Importance of the Baroclinic Mechanism for Vortexgenesis in Idealized Simulations of High-Shear, Low-CAPE Quasi-Linear Convective Systems

The mechanisms behind the development and maintenance of quasi-linear convective system (QLCS) mesovortices and tornado-like vortices have been the recent focus of both numerical and observational studies. However, it is not well understood if these results are generalizable to extreme environments, such as high-shear low-CAPE (HSLC; often defined as surface-based CAPE ≤ 500 J kg^-1 with 0-6 km bulk shear ≥ 18 m s^-1). The purpose of this study is to provide a robust examination of vortices that develop in four idealized HSLC QLCS simulations in CM1 (250-m horizontal grid spacing) with a focus on the importance of the baroclinic vorticity mechanism. Variations in both the mid-level shear magnitude and lower boundary condition (free-slip vs semislip) result in nontrivial changes in the evolution and characteristics of both the parent QLCSs and their vortices, allowing for a more inclusive investigation of how storm-scale effects influence the vorticity budget of parcels sourcing and maintaining the vortices.

While sustaining HSLC convection in an idealized model for long periods of time can be difficult, this study leverages a heat sink deep in the cold pool on the west side of the domain which allows convection to maintain itself through 12 hours of simulation time. The free-slip simulations have stronger cold pools and leading-edge cold-pool temperature gradients on average, resulting in stronger low-level updrafts, a greater number of vortices, and more intense vortices. Despite being generally weaker, the strongest semislip vortices are associated with stronger updrafts (between 1 and 3 km AGL) than their equivalent free-slip counterparts and tend to extend upwards to a greater height. However, while vortex evolution and appearance present quite differently across the four simulations, the parcels that enter the vortices tend to come from similar regions relative to the vortex: predominantly from the cold pool, but often with a steady stream of parcels from the environment as well. A minimum of 6.2 million parcels are released for each analyzed vortex, resulting in O(1000) parcels that enter and ascend within a 2 km x 2 km x 200 m (x, y, and z, respectively) box centered on the surface vortex. Such a large dataset allows for a comprehensive investigation of parcel source regions and behavior. Vortex-line slippage (caused by baroclinic vorticity generation) is present in parcels from vortices in all four simulations, and further vorticity-budget analyses and parcel composites will determine the relative contribution of baroclinic mechanisms.