The important points in the articles below are:
1) The rate of lightning activity decreases with a tornado touchdown.
2) "Most tornadic storms (80% or more) have an increase in total flash rates near the time of the tornado.
3) "The increase in total flash rates is often dominated by intracloud flashes.”
4) In one study 6 out of 42 thunderstorms exhibited a polarity reversal during tornado formation or touchdown.
5) The total number of flashes associated with each storm had no correlation with tornadogenesis.
"we acquired lightning and Doppler radar data on two tornados. The mesocyclone were measured with an L-band radar. In both storms, there was no clear relationship between tornado occurrence and ground flash rates of the storm as a whole, but the stroke rate of each storm was highest after it stopped producing tornadoes..... Intracloud lightning flash rates reached a peak of almost 14 min−1 approximately 10 min after the peak in cyclonic shear at the 6 km level and at the same time as the peak in cyclonic shear at the 1.5 km level. The peak in intracloud rates also occurred 5–10 min after the peak in the area within 40 and 45 dBZ contours at the 8 km level and at about the same time as the peak in the area within 50 dBZ contours at 8 km and within 40 dBZ at 6 km. However, ground flash rates in the mesocyclone region were usually less than 1 min−1 during periods when intracloud rates were high and were negatively correlated with cyclonic shear at both 1.5 and 6 km. The ground flash rate was the last parameter to peak, approximately 15 min after intracloud lightning and a few minutes after the latest reflectivity area (the area having >55 dBZ at the 1 km level).
We suggest that intracloud rates were governed, in part, by particle interactions during the growth in reflectivity at 7–9 km and, in part, by some process associated with the evolution of cyclonic shear at low altitudes. Earlier studies of tornado storms indicate that the evolution of updrafts and downdrafts affects the evolution of both reflectivity and low-altitude cyclonic shear and so, as in previous storm studies, updraft evolution will affect intracloud rates. We suggest that the peaks in ground flash rates resulted from increasing the distance between the main positive and negative charge centers, from the sedimentation of negative charge to lower altitudes, or from the generation or advection of positive charge below the main negative charge....1) most tornadic storms (80% or more) have an increase in total flash rates near the time of the tornado, and 2) the increase in total flash rates is often dominated by intracloud flashes.”
Source: “Lightning Rates Relative to Tornadic Storm Evolution on 22 May 1981,” by Donald R. MacGorman, et al., Journal of Atmospheric Sciences: Vol46,No2, pp221–251.
http://ams.allenpress.com/amsonline....1175/1520-0469(1989)046<0221:LRRTTS>2.0.CO;2
“Cloud-to-ground (CG) lightning patterns were analyzed in 42 violent tornado-producing (F4, F5) supercells that occurred between January 1989 and November 1992. The purpose of this analysis was to identify potential correlations between CG lightning patterns and tornadogenesis. Thirty-one of the storms were characterized by a peak in CG flash rate preceding tornado formation; 20 storms displayed a decrease in CG flash activity coincident with tornado touchdown. Six of the 42 storms exhibited a polarity reversal, from positive to negative, in the sign of the charge lowered to ground. Storms exhibiting a majority of positive flashes were generally associated with long-track tornadoes, F5 damage ratings, or severe weather outbreak conditions. The total number of flashes associated with each storm had no correlation with tornadogenesis (total number of flashes ranged from 16 to 3394).
Based on this analysis, it appears that using CG lightning flash patterns exclusively to detect tornado formation is not practical. The amplitudes of the flash rate changes are too variable to be used as a prognostic tool. The flash rate trends do, however, suggest a recurring relationship to tornado formation. Therefore when used in conjunction with other operational tools, CG flash rate analysis may provide additional information useful in identifying changes in thunderstorm intensity.”
Source: “Characteristics of Cloud-to-Ground Lightning Associated with Violent Tornadoes,” by Perez, Antony H., Wicker, Louis J., Orville, Richard E, Weather and Forecasting, Vol.12 Is.3 pp.428-437.
http://ams.allenpress.com/amsonline...&issn=1520-0434&volume=012&issue=03&page=0428