Numerical simulation of secondary vortex development in a tornadic vortex

A nested grid primitive equation model (RAMS version 3b) was used to simulate a classic supercell. Synoptic data from May 15, 1991, was used to initialize the model. A total of six movable nested grids where used to simulate both the synoptic scale features, and the storm initiation and development without the use of warm bubbles or cumulus parameterization.

The simulated storm developed along the dryline in the Texas panhandle around 2030 UTC. This storm developed into a classic supercell which in turn produced a tornado which lasted for 50 minutes in the simulation. During a ten minute period toward the end of the simulation, six secondary vortices developed within the parent vortex which was allowed to develop 'naturally' in relation to its parent storm without any axisymmetric constraints. The simulated secondary vortices had many features in common with multiple-vortex tornadoes and secondary vortices produced in the laboratory.

During the last 20 minutes of the simulation, three different downdraft ‘surges’ wrapped around the outside of the parent vortex. The first ‘surge’ significantly disrupted and weakened the vortex. After a period of time, the parent vortex re-established itself and two additional downdraft surges wrapped around the outside of the vortex. The development of the second RFD ‘surge’ coincided with the intensification of the occlusion downdraft in the center of the parent vortex. The onset of secondary vortex production occurred shortly after the parent vortex developed a well-defined two-celled structure through the depth of the boundary layer. Strong secondary vortex development occurred as the third surge wrapped around the vortex. The secondary vortices developed along the southeast quadrant and in the interior of the parent vortex where the radial shears of the tangential wind, vertical vorticity, and vertical velocity were generally largest. They dissipated along the north/northwest side of the parent vortex before making a complete revolution. The end of the secondary vortex development occurred shortly after the two-celled structure broke down along the north/northeast quadrant of the parent vortex.

An axisymmetric ‘base state’ was constructed by azimuthally averaging vortex quantities at many different radii. This axisymmtric base state was then subtracted from actual vortex quantities in order to clearly illustrate the characteristics of the secondary vortices (structure, growth rate, etc.), and asymmetries in parent vortex flow. These asymmetries elucidate structures in the parent vortex that are important for secondary vortex development and dissipation, including vortex flow perturbations associated with the RFD surges. Results from the simulation and these analyses will be presented.