The relationship between cloud-to-ground lightning polarity and surface equivalent potential temperature during three tornadic outbreaks

The relationship between cloud-to-ground (CGI lightning polarity and surfac e equivalent potential temperature (theta(e)) is examined for the 26 April 1991, Andover-Wichita, Kansas; the 13 March 1990, Hesston, Kansas; and the 28 August 1990, Plainfield, Illinois, tornadic storm events. The majority o f thunderstorms whose CG lightning activity was dominated by negative flash es (labeled negative storms) formed in regions of weak theta(e) gradient an d downstream of a theta(e) maximum. The majority of thunderstorms whose ini tial CG lightning activity was dominated by positive hashes formed in regio ns of strong theta(e) gradient, upstream of a theta(e) maximum. Some of the se storms moved adjacent to the theta(e) maximum and were dominated by posi tive CG lightning throughout their lifetimes (labeled "positive storms"). T he ether initially positive storms moved through the theta(e) maximum where their updrafts appeared to undergo intensification. The storms' dominant C G polarity switched from positive to negative after they crossed the theta( e) maximum (labeled reversal storms). Summary statistics based on this stor m classification show that all the reversal storms examined for these three events were severe and half of them produced tornadoes of F3-F5 intensity. By comparison, only 58% of the negative storms produced severe weather and only 10% produced tornadoes of F3-F5 intensity. It is suggested that the C G lightning reversal process may be initiated by rapid updraft intensificat ion brought about by an increase in the buoyancy of low-level inflow air as initially positive storms pass through mesoscale regions of high theta(e). As these storms move out of a theta(e) maximum, massive precipitation fall out may occur when their updrafts weaken and can no longer support the mass of liquid water and ice aloft. The fallout may in turn cause a major redis tribution of the electrical charge within the storm resulting in polarity r eversal and/or downdraft-induced tornadogenesis.