Observations of a Colorado Tornado. Part I: Mesoscale Environment and Tornadogenesis

Abstract

On 2 July 1987 a nonmesocyclone tornado was observed in northeastern Colorado during the Convection Initiation and Downburst Experiment (CINDE). This tornado, reaching FI?F2 intensity, developed under a rapidly growing convective cell, without a preceding supercell or midlevel mesocyclone being present. The pretornado environment on 2 July is described, including observations from a triangle of wind profilers, a dense surface mesonet array, and a special balloon sounding network. Important features contributing to tornado generation include the passage of a 700-mb short-wave trough; the formation of an ?70-km diameter, terrain-induced mesoscale vortex (the Denver Cyclone) and its associated baroclinic zone; the presence of a stationary low-level convergence boundary; and the presence of low-level azimuthal sheer maxima (misovortices) along the boundary. Vorticity budget terms are calculated in the lowest 2 km AGL using a multiple-Doppler radar analysis. These terms and their spatial distributions are compared with observations of mesocyclone-associated supercell tornadoes. Results show that vorticity associated with the 2 July nonsupercell tornado was generated in a more complicated manner than that proposed by previous nonsupercell tornadogenesis theory. In particular, tilting of baroclinically generated streamwise horizontal vorticity into the vertical was important for the formation of low-level rotation, in a manner similar to that previously proposed for supercell tornadic storms.