THE ARCADIA, OKLAHOMA, STORM OF 17 MAY 1981 - ANALYSIS OF A SUPERCELLDURING TORNADOGENESIS

On 17 May 1981, an extensive dataset was collected for a supercell thu nderstorm that produced an FZ tornado near Arcadia in central Oklahoma . Coordinated dual-Doppler scans of the storm by 10-cm research radars were collected at approximately 5-min intervals from 30 min before th e tornado touched down until 15 min after the tornado had dissipated. The Arcadia storm was also well sampled by a 444-m-tall instrumented t ower, The low-level inflow, updraft, mesocyclone, and rear precipitati on core of the supercell all passed across the tower. A comparison of the instrumented tower measurements with a dual-Doppler synthesis reve als that the latter qualitatively resolved the low-level flow. However , the magnitudes of the low-level horizontal winds and updraft speed w ere underestimated. In addition, the vertical shear of the horizontal wind in the lowest kilometer was unresolved in the Doppler winds. In t he storm environment, horizontal vorticity was strong (similar to 1.5 x 10(-2) s(-1)) and approximately streamwise over the depth of the ins trumented tower. Just upstream (northeast) of the updraft, the magnitu de of horizontal vorticity was nearly twice this value and had likely been enhanced by baroclinic generation of horizontal vorticity and/or stretching of horizontal vorticity. Tilting of the resulting horizonta l vorticity into the vertical produced the pretornadic low-level mesoc yclone. Low-level mesocyclone inflow was primarily from the east, but during the tornadic stage, parcels approaching from the north and west were also drawn into the circulation. The tornado formed southeast of the mesocyclone center and near the tip of the reflectivity hook echo while low-level mesocyclone vorticity was increasing. Tornadogenesis occurred near the nose of the rear downdraft within a region of horizo ntal shear between southeasterly inflow into the storm and westerly ou tflow from the rear downdraft. Pressure retrievals suggest the rear do wndraft south of the mesocyclone center was associated with a downward -directed perturbation pressure gradient force. The tornado and the pa rent storm dissipated as outflow surged eastward ahead of the updraft. This case study is the first to include a comparison of independent m easurements of the wind field in and near the low-level mesocyclone of a supercell. The wind analysis is also complemented by the instrument ed tower thermodynamic measurements.