The influence of shear on deep convection initiation. Part 1: Theory

This article introduces a novel hypothesis for the role of vertical wind shear ("shear") in deep convection initiation (DCI). In this hypothesis, initial moist updrafts that exceed a width and shear threshold will 'root' within a progressively deeper steering current with time, increase their low-level cloudrelative flow and inflow, widen, and subsequently reduce their susceptibility to entrainment-driven dilution, evolving toward a quasi-steady self-sustaining state. In contrast, initial updrafts that do not exceed the aforementioned thresholds experience suppressed growth by shear-induced downward pressure gradient accelerations, will not root in a deep enough steering current to increase their inflow, will narrow with time, and will succumb to entrainmentdriven dilution. In the latter case, an externally-driven lifting mechanism is required to sustain deep convection, and deep convection will not persist in the absence of such lifting mechanism. A theoretical model is developed from the equations of motion to further explore this hypothesis. The model indicates that shear generally suppresses DCI, raising the initial sub-cloud updraft width that is necessary for it to occur. However, there is a pronounced bifurcation in updraft growth in the model after the onset of convection. Sufficiently wide initial updrafts grow and eventually achieve a steady state. In contrast, insufficiently wide initial updrafts shrink with time and eventually decay completely without external support. A sharp initial updraft radius threshold discriminates between these two outcomes. Thus, consistent with our hypothesis and observations, shear inhibits DCI in some situations, but facilitates it in others.