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Cloud-to-ground (CG) lightning data have been analyzed for the years 2001-09 for North America, which includes Alaska, Canada, and the lower 48 U.S. states. Flashes recorded within the North American Lightning Detection Network (NALDN) are examined. No corrections for detection efficiency variability are made over the 9 yr of the dataset or over the large geographical area comprising North America. There were network changes in the NALDN during the 9 yr, but these changes have not been corrected for nor have the recorded data been altered in any way with the exception that all positive lightning reports with peak currents less than 15 kA have been deleted. Thus, the reader should be aware that secular changes are not just climatological in nature. All data were analyzed with a spatial resolution of 20 km. The analyses presented in this work provide a synoptic view of the interannual variability of lightning observations in North America, including the impacts of physical changes in the network during the 9 yr of study. These data complement and extend previous analyses that evaluate the U.S. NLDN during periods of upgrade. The total (negative and positive) flashes for ground flash density, the percentage of positive lightning, and the positive flash density have been analyzed. Furthermore, the negative and positive first stroke peak currents and the flash multiplicity have been examined. The highest flash densities in Canada are along the U.S.-Canadian border (1-2 flashes per square kilometer) and in the United States along the Gulf of Mexico coast from Texas through Florida (exceeding 14 flashes per square kilometer in Florida). The Gulf Stream is ''outlined'' by higher flash densities off the east coast of the United States. Maximum annual positive flash densities in Canada range primarily from 0.01 to 0.3 flashes per square kilometer, and in the United States to over 0.5 flashes per square kilometer in the Midwest and in the states of Louisiana and Mississippi. The annual percentage of positive lightning to ground varies from less than 2% over Florida to values exceeding 25% off the West Coast, Alaska, and the Yukon. A localized maximum in the percentage of positive lightning in the NALDN occurs in Manitoba and western Ontario, just north of North Dakota and Minnesota. When averaged over North America, first stroke negative median peak currents range from 19.8 kA in 2001 to 16.0 kA in 2009 and for all years, average 16.1 kA. First stroke positive median peak currents range from a high of 29.0 kA in 2008 and 2009 to a low of 23.3 kA in 2003 with a median of 25.7 kA for all years.There is a relatively sharp transition from low to high median negative peak currents along the Gulf and Atlantic coasts of the United States. No sharp transitions are observed for the median positive peak currents. Relatively lower positive peak currents occur throughout the southeastern United States. The highest values of mean negative multiplicity exceed 3.0 strokes per flash in the NALDN with some variation over the 9 yr. Lower val...

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IntroductionContinuous real-time coverage of lightning occurrence across southern Canada has been available since the installation of the Canadian portion of the integrated North American Lightning Detection Network (NALDN) was completed in February 1998. The Canadian Lightning Detection Network (CLDN) consists of 82 sensors that detect lightning over most of Canada to approximately 65°N in the far west, 55°N in the far east, and offshore to about 300 km. Previous information on lightning occurrence was available only in the vicinity of staffed meteorological observing sites or in limited areas covered by provincial agencies (e.g., Clodman and Chisholm, 1996;Kochtubajda et al., 2002). There were vast stretches of the country where knowledge was lacking. The incidence of lightning in Canada has likely been underestimated in traditional climatologies based on observations available before the CLDN was installed. For example, in a study of Atlantic Canada using CLDN data, Lewis (2000) found that during two winters, December to April 1998April -1999April and 1999April -2000 separate thunderstorm days were identified by the ten-station surface weather network but the CLDN network detected 41 separate thunderstorm days over land for the same period. He further noted that since the installation of the CLDN it is apparent that winter lightning occurs far more frequently off the coast of Atlantic Canada than had been previously suspected or forecast. While a true lightning climatology is probably several years away from definition, we believe there are now sufficient data to identify significant features of lightning occurrence patterns over the portion of Canada covered by the CLDN. We show national summary results, and expand in greater detail for three diverse regions. Where possible we suggest physical reasons for significant observed features. Emphasis is on Canadian lightning patterns but we do comment on some interesting features found over the northern United States. In discussions we assume that the reader is familiar with locations, boundaries, and names of political jurisdictions, and major geographical features in both countries.

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. RéSuMé [Traduit par la rédaction] Nous analysons les caractéristiques de densité et d'occurrence de la foudre à partir de plus de 23,5 millions d'éclairs nuage-sol détectés par le Réseau canadien de détection de la foudre (RCDF) entre 1999 et 2008 dans des cellules de surface uniforme de 20 km de côté au Canada. Cette étude a été réalisée pour mettre à jour une analyse effectuée en 2002 avec seulement trois années de données. Les éclairs o...