A Decade of Cloud-to-Ground Lightning in Canada: 1999-2008. Part 1: Flash Density and Occurrence

Flash density and occurrence features for more than 23.5 million cloud-to-ground (CG) lightning flashes detected by the Canadian Lightning Detection Network (CLDN) from 1999 to 2008 are analyzed on 20 20 km equal area squares over Canada. This study was done to update an analysis performed in 2002 with just three years of data. Flashes were detected throughout the year, and distinct geographic differences in flash density and lightning occurrence were observed. The shape and locations of large scale patterns of lightning occurrence remained almost the same, although some details were different. Flash density maxima occurred at the same locations as found previously: the Swan Hills and Foothills of Alberta, southeastern Saskatchewan, southwestern Manitoba and southwestern Ontario. A region of greater lightning occurrence but relatively low flash density south of Nova Scotia occurred at the same location as reported previously. New areas of higher flash density occurred along the US border with northwestern Ontario and southern Quebec. These appear to be northward extensions of higher flash density seen in the previous study. The greatest average CG flash density was 2.8 flash km(-2) y(-1) in southwestern Ontario, where the greatest single-year flash density (10.3 flash km(-2) y(-1)) also occurred. Prominent flash density minima occurred east of the Continental Divide in Alberta and over the Niagara Escarpment in southern Ontario. Lightning activity is seen to be highly influenced by the length of the season, proximity to cold water bodies and elevation. The diurnal heating and cooling cycle exerted the main control over lightning occurrence over most land areas; however, storm translation and transient dynamic features complicated the time pattern of lightning production. A large portion of the southern Prairie Provinces experienced more than 50% of flashes between 22: 30 and 10: 30 local solar time. The duration of lightning over a 20 20 km square at most locations in Canada is 5-10 h y(-1), although the duration exceeded 15 h y(-1) over extreme southwestern Ontario. Lightning occurred on 15-30 days each year, on average, over most of the interior of the country. The greatest number of days with lightning in a single year was 47 in the Alberta foothills and 50 in southwestern Ontario. Beginning and ending dates of the lightning season show that the season length decreases from north to south; however, there are considerable east-west differences between regions. The season is nearly year-round in the Pacific coastal region, southern Nova Scotia, southern Newfoundland and offshore.