Effects of Surface Drag and Mid-level Shear Magnitude on Idealized High-Shear, Low-CAPE Quasi-Linear Convective System Cold Pools and Vortex Development

High-shear low-CAPE (HSLC) tornadic convective events have been of particular interest in recent years, given their complex environments and propensity to occur at times and locations that increase threats to life and property. Past numerical and observational studies have identified the potential for baroclinically generated vorticity that develops in quasi-linear convective system (QLCS) downdrafts to play a role in the development of mesovortices and tornadoes, but it is not certain how prevalent these mechanisms are in HSLC QLCSs. To investigate the importance of baroclinic vorticity generation for vortexgenesis, this study presents six idealized QLCS simulations, four of which are HSLC. These simulations are initialized with varying mid-level shear and lower boundary conditions (free-slip and semislip) that affect cold-pool characteristics and thus baroclinic vorticity generation. Increasing mid-level shear reduces the number of TLVs that develop over a 3-hour period, due to weaker updrafts and possible inhibition of horizontal shearing instability release. Semislip simulations also have fewer and less intense TLVs than the free-slip simulations due to weaker convergence at the surface which results in broader vortices. Vortices develop and are maintained due to various combinations of the following mechanisms: environmental vorticity being depressed by a downdraft ahead of the surface cold pool, horizontal shearing instability release, tilting of frictionally generated vorticity into the vertical, and baroclinic vorticity generation. Additionally, a novel mechanism is identified that involves baroclinic and frictional vortex lines being tilted in opposite directions at different heights via cold-pool lobes and clefts, resulting in positive and negative vertical vorticity stacked in the same locations, leading to deeper vortices. The array of vorticity sources identified here suggest that while baroclinic vorticity generation is playing both direct and indirect roles in vortexgenesis in these simulations, the relative importance of specific baroclinic mechanisms is difficult to deduce as several vorticity genesis mechanisms can be concurrently active in a single vortex.