Convection initiation by density currents: Role of convergence, shear, anddynamical organization

Steady-state analytic models establish two key points concerning the impact of vertical shear on density currents and the implication for convection i nitiation. First, shear decreases the horizontal convergence, and therefore the mean ascent, associated with downshear propagating currents. Second, s hear has a basic effect on the dynamical organization. If the downshear cur rent travels at the speed of the ambient how 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 liftin g is comparatively shallow because jumplike ascent occurs rather than deep overturning. The convection initiation mechanism involving the downshear cu rrent is therefore very different from the upshear case. These basic principles are borne out in two-dimensional numerical simulatio ns. Density currents generated by a stationary cold source imposed on an in itially horizontally homogeneous, sheared, and neutrally stratified ambient how 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 on e another; and (ii) if they oppose one another, as in a surface jet, both c onspire 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 a pproximately equal but weak preference for convection exists on the leeward and windward coasts. Point (ii) shows that initiation is strongly suppress ed on the windward coast, but strongly enhanced on the leeward one. The hyp othesis that sea breezes are more intense in offshore flow therefore holds only if shear and surface how have opposite sign or if the flow is unsheare d. Concerning convection initiation by thunderstorm outflows, downshear propag ating 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 converge nce is decreased by shear. While shear strengthens the mean ascent in upshe ar propagating outflows there is no steering level to anchor the incipient convection to the organized ascent.