Oklahoma Thunderstorms on 29–30 April 1970. Part II: Radar-Observed Merger of Twin Hook Echoes

Abstract

National Severe Storms Laboratory radar film reveals twin hook echoes on the southeast flank of a tornadic thunderstorm (Storm G) that struck Oklahoma City on 30 April 1970. Only one mesocyclone is apparent in surface wind analyses (that associated with G's supercell updraft), but both tornado cyclones produced tornadoes. As the two hook echoes merged into one larger hook, damage diminished. Divergence (?2.6 ? 10?3 s?1) shows an overall decrease in updraft strength during the merger. Maximum vorticity (1.3 ? 10?3 s?1) overtook the developing hook echoes and increased slightly in response to an accelerating downdraft impulse from the storm's southwest flank. The multiple-tornado cyclones that developed along the accelerating gust front are believed due to vortex sheet rollup induced partly by the horizontal momentum of the downdraft air and partly by an unusual distribution of pressure-gradient forces behind the gust front. Merger of the two tornado cyclones is believed related to updraft propagation along convergent axes of maximum pressure gradients. One tornado cyclone contained multiple tornadoes orbiting about its center. Analyzed maximum vorticity/convergence ratio, updraft radius estimated from radar signatures and inflow layer depth are used to approximate the ratio of updraft perimeter tangential velocity to mean updraft speed (swirl ratio) which in turn is used to estimate radius of turbulent vortex core (about which multiple vortices orbit). The relationship used to estimate core radii from swirl ratio is that published by Davies-Jones who used Ward's experimental results. For the observed tornado cyclone, independent radius estimates range from 0.68 to 0.84 km, and bracket the 0.7 km radius deduced from damage path characteristics. It is concluded that divergence and vorticity maxima analyzed from wind observations 11 km apart reflect the kinematic properties of the inflowing air that determine tornado character. Evolutions of convergence and vorticity centers for this and three other tornado cyclones suggest that a tornado cyclone's ability to produce vortices in the surface (friction) layer depends on ambient vorticity exceeding a threshold value 10?3 to 10?2 s?1 on a scale from 25 to 2.5 km.