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  • Author or Editor: David A. R. Kristovich x
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Luke Bard
and
David A. R. Kristovich

Abstract

One of the most notable ways the Laurentian Great Lakes impact the region’s climate is by augmenting snowfall in downwind locations during autumn and winter months. Among many negative consequences, this surplus of snow can cause substantial property damage to homes and can escalate the number of traffic accident–related injuries and fatalities. The consensus among several previous studies is that lake-effect snowfall increased during the twentieth century in various locations in the Great Lakes region. The goal of this study is to better understand variability and long-term trends in Lake Michigan’s lake-contribution snowfall (LCS). LCS accounts for both lake-effect and lake-enhanced events. In addition, this study updates findings from previous investigations using snowfall observations found by a recent study to be appropriate for climate studies. It is demonstrated that considerable variability exists in 5-yr periods of LCS east and south of Lake Michigan from 1920 to 2005. A general increase in LCS from the early 1920s to the 1950–80 period at locations typically downwind of the lake was found. Thereafter, LCS decreased through the early 2000s, indicating a distinct trend reversal that is not reported by earlier studies. The reasons for this reversal are unclear. The reversal is consistent with observed increasing minimum temperatures during winter months after the 1970s, however.

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David A. R. Kristovich
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David A. R. Kristovich
Open access
Sudheer R. Bhimireddy
and
David A. R. Kristovich

Abstract

This study evaluates the methods of identifying the height zi of the top of the convective boundary layer (CBL) during winter (December and January) over the Great Lakes and nearby land areas using observations taken by the University of Wyoming King Air research aircraft during the Lake-Induced Convection Experiment (1997/98) and Ontario Winter Lake-effect Systems (2013/14) field campaigns. Since CBLs facilitate vertical mixing near the surface, the most direct measurement of zi is that above which the vertical velocity turbulent fluctuations are weak or absent. Thus, we use zi from the turbulence method as the “reference value” to which zi from other methods, based on bulk Richardson number (Ri b ), liquid water content, and vertical gradients of potential temperature, relative humidity, and water vapor mixing ratio, are compared. The potential temperature gradient method using a threshold value of 0.015 K m−1 for soundings over land and 0.011 K m−1 for soundings over lake provided the estimates of zi that are most consistent with the turbulence method. The Ri b threshold-based method, commonly used in numerical simulation studies, underestimated zi . Analyzing the methods’ performance on the averaging window z avg we recommend using z avg = 20 or 50 m for zi estimations for lake-effect boundary layers. The present dataset consists of both cloudy and cloud-free boundary layers, some having decoupled boundary layers above the inversion top. Because cases of decoupled boundary layers appear to be formed by nearby synoptic storms, we recommend use of the more general term, elevated mixed layers.

Significance Statement

The depth zi of the convective atmospheric boundary layer (CBL) strongly influences precipitation rates during lake-effect snowstorms (LES). However, various zi approximation methods produce significantly different results. This study utilizes extensive concurrently collected observations by project aircraft during two LES field studies [Lake-Induced Convection Experiment (Lake-ICE) and OWLeS] to assess how zi from common estimation methods compare with “reference” zi derived from turbulent fluctuations, a direct measure of CBL mixing. For soundings taken both over land and lake; with cloudy or cloud-free conditions, potential temperature gradient (PTG) methods provided the best agreement with the reference zi . A method commonly employed in numerical simulations performed relatively poorly. Interestingly, the PTG method worked equally well for “coupled” and elevated decoupled CBLs, commonly associated with nearby cyclones.

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David A. R. Kristovich
and
Ronald A. Steve III

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.

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Jason M. Keeler
and
David A. R. Kristovich

Abstract

Predictions of lake and sea breezes are particularly important in large coastal population centers because of the circulations’ influence on heat-wave relief, energy use, precipitation, and dispersion of pollutants. While recent numerical modeling studies have suggested that sea or lake breezes should move more slowly through urban areas than in the surrounding suburbs because of urban heat island (UHI) circulations, there have been few quantitative observational studies to evaluate these results. This study utilizes high-resolution Weather Surveillance Radar-1988 Doppler (WSR-88D) observations to determine the effect of the UHI on lake-breeze frontal movement through Chicago, Illinois, and nearby suburban areas. A total of 44 lake-breeze cases from the April–September 2005 period were examined. The inland movement of the lake-breeze front (LBF) was calculated by tracking “fine lines” of radar reflectivity along several cross sections perpendicular to the Lake Michigan shoreline. The average inland propagation speed of the LBF was 5.0 km h−1; there was substantial spatial and temporal variability in LBF propagation, however. Chicago’s UHI magnitude on lake-breeze days exhibited an average nighttime maximum urban–rural temperature difference near 4.5°C and an afternoon minimum near 0°C. The observed daytime UHI magnitude did not have a significant relationship with lake-breeze frontal movement through Chicago. However, the maximum magnitude of the nighttime UHI preceding lake-breeze development was found to be strongly related to a decrease in speed of LBF movement through Chicago’s southwest (inland) suburbs. This relationship is consistent with previous studies of the diurnal evolution of UHI circulations and may represent a useful method for predicting lake-breeze inland movement.

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Nancy E. Westcott
and
David A. R. Kristovich

Abstract

This study focuses on dense fog cases that develop in association with low clouds and sometimes precipitation. A climatology of weather conditions associated with dense fog at Peoria, Illinois, for October–March 1970–94 indicated that fog forming in the presence of low clouds is common, in 57% of all events. For events associated with low pressure systems, low clouds precede dense fog in 84% of cases. Therefore, continental fogs often do not form under the clear-sky conditions that have received the most attention in the literature. Surface cooling is usually observed prior to fog on clear nights. With low cloud bases, warming or no change in temperature is frequent. Thus, fog often forms under conditions that are not well understood, increasing the difficulty of forecasting fog. The possible mechanisms for fog development under low cloud-base conditions were explored for an event when dense fog covered much of Illinois on 7 November 2006. Weather Surveillance Radar-1988 Doppler (WSR-88D) and rawinsonde observations indicated that evaporating precipitation aloft was important in moistening the lower atmosphere. Dense fog occurred about 6 h following light precipitation at the surface. The surface was nearly saturated following precipitation, but relative cooling was needed to overcome weak warm air advection and supersaturate the lower atmosphere. Surface (2 m) temperatures were near or slightly cooler than ground temperatures in most of the region, suggesting surface sensible heat fluxes were not important in this relative cooling. Sounding data indicated drying of the atmosphere above 800 hPa. Infrared satellite imagery indicated deep clouds associated with a low pressure system moved east of Illinois by early morning, leaving only low clouds. It is hypothesized that radiational cooling of the low cloud layer was instrumental in promoting the early morning dense fog.

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Mathieu R. Gerbush
,
David A. R. Kristovich
, and
Neil F. Laird

Abstract

The development of extensive pack ice fields on the Great Lakes significantly influences lake-effect storms and local airmass modification, as well as the regional hydrologic cycle and lake water levels. The evolution of the ice fields and their impacts on the atmospheric boundary layer complicates weather forecasters’ ability to accurately predict late-season lake-effect snows. The Great Lakes Ice Cover–Atmospheric Flux (GLICAF) experiment was conducted over Lake Erie during February 2004 to investigate the surface–atmosphere exchanges that occur over midlatitude ice-covered lakes. GLICAF observations taken by the University of Wyoming King Air on 26 February 2004 show a strong mesoscale thermal link between the lake surface and the overlying atmospheric boundary layer. Mesoscale atmospheric variations that developed over the lake in turn influenced heat exchanges with the surface. Boundary layer sensible and latent heat fluxes exhibited different relationships to variations in surface pack ice concentration. Turbulent sensible heat fluxes decreased nonlinearly with increases in underlying lake-surface ice concentration such that the largest decreases occurred when ice concentrations were greater than 70%. Latent heat fluxes tended to decrease linearly with increasing ice concentration and had a reduced correlation. Most current operational numerical weather prediction models use simple algorithms to represent the influence of heterogeneous ice cover on heat and moisture fluxes. The GLICAF findings from 26 February 2004 suggest that some currently used and planned approaches in numerical weather prediction models may significantly underestimate sensible heat fluxes in regions of high-concentration ice cover, leading to underpredictions of the local modification of air masses and lake-effect snows.

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Robert M. Rauber
,
Michael Garstang
, and
David A. R. Kristovich
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David A. R. Kristovich
,
Luke Bard
,
Leslie Stoecker
, and
Bart Geerts

Abstract

Annual lake-effect snowstorms, which develop through surface buoyant instability and upward moisture transport from the Laurentian Great Lakes, lead to important local increases in snowfall to the south and east. Surface wind patterns during cold-air outbreaks often result in areas where the air is modified by more than one Great Lake. While it is known that boundary layer air that has crossed multiple lakes can produce particularly intense snow, few observations are available on the process by which this occurs. This study examines unique observations taken during the Ontario Winter Lake-effect Systems (OWLeS) field project to document the process by which Lake Erie influenced snowfall that was produced over Lake Ontario on 28 January 2014. During the event, lake-effect clouds and snow that developed over Lake Erie extended northeastward toward Lake Ontario. OWLeS and operational observations showed that the clouds from Lake Erie disappeared (and snow greatly decreased) as they approached the Lake Ontario shoreline. This clear-air zone was due to mesoscale subsidence, apparently due to the divergence of winds moving from land to the smoother lake surface. However, the influence of Lake Erie in producing a deeper lake-effect boundary layer, thicker clouds, increased turbulence magnitudes, and heavier snow was identified farther downwind over Lake Ontario. It is hypothesized that the combination of a low-stability, high-moisture boundary layer as well as convective eddies and limited snow particles crossing the mesoscale subsidence region locally enhanced the lake-effect system over Lake Ontario within the plume of air originating over Lake Erie.

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