21st Conf. on Severe Local Storms


Tornadogenesis in a Simulated HP Supercell

Catherine A. Finley, University of Northern Colorado, Greeley, CO; and W. R. Cotton and R. A. Pielke

A nested grid primitive equation model (RAMS version 3b) was used to simulate a High-Precipitation supercell / bow echo which produced two weak tornadoes. The model was initialized with synoptic data from June 30, 1993 and six telescoping nested grids allowed atmospheric flows ranging from the synoptic-scale down to the tornadic vortices to be represented. All convection in the simulation was initiated with resolved vertical motion and subsequent condensation/latent heating from the model microphysics; no warm bubbles or cumulus parameterizations were used.

The first tornado (T1) developed at the southern end of a shear zone present along the flanking line of the initial supercell storm. Tornadogenesis corresponded to the merger between the flanking line of the parent HP supercell, and another storm to the south. The possible connection between cell merger and tornadogenesis in this case will be discussed. Although significant vertical vorticity existed above cloud base prior to tornadogenesis, the model results indicated that the tornadic circulation formed first near the surface and developed upward in time to a height of 5-6 km. Two separate downdrafts were present just prior to tornadogenesis - an 'occlusion' downdraft and the rear flank downdraft (RFD). As the tornado developed, these downdrafts merged into one continuous downdraft which wrapped around the southern and eastern sides of the tornado. The possible role that these downdrafts played in tornadogenesis will also be discussed.

The second tornado (T2) developed along a strong horizontal shear zone beneath the rotating comma-head structure during the bow-echo phase of the HP supercell. Although T2 formed beneath the mid-level mesocyclone, the two were not clearly linked. T2 developed from the ground upward and only extended to a height of 2 km, well below the mid-level mesocyclone. The development of T2 appeared very similar to that of nonsupercell tornadoes (NSTs) which arise from shearing instabilities. Periodic local maxima in the vorticity field were present along the horizontal shear zone at the time tornadogenesis occurred.

Circulation and vorticity analyses were used to investigate the tornadogenesis process in both tornadoes in this case. Results from the simulation and these analyses will be presented.

extended abstract  Extended Abstract (444K)

Poster Session 10, Tornadogenesis
Thursday, 15 August 2002, 3:00 PM-4:30 PM

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