The Impact of Variations in Upper-Level Shear on Simulated Supercells

Numerous observational and numerical studies have highlighted the critical role that vertical wind shear plays in the organisation of deep moist convection. However, the vast majority have focused on shear in the lowest 6 km of the atmosphere. It has been suggested that upper-level storm-relative flow (a function both of the deep-layer shear and storm motion) is an important control on the precipitation distribution in supercell storms, giving rise to the familiar ‘classic', ‘low-precipitation' (LP), and ‘high-precipitation' (HP) morphologies. In the present study, the specific role of upper-level shear (ULS) is investigated using idealized numerical simulations. It is found that as ULS increases, storms become larger, with more intense precipitation and associated stronger outflow. These changes, which are consistent across a range of model and environmental configurations, result from an increase in storm motion which strengthens the inflow at low-levels, promoting wider updrafts and enhanced condensation. Despite the increase in storm motion, stronger ULS still promotes stronger upper-level storm-relative flow. Our findings thus contradict the generally accepted hypothesis that strong storm-relative winds at upper levels favour the formation of LP supercells. In addition, they suggest that winds above 6 km should be considered when estimating storm motion from sounding data.