Mobile Doppler radar observations of a tornado in a supercell near Bassett, Nebraska, on 5 June 1999. Part I: Tornadogenesis

This two-part paper details an analysis of high-resolution wind and reflectivity data collected by a mobile, W-band Doppler radar: The dataset captures the near-surface life history of a tornado in a supercell in north-central Nebraska on 5 June 1999. The formation of the tornado vortex near the ground is described from a sequence of sector scans ranging from 30-s intervals prior to tornadogenesis to 10-15-s intervals during much of the lifetime of the tornado. Cyclonic vortices of 100-200 m width were found along a bow-shaped line of enhanced radar reflectivity, at what appears to have been the leading edge of a rear-flank gust front. At the time of tornadogenesis, one of these vortices was located just ahead of the nose of the bow-shaped radar echo and a jet, which were embedded within a larger-scale cyclone. At other times, small-scale cyclonic vortices coexisted with the tornado along an arc-shaped line extending to its north and northeast but did not appear to interact with the tornado. The evolution of all vortices and their associated reflectivity signatures was on a timescale shorter than 30 s, indicating that during tornadogenesis the flow pattern was highly unsteady. Mechanisms by which a smaller-scale vortex or vortices and a bow-shaped echo may have played a role in tornadogenesis are suggested. The structure of the tornado vortex near the ground, as a function of time, is discussed in Part II.