Abstract
The moistening of cold air passing over the Great Lakes of North America has a profound impact on the cool season climate of regions downwind, from relatively benign air mass modification to highly-impactful snowfall events. The importance of lake effects has led to the development of varying techniques for systematically identifying lake-effect days. The results of two such methods are merged here to yield a more thorough record of lake-effect days for the eastern Great Lakes. Comparative analysis of the data sets illustrates the different objectives of the two methodologies, where one identifies days with a synoptic setup conducive to lake-effect snowfall, and the other identifies days with lake-effect modification of the overlying air mass. A smaller population of “absolute” lake-effect days are those identified by both methods, while a larger population of “hybrid” lake-effect days are absolute days plus those identified by one method but not the other. For a 51-year study period ending with the 2014–15 cool season, the absolute data set yields a mean of about 15 lake-effect days per year, or 8% of the November through April season, while the hybrid data set yields a mean of 56 lake-effect days per year, or 31% of the season. The frequencies of absolute, air mass modification-defined, and hybrid lake-effect days decreased through the study period, with days within the hybrid data set declining at a statistically significant rate of 2.8 days per decade, although most obviously from the late 1970s through the early 2000s. The result is a general drying of the cool-season lake-effect hydroclimate. The merged data set offers a more thorough historical record of days available for atmospheric and hydroclimatic study of the lake-effect phenomenon within the eastern Great Lakes region.
Highlights
Like a number of water bodies globally, the Great Lakes of North America (Figure 1) are capable of modifying the thermal and moisture characteristics of the lower atmosphere, altering the weather and climate of areas downwind (Andresen, 2012; Notaro et al, 2013)
We further evaluated the three sets of lake-effect days (TSI+Synoptic Classification (SSC), Temporal Synoptic Index (TSI)-only, and SSC-only) by portraying the magnitude and spatial pattern of precipitation frequency derived from the station-level Global Historical Climate Network (GHCN)Daily database of the National Centers for Environmental Information (NCEI)
The two methods agree on 742 days, or 35.8% of TSI lake-effect days and 48.5% of those identified by the SSC method
Summary
Like a number of water bodies globally, the Great Lakes of North America (Figure 1) are capable of modifying the thermal and moisture characteristics of the lower atmosphere, altering the weather and climate of areas downwind (Andresen, 2012; Notaro et al, 2013). Great Lakes “lake effects” are most distinct early within the cool season, when energy that has accumulated within the lakes during the warmth of the year interacts with southward moving cold air. The advection of cold air across the lakes is typically associated with a rather distinct weather pattern, most often involving some variation of a surface low-pressure center to the east and surface high-pressure to the west (Ellis and Leathers, 1996; Suriano and Leathers, 2017a). The integrated portrayal of atmospheric pressure centers and the air masses arranged around them is referenced as the synoptic atmosphere; the cool season effect of the Great Lakes, with distinct cold air advection between opposing pressure centers, is a phenomenon that lends itself to synoptic atmospheric classification techniques
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