The Relationship between Cloud-to-Ground Lightning Polarity and Surface Equivalent Potential Temperature during Three Tornadic Outbreaks

Abstract

The relationship between cloud-to-ground (CG) lightning polarity and surface equivalent potential temperature (?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 of thunderstorms whose CG lightning activity was dominated by negative flashes (labeled negative storms) formed in regions of weak ?e gradient and downstream of a ?e maximum. The majority of thunderstorms whose initial CG lightning activity was dominated by positive flashes formed in regions of strong ?e gradient, upstream of a ?e maximum. Some of these storms moved adjacent to the ?e maximum and were dominated by positive CG lightning throughout their lifetimes (labeled ?positive storms?). The other initially positive storms moved through the ?e maximum where their updrafts appeared to undergo intensification. The storms? dominant CG polarity switched from positive to negative after they crossed the ?e maximum (labeled reversal storms). Summary statistics based on this storm 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 CG lightning reversal process may be initiated by rapid updraft intensification brought about by an increase in the buoyancy of low-level inflow air as initially positive storms pass through mesoscale regions of high ?e. As these storms move out of a ?e maximum, massive precipitation fallout 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 redistribution of the electrical charge within the storm resulting in polarity reversal and/or downdraft-induced tornadogenesis.