The Sensitivity of Hail Production in Supercell Storms to the Distribution of Vertical Wind Shear

There is an emerging consensus that strong deep-layer shear, particularly in the 1-6 km altitude layer, favors hail production in supercells, whereas strong low-level (i.e., 0-1- km) shear does not. However, the importance of the distribution of shear within the 1-6 km layer is unknown, yet that distribution of shear will affect the storm-relative wind magnitude and direction in the lowest few kilometers of the troposphere. The lower-tropospheric storm-relative winds play an important role in creating optimal hail growth pathways via the mesocyclone.

 To understand how the distribution of shear in the 1-6-km layer affects hail production in supercell storms, we perform two sets of idealized simulations using CM1 and our hail growth trajectory model. The first quantifies the impact of the storm-relative wind magnitude on the midlevel mesocyclone and hail production. We do this by modifying the hodograph shape to increase the storm-relative wind in the 1-3.5 km layer and decrease the storm-relative wind in the 3.5 – 6 km layer (and vice versa), while generally maintaining the overall storm-relative wind direction, 0-1-km SRH, 0-6-km and 1-6-km shear vectors, and Bunkers storm motion vector.  The second set of experiments quantifies the impact of the vertical distribution of shear within the 1-6 km layer on the mesocyclone and hail production. We do this by adjusting the hodograph such that, while the total 1-6 km shear remains fixed, shear in the 1-3.5 km layer is increased and 3.5 km-6 km layer is decreased (and vice versa). This results in changes to the storm-relative wind direction while generally maintaining the storm-relative wind magnitude, 0-1-km SRH, 0-6-km and 1-6-km shear vectors, and Bunkers storm motion vector.

We will discuss how these hodograph modifications affect mesocyclone properties, storm behavior (cycling, splitting), updraft width and intensity, and how all these storm structural changes affect hail production.