organization

Abstract

Steady-state analytic models establish two key points concerning the impact of vertical shear on density currents and the implication for convection initiation. First, shear decreases the horizontal convergence, and therefore the mean ascent, associated with downshear propagating currents. Second, shear has a basic effect on the dynamical organization. If the downshear current travels at the speed of the ambient flow at a critical (steering) level, an overturning circulation provides deep lifting. Although mean ascent is increased by shear in the case of upshear propagating currents, the lifting is comparatively shallow because jumplike ascent occurs rather than deep overturning. The convection initiation mechanism involving the downshear current is therefore very different from the upshear case. These basic principles are borne out in two-dimensional numerical simulations. Density currents generated by a stationary cold source imposed on an initially horizontally homogeneous, sheared, and neutrally stratified ambient flow are explored. Results show that (i) if the surface flow and low-level shear vectors are in the same direction, as in a low-level jet, the effects of shear and surface flow on the density current head height counteract one another; and (ii) if they oppose one another, as in a surface jet, both conspire to lower the density current head on the downwind side but raise it on the upwind side. As regards convection initiation by sea breezes, point (i) above shows an approximately equal but weak preference for convection exists on the leeward and windward coasts. Point (ii) shows that initiation is strongly suppressed on the windward coast, but strongly enhanced on the leeward one. The hypothesis that sea breezes are more intense in offshore flow therefore holds only if shear and surface flow have opposite sign or if the flow is unsheared. Concerning convection initiation by thunderstorm outflows, downshear propagating outflows provide the deepest lifting if they move at the speed of the ambient flow at a critical level, despite the fact that low-level convergence is decreased by shear. While shear strengthens the mean ascent in upshear propagating outflows there is no steering level to anchor the incipient convection to the organized ascent.