Lightning occurrence patterns over Canada and adjacent United States from Lightning Detection Network observations

Since February 1998 the Canadian Lightning Detection Network has provided continuous lightning detection in space and time to about 65degreesN in the west, 55degreesN in the east, and offshore to about 300 km. From February 1999 the distinction between cloud-to-ground and cloud-to-cloud lightning discharges was added. Approximately 21 million lightning flashes from February 1998 to December 2000 were sorted by equal-area square grid cells with 20-km sides and analyzed for Canada and the adjacent United States. Both cloud-to-ground lightning and cloud-to-cloud lightning were studied, as well as total activity irrespective of lightning discharge type. We derived mean patterns of number of lightning occurrence days, flash density, fraction of cloud-to-cloud lightning, fraction of nocturnal-morning lightning, duration, ratio of positive to negative cloud-to-ground flashes, peak current, and mean multiplicity of cloud-to-ground flashes. A complex national pattern of lightning occurrence was revealed, showing strong regional, diurnal and seasonal dependencies. There are substantial influences by elevated terrain features and major land-water boundaries. Three regions where significant features were seen in lightning patterns were studied in greater detail: the Alberta foothills to western Saskatchewan, southern Ontario and vicinity, and south-east of Nova Scotia. For these areas there were several features observed in all three years or in two of the three years. For those fields which we could compare with results of previous studies for the northern United States, our conclusions were similar even though different years were analyzed. While definition of a true Canadian lightning climatology is likely several years away, we believe we have identified many of its aspects, and where possible we suggest physical reasons for significant features seen.