A decade of cloud-to-ground lightning in Canada: 1999-2008. Part 2: Polarity, multiplicity and first-stroke peak current

We summarize the temporal and spatial characteristics of polarity, multiplicity and first-stroke peak current of approximately 23.5 million cloud-to-ground (CG) lightning flashes detected by the Canadian Lightning Detection Network for the period 1999-2008. Regional differences in these parameters reflect the complex nature and structure of thunderstorms across the country. The annual mean percentage of positive CG flashes was found to be lowest in eastern Canada (11%) and highest in northern Canada (17%). The data do not show any trends over the years in any region. The monthly distribution of positive CG flashes reflects a strong seasonality in all regions, with higher values in winter than in summer. Areas of more than 25% positive flashes are observed along the west coast of British Columbia, in Yukon extending southeast into central British Columbia, in southern Manitoba, northern Quebec, Newfoundland and off the coast of Nova Scotia. The percentages of single-stroke positive and negative flashes for northern (western, eastern) Canada are 93% and 63%, (89% and 48%, 90% and 50%), respectively. The monthly distribution of multiplicity for negative CG flashes peaks between 2 and 2.4 strokes per flash in the summer and early fall in all regions. The multiplicity of positive flashes (slightly higher than 1 stroke per flash) shows little variation throughout the year in all regions. The annual variation of median negative and positive first-stroke peak currents reflects a latitudinal dependence over the past decade. The lowest values for each polarity are observed in southern Canada and the highest values occur in the North. The data do not show any trends in peak currents over the years in the eastern or western regions of Canada. The monthly median first-stroke peak currents for both polarities are strongest in winter and reach a minimum during summer in all regions. Large current flashes ≥100 kA are usually detected in summer and comprise less than 1% of the average annual CG flashes detected in Canada. Large current flashes with stroke multiplicity ≥10 are usually associated with negative polarity. These CG flashes are mostly detected in western Canada.