Exceptional cloud-to-ground lightning during an unusually warm summer in Yukon, Canada

摘要

The 2004 lightning season and related wildfire activity in Yukon, Canada, was exceptional in many aspects. The synoptic environment during the summer was dominated by a persistent upper level ridge over Alaska and Yukon, bringing above-normal temperatures and below-normal precipitation to Yukon. The number of cloud-to-ground (CG) flashes, lightning-initiated forest fires, and extent of the area burned exceeded historic records. Forest fire smoke affected most of Yukon during the summer. Thunderstorms forming in this northern environment in July exhibited unusual lightning characteristics as detected by Canadian Lightning Detection Network. Changes in the frequency of extreme lightning days and in the fraction of nocturnal lightning occurrence were observed. Lightning properties in July differed from climatology in several ways. The central regions of Yukon experienced enhanced positive CG lightning activity. First-stroke positive peak currents were found to be stronger, while negative peak currents were weaker than climatology. Our observations are consistent with the previous findings reported in southerly climates. Although thunderstorms related to the diurnal heating and cooling cycle influenced positive lightning occurrences in Yukon, other possible sources, including pyrocumulonimbus clouds and inverted-polarity thunderstorms, cannot be overlooked. Evidence is presented suggesting that both atmospheric conditions and smoke from the fires may have influenced the electrification process of thunderstorms to enhance +CG production. The extreme summer of 2004 experienced in Yukon may provide a hint of future impacts due to climate change.