July 14, 2022
Journal Article

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 cloud relative 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 entrainment driven 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 width threshold that is necessary for it to occur. However, there is a pronounced bifurcation in updraft growth behavior in our model. Sufficiently wide initial updrafts grow and eventually achieve quasi-steady characteristics. In contrast, insufficiently wide initial updrafts shrink with time and eventually decay completely without external support. A sharp initial radius threshold discriminates between these two outcomes. Thus, consistent with our hypothesis, shear inhibits DCI in some situations, but facilitates it in others.

Published: July 14, 2022


Peters J.M., H. Morrison, T.C. Nelson, J.N. Marquis, J. Mulholland, and C. Nowotarski. 2022. The influence of shear on deep convection initiation. Part 1: Theory. Journal of the Atmospheric Sciences 79, no. 6:1669–1690. PNNL-SA-162700. doi:10.1175/JAS-D-21-0145.1