A Satellite Study of Cloud-Band Frequencies over the Great Lakes

Abstract

Lake-effect cloud bands over each of the North American Great Lakes were studied, using five winters of visible satellite data (1988?93) in order to better document the frequency of mesoscale boundary layer flows that led to their development. Several cloud-band classifications, based on boundary layer circulations identified by past authors, were used. The two most common cloud features over the Great Lakes were widespread lake-effect clouds, usually exhibiting multiple wind-parallel bands, and single or double bands parallel to the long axis of the lakes. Wind-parallel bands of lake-cited clouds have been shown in previous studies to form in the updraft regions of boundary layer roll vortices. Cloud bands parallel to the long axis of each of the Great Lakes have been shown to be organized primarily by land breezes. October?March frequencies revealed that clouds were more prevalent over the western lakes (Superior, Michigan, and Huron) than over the eastern lakes (Erie, Ontario) due to differences in the frequencies of lake-induced clouds. The frequency of clouds due to larger-scale systems did not vary appreciably from lake to lake. Lake-induced cloudiness ranged from about 16% of the days over Lake Ontario to about 30% of the days over Lake Superior. Widespread cloudiness was the most frequent lake-effect cloud organization over the Great Lakes, with the exception of Lake Ontario where they occurred about as often as shore-parallel bands. However, their frequency decreased from west to east, with wind-parallel bands occurring nearly twice as often over Lake Superior as over Lake Erie. Bands parallel to the long axis of the lakes were much more common over the eastern lakes than the western lakes. Variations in monthly mean convection band frequencies were documented. Observed frequencies were consistent with the annual cycle of air-lake temperature difference and wind direction trends.