Electrical Evolution of Thunderstorms and Lightning

This presentation uses data collected during the 1999 Study of Electrical Evolution of Thunderstorms (SEET) to investigate two types of interactions between lightning flashes and the evolving electrical structure of a thunderstorm. The first type of interaction involves the effects of lightning on the evolution of the charge structure. The SEET data suggest that lightning charge deposition tends to make the overall charge distribution inside a storm more complicated. This finding helps explain the complicated charge distributions commonly observed with balloon-borne instruments. In the second type of interaction the evolving charge structure influences the lightning activity. It is known that the lightning flashes at the beginning of many thunderstorms are exclusively intracloud flashes. The SEET data suggest that the lack of cloud-to-ground (CG) flashes at the beginning of a storm is due to a relatively weak electric field in the lower part of the storm: the field below the main negative charge region is not strong enough to initiate a lightning flash. As the storm's electrical structure evolves and a sufficiently large region of lower positive charge develops, then initiation of CG flashes can occur. Storms that develop a substantial lower positive charge region early in their life-cycle have CG flashes early, too. Examples of these two types of interactions between electrical evolution and lightning will be presented.