Characteristics of a tall-structure severe lightning storm

Although initial observations of lightning strikes to the CN Tower commenced in 1978, the comprehensive phase of CN Tower lightning investigations emerged in 1989. By the summer of 1990, measurement stations started to operate to simultaneously record all important parameters of CN Tower lightning flashes, including flash-optical characteristics, return-stroke currents and the associated lightning electromagnetic pulses. This paper investigates a remarkably severe CN Tower lightning storm that contained 52 flashes and lasted for 84 minutes. Flash trajectory images showed that each of the 52 flashes contained an initial continuous current (ICC), proving that all flashes were upward initiated. Current records indicated that 32 flashes (61.5% of all flashes) contained return strokes. Only one of these 32 flashes shows a one-stroke positive flash. The remaining 31 flashes are found to be negative with 1-14 strokes/flash. Statistical analysis clearly indicates that flashes with ICCs and return strokes have higher flash durations in comparison with those for flashes that only contain ICCs. It is also shown that ICCs in flashes containing return strokes have statistically much lower durations in comparison with those in flashes containing only ICCs.