INFLUENCE OF ATMOSPHERIC STABILITY AND OVER-WATER FETCH ON WINDS OVER THE LOWER GREAT LAKES

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  • 1 Meteorological Service of Canada
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Abstract

Five years of wind observations taken by the research vessel C.C.G.S. Porte Dauphine on Lake Erie and Lake Ontario have been compared to simultaneous observations taken at land stations upwind of the ship. The resultant ratios of over-lake winds/over-land winds have been sorted by speed classes, conditions of atmospheric stability, and the length of the over-water fetch to assess the relative influence of each factor on the over-lake wind.

The effect of changes in atmospheric stability as created by air-water temperature differences is shown quantitatively with results confirming that wind speeds increase over water during unstable conditions and decrease during stable conditions. It is also shown that these changes are greatest in low winds and least in high winds.

An increase in the length of the over-water fetch up to about 25 mi. contributes to an increase in wind speeds during unstable conditions. Under very stable conditions the lake winds become lighter with longer fetches but the changes are highly erratic. This is likely due to intermittent breakdowns of the shallow atmospheric inversion layer associated with the stable conditions. An increase in fetch beyond 25 mi. does not appear to contribute to any further change in speeds.

*Messrs. Dragert's and McIntyre's contributions were made while they were employed with the Meteorological Service as student assistance during the summer of 1965. Both are Honours Mathematics and Physics undergraduates at the University of Toronto.

Abstract

Five years of wind observations taken by the research vessel C.C.G.S. Porte Dauphine on Lake Erie and Lake Ontario have been compared to simultaneous observations taken at land stations upwind of the ship. The resultant ratios of over-lake winds/over-land winds have been sorted by speed classes, conditions of atmospheric stability, and the length of the over-water fetch to assess the relative influence of each factor on the over-lake wind.

The effect of changes in atmospheric stability as created by air-water temperature differences is shown quantitatively with results confirming that wind speeds increase over water during unstable conditions and decrease during stable conditions. It is also shown that these changes are greatest in low winds and least in high winds.

An increase in the length of the over-water fetch up to about 25 mi. contributes to an increase in wind speeds during unstable conditions. Under very stable conditions the lake winds become lighter with longer fetches but the changes are highly erratic. This is likely due to intermittent breakdowns of the shallow atmospheric inversion layer associated with the stable conditions. An increase in fetch beyond 25 mi. does not appear to contribute to any further change in speeds.

*Messrs. Dragert's and McIntyre's contributions were made while they were employed with the Meteorological Service as student assistance during the summer of 1965. Both are Honours Mathematics and Physics undergraduates at the University of Toronto.

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