Conceptual models of cyclic supercell tornadogenesis

At the previous Severe Local Storms Conference, we provided examples of a preliminary analysis of a storm that produced a family of tornadoes near McLean, Texas on 8 June 1995. Our analysis of this case from VORTEX-95 is now complete. A comparison of the 8 June 1995 case to previous studies suggests that there are two common modes of cyclic supercell tornadogenesis (where "cyclic tornadogenesis" refers to the production of a series of tornadoes by a single supercell thunderstorm).

In the first conceptual model of cyclic tornadogenesis, which we call an "outflow dominated" mode, the main storm updraft moves discretely. Each successive updraft and its associated tornado do not persist indefinitely because outflow eventually surrounds them, cutting off the supply of liftable, potentially buoyant air. The storm itself may persist, however, if a new updraft forms at the leading edge of the outflow that has surged ahead of the previous tornado. This traditional model of cyclic vortex formation has been described previously by Burgess et al. (1982), Klemp (1987), and Adlerman et al. (1999).

In our second conceptual model, which we call an "inflow dominated" mode, the main storm updraft essentially propagates continuously. In this case, the low-level features are simply unable to keep up with the motion of the updraft. Each individual tornado that forms is carried rearward, away from the main updraft, by the low-level front-to-rear storm inflow. We believe this process often occurs in relatively young cyclic supercells in which a strong cold pool is not yet established. This conceptual model appears to be applicable to the early evolution of the 8 June 1995 McLean storm.

Later in the life of the McLean storm, the mean air velocity at low levels beneath the west side of the updraft more closely matched the velocity of the main storm updraft aloft. This apparently led to a "balanced" state in which a long-lived tornado was possible.