Oklahoma Thunderstorms on 29–30 April 1970. Part I: Morphology of a Tornadic Storm

Abstract

Data collected at the National Severe Storms Laboratory reveal the mesogamma-scale (2.5?25 km) features of two severe thunderstorms that struck Oklahoma City within 1 h of each other. This paper discusses the surface, upper air and radar data obtained during the passage of the first tornadic storm (F). Companion papers deal with the second storm (G) which exhibited twin tornado cyclones (Parts II and III), and another discusses the environmental conditions which led to the demise of an earlier hailstorm (Part IV). At the surface, the tornadic supercell storms were characterized by mesocyclonic sinks beneath the main updrafts with convergence values greater than 2 ? 10?3 s?1 and vorticity about half as large. Lowest pressures preceded the mesocyclones by several kilometers and are believed to be dissociated from the wind centers because of the storms' rapid translational speeds (25?32 m s?1). Highest pressures were found near the rainy cores but not coincident. Middle-tropospheric air descended on the southwest (rear) flanks and produced accelerators in the cold air behind the gust fronts. Changes in characteristics of tornadoes associated with three tornado cyclones seem closely related to an evolving interaction between each updraft's mesocyclone and the downdraft-induced vorticity maxima. Just ahead of Storm F, rawinsoundings indicated both dry adiabatic descent near the surface and ascent aloft. Boundary layer divergence associated with the former was caused by differential acceleration of surface layer air as the intense 1.5 hPa mesolow rapidly approached. Dry-adiabatic ascent aloft is believed to be driven by the quasi-steady updraft and is a feature similar to that found in several numerical simulation models. One balloon penetrated Storm F's anvil from 7.5 to 9.9 km some 35?40 km downwind of the main updraft and experienced 71 m s?1 winds in a 4 m s?1 ?residual? updraft. A Richardson number of about 0.3 indicates such convective perturbations may feed from mean flow energy as opposed to residual buoyancy alone.