New Perspectives on the Influence of Lifting Condensation Level on Low-Level Outflow and Rotation in Simulated Supercells

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 were analyzed. Previous studies have shown that LCL can influence the outflow buoyancy, which can in turn influence generation and stretching of rotation within the supercell environment. A less explored hypothesis is tested: that the LCL affects the relative positioning of near-surface circulation and the overlying mesocyclone, thus influencing the dynamic lifting and intensification of near-surface vertical vorticity. To test this hypothesis, a set of three base-state thermodynamic profiles corresponding to different LCLs are tested, with the 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. However, the amount of near-surface vertical vorticity generated ultimately depends on numerous 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.