P10.4
Investigating the role of the dynamic pipe effect in tornadogenesis by using a laboratory simulator
Ashley E. Tidwell, Westmoore High School, Oklahoma City, OK
Experimental results using a laboratory model that simulates the flow structure of the supercell thunderstorm have shown a strong dependency on the simulated rear-flank downdraft (RFD) in vortexgenesis, and a high correlation between the strength of the simulated RFD and the strength of the generated vortex. The above experimental results do not necessarily disprove that tornadogenesis could occur with the Dynamic Pipe Effect (DPE), a tornadogenesis model believed to trigger the tornado without the RFD. This is because the strength of mid-level rotation in the simulator, with only the updraft propeller running, may have been too weak for a DPE to form of sufficient strength to trigger vortexgenesis. It is not known for sure if vortexgenesis occurred with the initiation of the simulated RFD and its outflow; or from increased mid-level cyclonic rotation (TVS/DPE) created by the opposing flow of the downdraft propeller; or both. To solve this problem, a new experiment has been designed which uses a reconfiguration of the original laboratory model or Supercell Simulator. This reconfiguration simulates the DPE, modified to agree with the Dynamic Pressure Distribution model for thunderstorms. The results of this new experiment showed that vortices developed primarily when the propeller speeds were high and when the distance between the propellers was small. For large propeller heights (Mode I: DPE), no circulation or radial flow was observed at the surface until just prior to vortexgenesis and then a narrow, weak, short-lived or sporadic vortex was observed. For small propeller heights (Mode II: non-DPE), a large wedge-shaped, intense, long lasting vortex was observed immediately after the propellers came on speed. The conclusion of this experiment is that tornadogenesis can occur with the DPE without the presence of the RFD. The above results indicate that in order to generate DPE-only, Mode I tornadoes, the mesocyclone and associated Tornadic Vortex Signature (TVS) must survive long enough (with no adverse RFD outflow) to allow time for the DPE to intensify and lower to the surface mesocyclone and TVS rotation. Using the additional results of the original simulator experiment, Mode II tornadogenesis is believed to occur primarily as a combination of the TVS/DPE model and the shearing instability (SI) model occurring below the principal updraft and mesocyclone between the RFD outflow and the thunderstorm's inflow (DPE/SI model). Although DPE/SI tornadogenesis may start out as Mode I with the formation and descent of the TVS via the DPE, it ends up Mode II with increased surface convergence created by the strengthening RFD outflow. If the TVS/DPE and RFD/SI combination is especially strong, then the tornado (although rare) could reach F4 to F5 strength.
Poster Session 10, Tornadogenesis
Thursday, 15 August 2002, 3:00 PM-4:30 PM
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