LIGHTNING FLASH CLIMATOLOGY IN THE SOUTHERN GREAT-LAKES REGION

We have made a preliminary study of cloud-to-ground lightning over sou thern Ontario and the adjoining Great Lakes region. The lightning data set, using magnetic direction finding, is sufficiently accurate to st udy lightning climatology. Cloud-to-ground flash totals have been foun d for the three warm seasons 1989-91. A large variation in flash total , lightning-day frequency and number of high flash density storms occu rs over the area, with the maximum in southwestern Ontario. The area o f the maximum also has a strong diurnal cycle and relatively few posit ive flashes. Several physical causes may contribute to this. Lake area s usually have slightly fewer flashes than nearby land areas and warm water usually has more flashes than cold water. The Great Lakes do pro duce more lightning than ocean areas. Convergence lines of lake breeze s and other lake circulations can, however, be sites for storms with i ntense lightning. High surface temperature and moisture leads to an in crease in lightning generation. Over land upslope flow increases light ning-producing storms and downslope flow decreases them. High flash de nsity storms may be favoured by smooth rather than rough ground, and b y open farmland rather than forest. On the other hand, there does not seem to be a clear urban effect increasing lightning in the Great Lake s area.