Impact of increasing low-level shear on supercells during the evening transition

A commonly observed characteristic of severe weather events in the central United States is an increase in shear in the lowest one kilometer during the late afternoon and early evening hours. It is unclear how storms directly respond to these changes in shear during the early evening transition (EET), nor how changes in shear interplay with changes in stability. We hypothesize that the evolving low-level shear during the EET leads to changes in the low-level dynamic lifting of air in addition to changes in the storm's profile of vertical vorticity. Using the Bryan Cloud Model 1 (CM1), we have performed idealized supercell simulations with horizontally homogeneous initial conditions, using soundings from three VORTEX2 cases as a basis for the simulated changes in the environmental wind profile. The primary effect of the increase in low-level shear is a non-linear response in the updraft strength due to the enhanced low pressure in the updraft from the spin of the mesocyclone. This is particularly important at low-levels, as the increased shear helps to establish the base of the low-level mesocyclone at lower altitudes. This in turn increases the storm's ability to lift cool outflow air, which may have implications for tornadogenesis. Trajectories launched in developing vortices support this preliminary conclusion.