the evolution of mesocyclone vorticity lines prior to and proceeding tornadogenesis. The supercell was simulated using the compressible mode of the nonhydrostatic Bryan Cloud Model 1, CM1 [@bryanfritsch02]. The simulation proceeded on a $112.5$ km $\times$ $112.5$ km $\times$ $20.0$ km domain with horizontal grid spacing of $75$ m and vertical grid spacing increasing from $50$ m at the lowest layer to $750$ m at the highest layer. The large and small time steps were $1/4$ s and $1/16$ s, respectively. Typical of idealized storm simulations, a horizontally uniform analytical base state was used (see Figure \[skewt-ok-nssl\]);

![Base state for a CM1 simulation. Wind barbs are in knots. The hodograph in the upper left corner is in m s$^{-1}$, with marked heights in km.[]{data-label="skewt-ok-nssl"}](skewt-ok-nssl_cropped.pdf){width="75.00000%"}

terrain, surface fluxes, radiative transfer, and Coriolis acceleration were omitted; and radiative (free slip) lateral (vertical) boundary conditions were imposed. Microphysical processes were parameterized using the double-moment scheme [@morrison05]. The subgrid turbulence scheme was similar to that in [@deardorff80]. The simulated supercell exhibits features commonly observed in real supercells, including a hook echo reflectivity signature with a cyclonic–anticyclonic vorticity couplet shown in Figure \[fig:simulated\_features\].

In Figure \[fig:vortex\_image\_time\_series1.pdf\], a two-minute time interval of the CM1 simulation is 