We are grateful for the discussions with our collaborators, especially Drs. George Bryan, Johannes Dahl, Bob Davies-Jones, David Dowell, Ryan Hastings, Karen Kosiba, Mario Majcen, Jim Marquis, Chris Nowotarski, Matt Parker, Erik Rasmussen, Jerry Straka, Lou Wicker, and Josh Wurman. We thank Dr. Johannes Dahl for sharing some of his analysis code, Dr. George Bryan for his ongoing support of CM1, and three anonymous reviewers. Support from NSF Grants ATM-0801035 and AGS-1157646 also is acknowledged.
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By intermediate, we are referring to density potential temperature deficits in the near-surface mesocyclone region of, very roughly, only 1–4 K, which most would probably regard as relatively mild outflow.
Throughout the paper, near surface refers to the lowest model grid level for horizontal winds.
The streamwise (antistreamwise) vorticity is the horizontal vorticity component aligned with (pointed in the opposite direction as) the updraft-relative wind. The crosswise (anticrosswise) vorticity is the horizontal vorticity component 90° to left (right) of the updraft-relative wind.
We deliberately refer to this vorticity as initial vorticity rather than environmental vorticity because the initial conditions of these simulations include a vertical vortex that resembles a midlevel mesocyclone.
The forward trajectories are computed via the first-order Euler scheme. Though it is not as accurate as higher-order schemes, the large model time step of 1 s results in sufficiently accurate trajectories. Trajectories computed using a time step of 0.1 s are virtually indistinguishable from the 1-s trajectories.
In contrast, virtually all trajectories closer to ζmax drop below the lowest scalar for significant periods of time (e.g., the trajectory that passes nearest to ζmax drops below z = 50 m nearly 5 km upstream of ζmax). Though there is justification for extrapolating horizontal velocity components to parcels below the lowest scalar level when the lower boundary is free slip, the extrapolation of vorticity forcings is problematic.
The circulation of a material circuit, following the circuit, is unaffected by the reorientation of vortex lines that thread a surface bounded by the circuit. However, the circulation about fixed points in a horizontal plane (such as in Figs. 13a–c, 16a, 19a, and 22a) also depends on the degree to which baroclinically generated horizontal vorticity is tilted into the vertical.
Though there is a low-LCL cutoff for tornadoes at approximately 500 m in Craven and Brooks’s (2004) dataset [see Fig. 10.13 in Markowski and Richardson (2010), which is an updated version of Craven and Brooks’s Fig. 12, which shows a scatterplot of tornadic versus nontornadic supercell environments as a function of low-level shear and LCL], we suspect that the tornado-scarce low-LCL regime probably represents a regime in which convective available potential energy (CAPE) is rarely present rather than an LP supercell regime in which cold pools are very weak.