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Impacts of the Great Lakes on regional climate conditions

Title
Publication TypeManual Entry
Year of Publication1996
AuthorsScott, R. W., and F. A. Huff
Journal of Great Lakes Research
Volume22
Pagination845-863
Abstract

Estimates of lake-induced spatial changes of six climate variables (precipitation, mean minimum and mean maximum temperatures, cloud cover, vapor pressure, and wind speed) were derived for the entire Great Lakes basin. These patterns were estimated by a comparison of maps of each weather variable using: (1) all regional climate data, and (2) regional data when observations within an 80-km zone around the lakes were removed. Results generally confirm expectations and prior findings, but point to inadequacies in data collection that limit a highly precise analysis. Lake effects are most noticeable in precipitation and temperature and vary considerably by season, time of day, and lake size. Greatest lake influences are found near Lake Superior where up to 100% more precipitation falls downwind of the lake in winter compared to that expected without its presence. During summer, all lakes cause a downwind decrease in rainfall of 10% to 20%. Mean minimum temperatures in the basin are higher in all seasons and over all lakes. Lake-induced reductions in mean maximum temperatures in the region are observed during spring and summer. Effects on cloud cover are greatest during winter and show increases of approximately 25% in areas downwind of Lakes Superior and Michigan. Conversely, the cool summertime waters of Lakes Michigan and Huron reduce cloudiness roughly 10%. Variations in vapor pressure are consistent with observed changes in temperature. Amounts in winter are estimated to be 10% to 15% higher across the center of the basin, but decrease by roughly 5% to 10% at many lake shore sites in summer. Seasonal wind speed data were considered to lack an appropriate number of quality long-term climate stations to determine spatial lake effects. Surface elevations, increasing east of the basin, complicated detection of effects due solely to the lakes.

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Lake Influences

Comments

Much of this influence is due to differences in the heat capacities between the water and land surfaces of the area, and to the large source of moisture the lakes provide to the lower atmosphere.

 

Major effects of the lakes are:

1) To moderate maximum and minimum temperatures of the region in all seasons (with perhaps little net effect on mean temperatures).

2) To increase cloud cover and precipitation over and just downwind of the lakes during winter due to the relatively large heat and moisture source present.

3) To decrease summertime convective clouds and rainfall over the lakes because of the greater atmospheric stability imparted at the surface by the relatively cooler water.

 

Precipitation:

Winter precipitation: A general increase in precipitation is observed from northwest to southeast across the entire area.

Terrain increase, influence increase

Greatest effects are found over eastern Lake Superior and over the land just east of the lake.

 

Summer precipitation: Long-term summer precipitation patterns reveal a general trend of higher precipitation in all directions away from the Great Lakes basin. Distinct minima of precipitation are seen over each lake. The cooler lake waters help stabilize the surface boundary layer above them and inhibit surface-based convection that commonly accompanies summertime precipitation.

Rainfall near Lake Erie shows a less well defined rainfall minimum. Since it is the shallowest of the Great Lakes, it warms faster seasonally. Therefore, its water surface temperature more quickly approximates the air temperature of adjacent land areas, and thus, reduces the negative effect of a cooler lake surface on convective precipitation.

In general, lake-induced precipitation is highest during winter and autumn when lake waters are typically much warmer than the air flowing over them. This scenario provides moisture for enhancing clouds and precipitation. Conversely, beginning in spring over Lakes Michigan and Ontario and spreading to all lakes in summer, the lakes act as a stabilizing force on the lower atmosphere, decreasing convective rainfall.

In all seasons, effects are greatest over Lake Superior, a result of its large size, and east-west orientation. Conversely, being the smallest water body, Lake Erie, as would be expected, creates the least influence.

 

Temperature:

Lake effects on mean seasonal temperatures are very evident due to differences in the heat capacities of land and water. The Great Lakes provide a significant source of warmth to the atmosphere during colder seasons, but conversely, they generally cool the region in summer.

Not unexpectedly, largest and smallest absolute changes in temperature are found over Lakes Superior and Erie, respectively, largely a function of their respective sizes and, thus, the amount of water mass available for air temperature modification.

Cloud:

Cloud frequency is most affected by the lakes during winter due to the vast heat and moisture source the lakes provide to the lower atmosphere into the cold, dry, continental polar air masses moving over them. Maximum effects occur over the larger lakes and tend to reach greatest magnitudes on their eastern sides. Conversely, summertime cloudiness is reduced due to the increased stability imparted to the lower atmosphere from the relatively cooler lake waters

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