Impacts of Low-Level Shear Orientation and Magnitude on Structure and Mesovortex Production in Idealized Squall Lines

Observational and modeling efforts have sought to characterize the processes relevant for the formation and maintenance of mesovortices, which are known to contribute to severe hazards in quasi-linear convective systems (QLCS). Though these processes are complex and numerous, there exists an important interplay between environmental shear (often 0-3 km shear) and cold pool-induced circulations which, when balanced, allows for upright updrafts with maximized lift along storm outflow boundaries. Numerical simulations have primarily tested the sensitivity of idealized squall lines to zonally-varying low-level (LL) shear profiles (i.e., line-normal, assuming a north-south oriented system), but observed near-storm environments of intense QLCSs with prolific mesovortexgenesis exhibit substantial hodograph curvature (i.e., line-parallel shear) below 1 km. Therefore, previous QLCS simulations may fail to capture the full impacts of LL shear variability on mesovortex characteristics.

As such, this study employs an ensemble of idealized squall line simulations with systematic variations in the orientation and length of the ambient LL shear vector all while holding 0-3 km line-normal shear constant, such that storms are initialized in the same cold pool-shear balance. This allows for a nuanced examination of how LL curvature subsequently modulates QLCS updraft characteristics, as well as related mesovortex attributes, including strength, size and longevity. Results indicate that LL hodograph curvature (and lengthening) contributes to stronger and wider QLCS updrafts, which in turn supports larger, more intense and persistent mesovortices. Furthermore, increased hodograph curvature favors predominantly cyclonic mesovortices compared to straight-line hodographs, which produce comparable populations of both cyclonic and anticyclonic vortices. These differences suggest that LL shear orientation may impact the physical pathways by which squall lines generate mesovortices. This hypothesis is explored through parcel trajectories and a meta-analysis of mesovortex-related studies spanning the last 40 years. Altogether, these results have important implications for both our physical understanding and prediction of mesovortexgenesis in squall lines.