The Role of the Streamwise Vorticity Current in Tornado Genesis and Maintenance

The streamwise vorticity current (SVC) is a helically flowing tube of air ingested by the low-level mesocyclone of a supercell, originating within the forward flank cold pool. The SVC has been identified in a numerical simulation within the environment that created the 24 May 2011 El Reno, OK supercell. The simulated storm produced a long-track EF5-strength tornado, similar to the observed storm, and is described in Orf et al (2017). The SVC appeared several minutes prior to tornado genesis, and persisted throughout over an hour of tornado maintenance.

Several additional simulations within the same environment have been conducted on the Blue Waters supercomputer over the past year. A 30 m resolution control simulation has been conducted where the storm-relative motion is nearly constant and matched nearly perfectly with the chosen box motion parameters in the CM1 model. This allowed for the creation of temporally averaged fields that tend to smooth out noisy/transient features while revealing more persistent ones. Two-minute moving window averaged fields were constructed from model history files that were saved in 1 s intervals, hence each averaged field is comprised of the average of 121 sequential saved times. Averaged fields were saved in 1 s intervals, and were visualized/animated. Periodic gravity wave oscillations in the pressure field, caused by boundary reflections, are completely eliminated with this technique, revealing the more important dynamically relevant changes to the pressure field.

In this presentation, the SVC's role in tornado genesis and maintenance is explored. Temporally averaged model fields are examined in order to focus on the persistent features present within the simulation. A rapid drop in pressure, with maximum temporally averaged deficits exceeding 20 hPa, is observed to occur throughout the bottom 5 km of the mesocyclone over the 10 minutes period leading up to tornadogenesis. This drop in pressure is associated with an increase in streamwise vorticity in the cold pool being ingested by the storm's updraft. During this 10 minute period of rapid pressure drop the updraft is observed to lower towards the ground and strengthen, with vertical gradients of w that exceed 50 m/s per km below 1 km.

A lobe of low pressure perturbation to the north of the tornado, with temporally averaged values that, at times, exceed 20 hPa below 1 km, persists throughout genesis and maintenance and is associated with strong helical flow of the SVC as it rises into the low-level mesocyclone. We will explore the relationship between the SVC and the pressure field and updraft structure, and examine the horizontal and vertical vorticity forcing terms in this region throughout the simulation.