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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.
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.
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.
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.
Abstract
An investigation of sensible and latent heat fluxes and their relation to synoptic weather events was performed using hourly meteorological measurements from National Data Buoy Center buoy 45003, located in northern Lake Huron, during April–November of 1984. Two temporal heat flux regimes were found to exist over Lake Huron. The first period extended from April through July and was characterized by periods of modest negative (downward) heat fluxes. The second regime was marked by periods of large positive (upward) heat fluxes and occurred from August through November. This later period accounted for 95%–100% of both the total positive sensible and latent heat fluxes. In addition, a comparison of the seasonal evolution of sensible and latent heat fluxes showed the transition from the negative to the positive flux regime occurred 10–20 days earlier for latent heat flux than for sensible heat flux. A notable, statistically significant increase of the surface heat flux variability from the negative to positive flux regimes with a general decrease in the near-surface atmospheric stability during the positive flux regime was found. During both flux regimes, the magnitude of surface sensible and latent heat fluxes remained coupled to transient synoptic-scale weather events. On average, the occurrences of minimum (maximum) heat fluxes preceded time periods of low (high) sea level pressure by 0–3 h during the negative flux regime. For the positive flux regime, maximum (minimum) surface heat fluxes followed the passage of low (high) pressure by approximately 24 h. In addition, maximum (minimum) sensible and latent heat fluxes preceded synoptic high (low) pressure by approximately 16 h. Typical synoptic surface weather patterns were identified for both significant positive and negative heat flux events, time periods when the atmosphere–lake heat exchange was maximized.
Abstract
An investigation of sensible and latent heat fluxes and their relation to synoptic weather events was performed using hourly meteorological measurements from National Data Buoy Center buoy 45003, located in northern Lake Huron, during April–November of 1984. Two temporal heat flux regimes were found to exist over Lake Huron. The first period extended from April through July and was characterized by periods of modest negative (downward) heat fluxes. The second regime was marked by periods of large positive (upward) heat fluxes and occurred from August through November. This later period accounted for 95%–100% of both the total positive sensible and latent heat fluxes. In addition, a comparison of the seasonal evolution of sensible and latent heat fluxes showed the transition from the negative to the positive flux regime occurred 10–20 days earlier for latent heat flux than for sensible heat flux. A notable, statistically significant increase of the surface heat flux variability from the negative to positive flux regimes with a general decrease in the near-surface atmospheric stability during the positive flux regime was found. During both flux regimes, the magnitude of surface sensible and latent heat fluxes remained coupled to transient synoptic-scale weather events. On average, the occurrences of minimum (maximum) heat fluxes preceded time periods of low (high) sea level pressure by 0–3 h during the negative flux regime. For the positive flux regime, maximum (minimum) surface heat fluxes followed the passage of low (high) pressure by approximately 24 h. In addition, maximum (minimum) sensible and latent heat fluxes preceded synoptic high (low) pressure by approximately 16 h. Typical synoptic surface weather patterns were identified for both significant positive and negative heat flux events, time periods when the atmosphere–lake heat exchange was maximized.
Abstract
Forecasters in the Great Lakes region have for several decades recognized a general relationship of wind speed and overlake fetch to lake-effect snowstorm morphology. A recent study using idealized mesoscale model simulations of lake-effect conditions over circular and elliptical lakes showed the ratio of wind speed to maximum fetch distance (U/L) may be used to effectively predict lake-effect snowstorm morphology. The current investigation provides an assessment of the U/L criteria using observational datasets. Previously published Great Lakes lake-effect snowstorm observational studies were used to identify events of known mesoscale morphology. Hindcasts of nearly 640 lake-effect events were performed using historical observations with U/L as the predictor.
Results show that the quantity U/L contains important information on the different mesoscale lake-effect morphologies; however, it provides only a limited benefit when being used to predict mesoscale morphology in real lake-effect situations. The U/L criteria exhibited the greatest probability of detecting lake-effect shoreline band events, often the most intense, but also experienced a relatively large number of false hindcasts. For Lakes Erie and Ontario the false hindcasts and biases were reduced and shoreline band events that occurred under higher wind speed conditions were better identified.
In addition, the Great Lakes Environmental Research Laboratory ice cover digital dataset was used in combination with observations from past events to assess the impact of ice cover on the use of U/L as a predictor of lake-effect morphology. Results show that hindcasts using the U/L criteria were slightly improved when the reduction of open-water areas due to lake ice cover was taken into account.
Abstract
Forecasters in the Great Lakes region have for several decades recognized a general relationship of wind speed and overlake fetch to lake-effect snowstorm morphology. A recent study using idealized mesoscale model simulations of lake-effect conditions over circular and elliptical lakes showed the ratio of wind speed to maximum fetch distance (U/L) may be used to effectively predict lake-effect snowstorm morphology. The current investigation provides an assessment of the U/L criteria using observational datasets. Previously published Great Lakes lake-effect snowstorm observational studies were used to identify events of known mesoscale morphology. Hindcasts of nearly 640 lake-effect events were performed using historical observations with U/L as the predictor.
Results show that the quantity U/L contains important information on the different mesoscale lake-effect morphologies; however, it provides only a limited benefit when being used to predict mesoscale morphology in real lake-effect situations. The U/L criteria exhibited the greatest probability of detecting lake-effect shoreline band events, often the most intense, but also experienced a relatively large number of false hindcasts. For Lakes Erie and Ontario the false hindcasts and biases were reduced and shoreline band events that occurred under higher wind speed conditions were better identified.
In addition, the Great Lakes Environmental Research Laboratory ice cover digital dataset was used in combination with observations from past events to assess the impact of ice cover on the use of U/L as a predictor of lake-effect morphology. Results show that hindcasts using the U/L criteria were slightly improved when the reduction of open-water areas due to lake ice cover was taken into account.
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.
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.
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.
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.
Abstract
While the total snowfall produced in lake-effect storms can be considerable, little is known about how clouds and snow evolve within lake-effect boundary layers. Data collected over Lake Michigan on 10 January 1998 during the Lake-Induced Convection Experiment (Lake-ICE) are analyzed to better understand and quantify the evolution of clouds and snow. On this date, relatively cold air flowed from west to east across Lake Michigan, creating a quasi-steady-state boundary layer that increased from ≈675 to ≈910 m in depth over a distance of 80 km. Once a cloud deck formed 14–18 km from the upwind shoreline, maximum cloud particle concentrations and liquid water content increased from west to east across the lake. Correspondingly, maximum ice water contents, snowfall rates, and maximum snow particle diameters also increased across the lake. Maximum particle concentrations were found below the mean top of the boundary layer and above the cloud base for both cloud and snow particles.
Surprisingly, snow particles were observed 3–7 km upwind of the upwind edge of the lake-effect cloud deck. These snow particles were observed to be rather spatially uniform throughout the boundary layer. Based on available observations, it is hypothesized that of the mechanisms that could produce this snow, the majority of it originated from transient clouds located near the upwind shore. In addition, maximum snow particle concentrations peaked near the middle of the lake before decreasing toward the downwind shore, indicating the location after which aggregation became an important snow growth mechanism. These results show that the evolution of clouds and snow within lake-effect boundary layers may not occur in the uniform manner often depicted in conceptual models.
Abstract
While the total snowfall produced in lake-effect storms can be considerable, little is known about how clouds and snow evolve within lake-effect boundary layers. Data collected over Lake Michigan on 10 January 1998 during the Lake-Induced Convection Experiment (Lake-ICE) are analyzed to better understand and quantify the evolution of clouds and snow. On this date, relatively cold air flowed from west to east across Lake Michigan, creating a quasi-steady-state boundary layer that increased from ≈675 to ≈910 m in depth over a distance of 80 km. Once a cloud deck formed 14–18 km from the upwind shoreline, maximum cloud particle concentrations and liquid water content increased from west to east across the lake. Correspondingly, maximum ice water contents, snowfall rates, and maximum snow particle diameters also increased across the lake. Maximum particle concentrations were found below the mean top of the boundary layer and above the cloud base for both cloud and snow particles.
Surprisingly, snow particles were observed 3–7 km upwind of the upwind edge of the lake-effect cloud deck. These snow particles were observed to be rather spatially uniform throughout the boundary layer. Based on available observations, it is hypothesized that of the mechanisms that could produce this snow, the majority of it originated from transient clouds located near the upwind shore. In addition, maximum snow particle concentrations peaked near the middle of the lake before decreasing toward the downwind shore, indicating the location after which aggregation became an important snow growth mechanism. These results show that the evolution of clouds and snow within lake-effect boundary layers may not occur in the uniform manner often depicted in conceptual models.
Abstract
Large spatial and temporal variations were observed in the location of the upwind cloud edge over Lake Michigan during five westerly wind lake-effect events in November 1995 through January 1996. This study examines the impacts of variations of Lake Michigan surface water temperatures (and corresponding surface fluxes) and upwind static stability on the location of the upwind edge of lake-effect clouds, which develop as cold air crosses the lake during the winter. Data used in this study were collected during the 1995/96 National Weather Service Lake-Effect Snow study. Spatial variations in the location of the upwind lake-effect cloud edge are shown to be related to spatial variations in surface heat and moisture fluxes between the lake surface and overlying air. Surface fluxes are influenced by both the distribution of lake surface water temperatures and variations of surface wind speed, air temperature, and relative humidity. Temporal variations of heat and moisture fluxes from the lake surface and low-level static stability upwind of the lake correlate well with changes in locations of the upwind lake-effect cloud edge. In general, increases in total flux over a particular period tended to correspond with westward change in the position of the upwind cloud edge, whereas decreases in total flux corresponded to eastward shifts of the upwind cloud edge. Atmospheric static stability below the upwind inversion was found to be more important than the inversion height in controlling the location of the upwind cloud edge over the lake, with increases in stability corresponding to eastward shifts in its location.
Abstract
Large spatial and temporal variations were observed in the location of the upwind cloud edge over Lake Michigan during five westerly wind lake-effect events in November 1995 through January 1996. This study examines the impacts of variations of Lake Michigan surface water temperatures (and corresponding surface fluxes) and upwind static stability on the location of the upwind edge of lake-effect clouds, which develop as cold air crosses the lake during the winter. Data used in this study were collected during the 1995/96 National Weather Service Lake-Effect Snow study. Spatial variations in the location of the upwind lake-effect cloud edge are shown to be related to spatial variations in surface heat and moisture fluxes between the lake surface and overlying air. Surface fluxes are influenced by both the distribution of lake surface water temperatures and variations of surface wind speed, air temperature, and relative humidity. Temporal variations of heat and moisture fluxes from the lake surface and low-level static stability upwind of the lake correlate well with changes in locations of the upwind lake-effect cloud edge. In general, increases in total flux over a particular period tended to correspond with westward change in the position of the upwind cloud edge, whereas decreases in total flux corresponded to eastward shifts of the upwind cloud edge. Atmospheric static stability below the upwind inversion was found to be more important than the inversion height in controlling the location of the upwind cloud edge over the lake, with increases in stability corresponding to eastward shifts in its location.
Abstract
Lake-effect snowstorms generally develop within convective boundary layers, which are induced when cold air flows over relatively warm lakes in fall and winter. Mesoscale circulations within the boundary layers largely control which communities near the downwind shores of the lakes receive the most intense snow. The lack of quantitative observations over the lakes during lake-effect storms limits the ability to fully understand and predict these mesoscale circulations. This study provides the first observations of the concurrent spatial and temporal evolution of the thermodynamic and microphysical boundary layer structure and mesoscale convective patterns across Lake Michigan during an intense lake-effect event. Observations analyzed in this study were taken during the Lake-Induced Convection Experiment (Lake-ICE).
Aircraft and sounding observations indicate that the lake-effect snows of 13 January 1998 developed within a convective boundary layer that grew rapidly across Lake Michigan. Boundary layer clouds developed within 15 km and snow developed within 30 km of the upwind (western) shoreline. Near the downwind shore, cloud cover was extensive and snow nearly filled the boundary layer. Extensive sea smoke in the surface layer, with disorganized (or cellular) and linear features, was observed visually across the entire lake. Over portions of northern Lake Michigan, where airborne dual-Doppler radar observations were obtained, the mesoscale circulation structure remained disorganized (random or cellular) across the lake. Given observed shear and stability conditions in this region, this structure is consistent with past theoretical and numerical modeling results. To the south, where surface winds were slightly stronger and lake–air temperature differences were less, wind-parallel bands indicative of rolls were often present.
The horizontal scale of the observed mesoscale convective structures grew across Lake Michigan, in agreement with most previous studies, but less rapidly than the increase of the boundary layer depth. The decreasing ratio of convective horizontal size to boundary layer depth (aspect ratio) is contrary to many recent studies that found a positive correlation between boundary layer depth and aspect ratio.
Abstract
Lake-effect snowstorms generally develop within convective boundary layers, which are induced when cold air flows over relatively warm lakes in fall and winter. Mesoscale circulations within the boundary layers largely control which communities near the downwind shores of the lakes receive the most intense snow. The lack of quantitative observations over the lakes during lake-effect storms limits the ability to fully understand and predict these mesoscale circulations. This study provides the first observations of the concurrent spatial and temporal evolution of the thermodynamic and microphysical boundary layer structure and mesoscale convective patterns across Lake Michigan during an intense lake-effect event. Observations analyzed in this study were taken during the Lake-Induced Convection Experiment (Lake-ICE).
Aircraft and sounding observations indicate that the lake-effect snows of 13 January 1998 developed within a convective boundary layer that grew rapidly across Lake Michigan. Boundary layer clouds developed within 15 km and snow developed within 30 km of the upwind (western) shoreline. Near the downwind shore, cloud cover was extensive and snow nearly filled the boundary layer. Extensive sea smoke in the surface layer, with disorganized (or cellular) and linear features, was observed visually across the entire lake. Over portions of northern Lake Michigan, where airborne dual-Doppler radar observations were obtained, the mesoscale circulation structure remained disorganized (random or cellular) across the lake. Given observed shear and stability conditions in this region, this structure is consistent with past theoretical and numerical modeling results. To the south, where surface winds were slightly stronger and lake–air temperature differences were less, wind-parallel bands indicative of rolls were often present.
The horizontal scale of the observed mesoscale convective structures grew across Lake Michigan, in agreement with most previous studies, but less rapidly than the increase of the boundary layer depth. The decreasing ratio of convective horizontal size to boundary layer depth (aspect ratio) is contrary to many recent studies that found a positive correlation between boundary layer depth and aspect ratio.
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.
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.
