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Neil F. Laird
and
David A. R. Kristovich

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.

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Faye E. Barthold
and
David A. R. Kristovich

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.

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David A. R. Kristovich
,
Neil F. Laird
, and
Mark R. Hjelmfelt

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.

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Joshua J. Schroeder
,
David A. R. Kristovich
, and
Mark R. Hjelmfelt

Abstract

The first detailed observations of the interaction of a synoptic cyclone with a lake-effect convective boundary layer (CBL) were obtained on 5 December 1997 during the Lake-Induced Convection Experiment. Lake-effect precipitation and CBL growth rates were enhanced by natural seeding by snow from higher-level clouds and the modified thermodynamic structure of the air over Lake Michigan due to the cyclone. In situ aircraft observations, project and operational rawinsondes, airborne radar, and operational Weather Surveillance Radar-1988 Doppler data were utilized to document the CBL and precipitation structure for comparison with past nonenhanced lake-effect events. Despite modest surface heat fluxes of 100–200 W m−2, cross-lake CBL growth was greatly accelerated as the convection merged with an overlying reduced-stability layer. Over midlake areas, CBL growth rates averaged more than twice those previously reported for lake-effect and oceanic cold-air outbreak situations. Regions of the lake-effect CBL cloud deck were seeded by precipitation from higher-level clouds over the upwind (western) portions of Lake Michigan before the CBL merged with the overlying reduced-stability layer. In situ aircraft observations suggest that in seeded regions, the CBL was deeper than in nonseeded regions. In addition, average water-equivalent precipitation rates for all of the passes with seeded regions were more than an order of magnitude greater in seeded regions than nonseeded regions because of higher concentration of snow particles of all sizes. A maximum snowfall rate of 4.28 mm day−1 was calculated using aircraft particle observations in seeded regions, comparable to snowfall rates previously reported for lake-effect events, often with much larger surface heat fluxes, but not interacting with synoptic cyclones.

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Joseph A. Grim
,
Neil F. Laird
, and
David A. R. Kristovich
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Yarice Rodriguez
,
David A. R. Kristovich
, and
Mark R. Hjelmfelt

Abstract

Premodification of the atmosphere by upwind lakes is known to influence lake-effect snowstorm intensity and locations over downwind lakes. This study highlights perhaps the most visible manifestation of the link between convection over two or more of the Great Lakes lake-to-lake (L2L) cloud bands. Emphasis is placed on L2L cloud bands observed in high-resolution satellite imagery on 2 December 2003. These L2L cloud bands developed over Lake Superior and were modified as they passed over Lakes Michigan and Erie and intervening land areas. This event is put into a longer-term context through documentation of the frequency with which lake-effect and, particularly, L2L cloud bands occurred over a 5-yr time period over different areas of the Great Lakes region.

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Neil F. Laird
,
David A. R. Kristovich
, and
John E. Walsh

Abstract

An array of 35 idealized mesoscale model simulations was used to examine environmental and surface forcing factors controlling the meso-β-scale circulation structure resulting from cold flow over an isolated axisymmetric body of water at the midlatitudes. Wind speed, lake–air temperature difference, ambient atmospheric stability, and fetch distance were varied across previously observed ranges. Simulated meso-β-scale lake-effect circulations occurred within three basic regimes (e.g., vortices, shoreline bands, widespread coverage), similar to observed morphological regimes. The current study found that the morphological regimes of lake-effect circulations can be predicted using the ratio of wind speed to maximum fetch distance (U/L). Lake-effect environmental conditions producing low values of U/L (i.e., approximately < 0.02 m s−1 km−1) resulted in a mesoscale vortex circulation. Conditions leading to U/L values between about 0.02 and 0.09 m s−1 km−1 resulted in the development of a shoreline band, and U/L values greater than approximately 0.09 m s−1 km−1 produced a widespread coverage event. It was found that transitions from one morphological regime to another are continuous and within transitional zones the structure of a circulation may contain structural features characteristic of more than one regime. Results show that 1) the U/L criterion effectively classifies the morphology independently of the lake–air temperature difference for the parameter value combinations examined and 2) the Froude number, suggested as a potential lake-effect forecasting tool in previous studies, does not permit the unique classification of lake-effect morphology.

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Neil F. Laird
,
L. Jay Miller
, and
David A. R. Kristovich

Abstract

This article presents a detailed examination of the kinematic structure and evolution of the 5 December 1997 winter mesoscale vortex in the vicinity of Lake Michigan using the synthetic dual-Doppler (SDD) technique. When such a mesoscale event propagates a distance large enough that the viewing angle from a single-Doppler radar changes by about 30° and the circulation is sufficiently steady during this time period, then the SDD method can reveal reliable details about the circulation. One such detail of the observed vortex was a pattern of convergence and divergence associated with radial bands, where heavier snowfall was located. Another was the steadiness and vertical coherence of the derived vorticity and convergence patterns within the cyclonic circulation.

On 5 December 1997, the observed reflectivity field remained remarkably steady for nearly 2.5 h as the vortex moved southeastward allowing for the application of the SDD technique. The reflectivity field exhibited a pronounced asymmetric convective structure with at least three well-defined, inward-spiraling radial snowbands, and a distinct weak-reflectivity region or “eye” near the center of cyclonic circulation. The SDD results showed the vortex circulation was composed of a combination of rotation on the meso-β scale and convergence on the meso-γ scale associated with the embedded radial snowbands. Vertical profiles of derived meso-β-scale, area-mean convergence and vorticity suggest that this winter vortex was likely a warm-core system, similar to both tropical cyclones and polar lows.

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Bart Geerts
,
Binod Pokharel
, and
David A. R. Kristovich

Abstract

Winter storms are often accompanied by strong winds, especially over complex terrain. Under such conditions freshly fallen snow can be readily suspended. Most of that snow will be redistributed across the landscape (e.g., behind obstacles), but some may be lofted into the turbulent boundary layer, and even into the free atmosphere in areas of boundary layer separation near terrain crests, or in hydraulic jumps. Blowing snow ice crystals, mostly small fractured particles, thus may enhance snow growth in clouds. This may explain why shallow orographic clouds, with cloud-top temperatures too high for significant ice initiation, may produce (usually light) snowfall with remarkable persistence. While drifting snow has been studied extensively, the impact of blowing snow on precipitation on snowfall itself has not.

Airborne radar and lidar data are presented to demonstrate the presence of blowing snow, boundary layer separation, and the glaciation of shallow supercooled orographic clouds. Further evidence for the presence of blowing snow comes from a comparison between snow size distributions measured at Storm Peak Laboratory (SPL) on Mount Werner (Colorado) versus those measured aboard an aircraft while passing overhead, and from an examination of snow size distributions at SPL under diverse weather conditions. Ice splintering following the collision of supercooled droplets on rimed surfaces such as trees does not appear to explain the large concentrations of small ice crystals sometimes observed at SPL.

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Neil F. Laird
,
John E. Walsh
, and
David A. R. Kristovich

Abstract

Idealized model simulations with an isolated elliptical lake and prescribed winter lake-effect environmental conditions were used to examine the influences of lake shape, wind speed, and wind direction on the mesoscale morphology. This study presents the first systematic examination of variations in lake shape and the interplay between these three parameters. The array of 21 model simulations produced cases containing each of the three classic lake-effect morphologies (i.e., vortices, shoreline bands, and widespread coverage), and, in some instances, the mesoscale circulations were composed of coexisting morphologies located over the lake, near the downwind shoreline, or inland from the downwind shore.

As with lake-effect circulations simulated over circular lakes, the ratio of wind speed (U) to maximum fetch distance (L) was found to be a valuable parameter for determining the morphology of a lake-effect circulation when variations of lake shape, wind speed, and wind direction were introduced. For a given elliptical lake and strong winds, a morphological transform from shoreline band toward widespread coverage accompanied changes in ambient flow direction from along to across the major lake axis. For simulations with weak winds over a lake with a large axis ratio, the morphology of the lake-effect circulation changed from vortex toward shoreline band with a change in wind direction from along to across the major lake axis. Weak winds across lakes with smaller axis ratios (i.e., 1:1 or 3:1) produced mesoscale vortices for each wind direction. Across the array of simulations, a shift in mesoscale lake-effect morphology from vortices to bands and bands toward widespread coverage was attended by an increase in U/L. Last, the elliptical-lake results suggest that the widths of the lake-effect morphological transition zones in U/L parameter space, conditions favorable for the coexistence of multiple morphologies, were greater than for circular lakes.

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