The Influence of Lifting Condensation Level on Low-Level Outflow and Rotation in Simulated Supercell Thunderstorms

Results of idealized numerical simulations testing the influence of low-level humidity, and thus lifting condensation level (LCL), on the evolution, morphology, and development of low-level rotation in supercell thunderstorms will be presented. Previous studies have shown that LCL can influence outflow characteristics, which in turn can affect the generation and stretching of rotation within the supercell environment. In light of this research, a set of three base-state thermodynamic profiles corresponding to different LCLs are tested, with the supercell sensitivity to LCL compared over a variety of low-level wind profiles.

The thermodynamic properties of the simulations are sensitive to variations in LCL, with higher LCLs contributing to broader, more negatively buoyant cold pools. These outflow characteristics affect the positioning of near-surface rotation relative to the mid-level mesocyclone. Specifically, more negatively buoyant outflow allows for more forward propagation of near-surface circulation. When near-surface circulation becomes vertically aligned with the mesocyclone aloft, favorable dynamic updraft forcing provided by the mesocyclone stretches and intensifies preexisting near-surface rotation. Though the amount of near-surface vertical vorticity generated ultimately depends on a number of interrelated factors, including the amount of near-surface circulation generated within the cold pool available for subsequent intensification and the buoyancy of the outflow air. These simulations suggest that such an alignment is a necessary condition for the strengthening of near-surface vertical vorticity and that this positioning of circulation may be modulated by the ambient LCL. This alignment is also sensitive to the low-level wind profile, such that different LCLs result in the most favorable positioning, depending on the low-level gust-front opposing winds and/or shear.