12.5
Two Modes of Tornadogenesis In Storm A on 3 May 1999
Yvette P. Richardson, Penn State Univ., University Park, PA; and J. Wurman
On 3 May 1999, eight separate storms produced tornadoes over a wide swath of Oklahoma. The first of these, storm A, was associated with fourteen separate tornadoes. The first tornado, A1, formed near Richards Spur, Oklahoma at approximately 21:50 and lasted only a few minutes, causing F0 damage. The second tornado, A2, reportedly occurred minutes later with slightly shorter duration and similar intensity. Approximately an hour later, storm A produced a more significant sixth tornado, A6, that caused F3 damage and lasted over 20 minutes.
Two Doppler on Wheels (DOW) radars recorded the genesis of the initial tornado from distances of 20 km and 30 km. While this distance is too great to resolve detailed structure in the tornado, it is sufficient for examining the overall evolution of the mid- and low-level mesocyclone during tornadogenesis. Although both DOW radars were collecting data, the crossing angle precludes accurate dual-Doppler synthesis. Instead, this study focuses on single-Doppler measurements from the radar with the best data at a particular time. Tornadogenesis coincides with the merger of Storm A and a small cell to its south. Analysis of the changes in vorticity, measured as single-Doppler azimuthal shear of the radial velocity, during this merger will be presented. The azimuthal shear increases rapidly during merger but quickly dissipates, suggesting a simple stretching process.
One DOW radar recorded the genesis of tornado A6 near Cement, Oklahoma from a distance of 3 km approximately 15 minutes prior to tornadogenesis and approximately 7 km during the genesis period. Detailed analysis of the evolution of the reflectivity and velocity fields during this time period will be presented. In particular, we document the rapid evolution of the velocity field in the final minute of tornadogenesis, the relationship between the low- and mid-level mesocyclones, and the relationship between tornadogenesis and evolution of structures in the reflectivity field. These will be compared to those for the earlier, weaker tornadoes.
Session 12, tornadogenesis
Thursday, 15 August 2002, 8:00 AM-10:00 AM
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