OBSERVATIONS OF A COLORADO TORNADO .1. MESOSCALE ENVIRONMENT AND TORNADOGENESIS

On 2 July 1987 a nonmesocyclone tornado was observed in northeastern C olorado during the Convection Initiation and Downburst Experiment (CIN DE). This tornado, reaching F1-F2 intensity, developed under a rapidly growing convective cell, without a preceding supercell or midlevel me socyclone being present. The pretornado environment on 2 July is descr ibed, including observations from a triangle of wind profilers, a dens e surface mesonet array, and a special balloon sounding network. Impor tant features contributing to tornado generation include the passage o f a 700-mb short-wave trough; the formation of an approximately 70-km diameter, terrain-induced mesoscale vortex (the Denver Cyclone) and it s associated baroclinic zone; the presence of a stationary low-level c onvergence boundary; and the presence of low-level azimuthal shear max ima (misovortices) along the boundary. Vorticity budget terms are calc ulated in the lowest 2 km AGL using a multiple-Doppler radar analysis. These terms and their spatial distributions are compared with observa tions of mesocyclone-associated supercell tornadoes. Results show that vorticity associated with the 2 July nonsupercell tornado was generat ed in a more complicated manner than that proposed by previous nonsupe rcell tornadogenesis theory. In particular, tilting of baroclinically generated streamwise horizontal vorticity into the vertical was import ant for the formation of low-level rotation, in a manner similar to th at previously proposed for supercell tornadic storms.