Abstract
One of the distinguishing characteristics of a supercell thunderstorm is the presence of a rotating updraft. During the past 30 years, various hypotheses have been proposed to explain the initiation and maintenance of rotation. However, attempts to verify the initiation process have been frustrated by the lack of multiple-Doppler radar measurements at the time that the first rotating updraft appears. Discussed in this paper are dual-Doppler radar measurements that successfully captured the initiation and evolution of rotation in the Agawam, Oklahoma, storm of 6 June 1979, which occurred during the storm-scale phase of the Severe Environmental Storms and Mesoscale Experiment (SESAME). The process leading to updraft rotation appears to follow that proposed in 1968 by Fujita and Grandoso, whereby a middle-altitude vorticity couplet formed on the downwind flanks of a strong nonrotating updraft, with cyclonic vertical vorticity on the right-forward flank and anticyclonic vertical vorticity on the left-forward flank. With low-altitude flow approaching the storm from the right, a new updraft developed on the rightward-propagating gust front located along the right edge of the storm beneath the cyclonic vorticity region. The growing updraft acquired cyclonic rotation at middle altitudes by entraining and stretching the ambient vertical vorticity. Subsequent right-flank updrafts in the Agawam storm appear to have developed middle-altitude rotation in the same manner. Based on observations made within the Agawam storm and its immediate environment, the conventional hypothesis that employs low-altitude vertical shear of the horizontal wind as the vorticity source did not likely play a significant role in establishing updraft rotation.
