A numerical study of cyclic tornadogenesis: The 8 June 1995 VORTEX Case

Cyclic mesocyclone production in long-lived supercell thunderstorms has been observed as early as the 1950's. While numerous observational studies have documented this phenomena, few numerical studies have focused on the processes that generate and control the formation of multiple mesocyclone cores in supercells. A recent study by Alderman et al. (2000) indicated that the formation of the initial mesocyclone may require longer spin up time than subsequent meocyclones, suggesting that once the storm becomes organized at low levels, multiple mesocyclone cores can be produced readily. However few studies, if any, have focused on what environmental conditions are favorable for cyclic mesocyclogenesis and what parameters influence or control the low-level behavior of these features.

Dual-doppler observations from the Eldora radar of the Kellerville supercell from 8 June 1995 indicate a wide spectrum of behavior in the mesocyclone development during the storm's lifetime. In particular, the Kellerville supercell transitions from a cyclic tornadic storm to a nearly steady tornadic storm during its lifetime, producing several short-lived tornadoes early and subsequently a long-track tornado late during the storm's lifetime. The early Kellerville supercell periodically produces mesocyclones along an inflow flank updraft early in its lifetime. This is a radically different evolution than the cyclic mesocyclogenesis proposed by Burgess et al. (1982) and described in Alderman et al.'s numerical study where new mesocyclones form on the surging rear-flank gust front.

These observations have prompted Dowell and Bluestion (2000) to propose a new hieracrchy of mesocyclogenesis in supcells that has two modes of cyclic mesocyclone production with a long-lived "steady" mesocyclone mode between the two cyclic modes. This paper will study this evolution by idealizing the 8 June 1995 environment and varying the parameters of the environment in an attempt to generate the various modes of mesocyclone formation in a modeled supercell storm. Previous studies by the first author have generated this type of cyclic behavior in numerical simulations using environments similar to the 8 June case. Results from this work could help forecasters anticipate the timing and location for the formation of new mesocyclone cores in long-lived tornadic supercells.