Does Low-level Vertical Wind Shear Matter for Hail Production?

Vertical wind shear (or more precisely, bulk wind vector magnitude differences between specified altitudes) has long been used for severe convective storm science and forecasting, in part owing to its relative success in correlating to various storm behaviors and hazards. However, theoretical, modeling, and observational work has suggested that this success may arise because vertical wind shear is associated with and/or a proxy for other, more dynamically relevant environmental characteristics.

Recent research has explored various environmental controls on hail sizes in supercell storms, and found that vertical wind shear (and, by extension, hodograph shape) is an important determinant on a storm’s proclivity for hail production. Idealized numerical simulations of supercells showed that, as deep-layer shear increases (specifically, zonal shear in the 2-6 km AGL layer; this also means increased 0-6-km shear), the resulting broader updrafts increased hailstone residence time and thus size. Paradoxically, increasing the 0-2-km shear (predominantly, but not entirely, in the meridional direction) broadened the updraft but decreased hail size: increased southerly winds within the hail growth zone increased residence times along the main growth pathway. These studies left a lingering question: what is the underlying driver to changes in supercellular hail production, the low-level shear magnitude, its orientation relative to the deep-layer shear, or neither?    

To answer this question, we present the results of simple idealized numerical modeling experiments in which the low-level (0-2 km) vertical wind shear magnitudes and directions are systematically and independently varied, keeping all other environmental factors the same. The resulting storms are used to drive the Kumjian & Lombardo (2020) hailstone growth trajectory model. The simulations lead to differences in hail sizes produced, despite having identical 0-2-km shear values. Rather, the differences in storm motion and hodograph shape lead to markedly different low-level storm-relative wind profiles. As a consequence of varied low-level storm-relative wind speeds and directions, the mesocyclonic flow speeds and directions within the hail growth zone differ amongst the experiments, which directly affects residence times and thus hail growth.