Lightning flash climatology in the southern Great Lakes region

We have made a preliminary study of cloud-to-ground lightning over southern Ontario and the adjoining Great Lakes region. The lightning data set, using magnetic direction finding, is sufficiently accurate to study lightning climatology. Cloud-to-ground flash totals have been found for the three warm seasons 1989-91. A large variation in flash total, lightning-day frequency and number of high flash density storms occurs over the area, with the maximum in southwestern Ontario. The area of the maximum also has a strong diurnal cycle and relatively few positive flashes. Several physical causes may contribute to this. Lake areas usually have slightly fewer flashes than nearby land areas and warm water usually has more flashes than cold water. The Great Lakes do produce more lightning than ocean areas. Convergence lines of lake breezes and other lake circulations can, however, be sites for storms with intense lightning. High surface temperature and moisture leads to an increase in lightning generation. Over land upslope flow increases lightning-producing storms and downslope flow decreases them. High flash density storms may be favoured by smooth rather than rough ground, and by open farmland rather than forest. On the other hand, there does not seem to be a clear urban effect increasing lightning in the Great Lakes area.