Supercell-like Storm-Scale and Tornado-Scale Processes in Simulated and Observed Quasi-Linear Convective Systems

Abstract

While supercells are responsible for a majority of long-lived significant tornadoes, tornadoes occur frequently in both supercellular and linear convective modes. Quasi-linear convective systems (QLCSs) and embedded mesovortices are responsible for up to 25% of tornadoes in the United States, and this proportion is even higher in the Southeastern United States. Whether supercell mesocyclones and QLCS mesovortices possess fundamentally distinct characteristics and dynamics is an active topic of discussion. In this work, results from idealized simulations and observational datasets of Southeastern QLCSs suggest that the mechanisms involved in both storm-scale and tornado-scale rotation are widely variable, and many of these processes strongly resemble known mesocyclone and tornado processes in supercells. Idealized simulations are used to study the evolution of rotation in a QLCS-supercell merger in an observed high-shear, low-CAPE environment in the Southeast. Mergers between QLCSs and isolated supercells have been linked to the generation and intensification of low-level rotation. In this work, the post-merger mesocyclone is stronger, deeper, and more persistent than mesovortices in a non-merging QLCS, suggesting that the merged mesocyclone retains its supercellular characteristics after the merger. Trajectory analysis reveals that parcels from near the top of the boundary layer are a key supercell-like feature of the merged mesocyclone, with distinct vorticity generation processes from the other source regions. These parcels follow an up-down trajectory before entering the mesocyclone. The predominant mechanism for vertical vorticity generation is upward tilting of primarily crosswise horizontal vorticity via the updraft, but upward tilting and downward tilting in the downdraft are also present. Horizontal vorticity is generated during descent via tilting and baroclinic processes. After the merger, horizontal vorticity is also amplified during descent via strong stretching within a developing low-level rotor. The vertical vorticity generation processes are primarily those that are typically associated with supercells, but have been seen in QLCS mesovortices as well. By contrast, the horizontal vorticity generation mechanisms are those that are typically seen with QLCS mesovortices. These simulations reveal that the merged mesocyclone is characterized by a combination of supercell- and QLCS-like features and processes. The QLCS processes seen in the idealized simulations are contrasted by observations of comparable resolution obtained by the Propagation, Evolution, and Rotation in Linear Storms (PERiLS) field campaign. In particular, the Rapid-scanning X-band Polarimetric (RaXPol) mobile radar collected close-range, low-level observations of a nontornadic mesovortex and a cluster of transient tornado-scale vortices in a severe QLCS. The processes inferred from observations appeared to differ from those diagnosed in the simulations. As these nontornadic vortices (NTVs) form, they rapidly advect back into the cold pool and dissipate, with only one NTV persisting for >3 min after forming from the merging of two weaker NTVs. Radar observations show that the NTVs are substantially tilted from surface to cloud base, dissipate in a top-down direction, and never develop strong near-surface rotation. Mobile mesonet surface measurements reveal a weak cold pool (Theta_v' = -1.1 C), which may have caused tornadogenesis failure by (1) limiting baroclinic horizontal vorticity generation along the gust front, preventing the development of strong surface rotation, and (2) displacing the NTV from the leading updraft, preventing the amplification of vertical vorticity via surface convergence and upward vertical pressure gradient accelerations. Immediately prior to and during the genesis of the cluster of NTVs, low-level radar velocity fields reveal line-parallel streaks of enhanced vertical vorticity, or "vorticity worms," in the near inflow of the nontornadic mesovortex, suggesting their potential role in NTV genesis. The role of the cold pool in tornadogenesis failure and the implication of vorticity worms in NTV genesis are both similar to tornado-scale processes in supercells---the rearward displacement of a vortex from the parent updraft due to cold pool weakness is known to contribute to tornado dissipation in supercells, and vorticity worms have been found in the highly sheared near-surface inflow of simulated and observed tornadic supercells but have not yet been examined in QLCS environments or in nocturnal surface layers.