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Todd Miner
,
Peter J. Sousounis
,
James Wallman
, and
Greg Mann

An intense cutoff low developed over the Great Lakes during the period 11–15 September 1996. As the low deepened, height falls in the lower troposphere exceeded those at upper levels, the cold-core low evolved into a warm core system, and vertical wind (speed and directional) shear decreased dramatically. The low eventually developed an eye and spiral bands of convective showers. In addition, the cyclone briefly produced tropical storm force winds and excessive rain (>10 cm) that caused flooding. From a satellite perspective, this system bore a striking resemblance to a hurricane. It is believed to be the first time that such a feature has been documented over the Great Lakes.

Because the initially cold-core cyclone moved slowly across the Great Lakes when they were near climatological peak temperature, heat fluxes, particularly latent heat fluxes, were unusually large. For this reason, it is hypothesized that the lakes, especially Lake Huron, played an integral role in the system's development. An analysis of the static stability present during the event suggests that a deep layer of conditional instability allowed lake-modified air parcels to reach altitudes not normally associated with lake-forced convection.

The hypothesis that the heat and moisture fluxes from the Great Lakes played a significant role in the system's development is supported by the following: 1) The cyclone deepened considerably in the presence of very weak baroclinicity, with the most substantial height falls occurring after the system reached Lake Huron. 2) The combination of surface sensible (Fs ) and latent (Fh ) heat fluxes exceeded 700 W m−2 during the low's development. This value is comparable to flux calculations during wintertime arctic air outbreaks over the Great Lakes as well as for polar low cases and category one hurricanes. 3) The low strengthened considerably more at lower levels than at upper levels. 4) The thermal structure of the cyclone appeared to evolve into a warm-core feature from its original cold-core structure, with a significant positive tropospheric thickness anomaly observed over the system's center.

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Greg E. Mann
,
Richard B. Wagenmaker
, and
Peter J. Sousounis

Abstract

Mesoscale disturbances in close proximity to one another typically undergo process interactions, which ultimately may result in the formation of a disturbance on the scale of the combined mesoscale disturbances. Embedded within this combined disturbance, some semblance of the incipient individual mesoscale disturbances may be preserved, especially in instances when the individual forcing mechanisms are fixed in space, as in the case of the Great Lakes. Studies have shown that during prolonged cold air outbreaks, collective lake disturbances can originate from the organization of individual lake-scale disturbances. These collective lake disturbances may, through scale interactions, alter the behavior of the contributing individual lake-scale disturbances and the embedded lake-effect storms. Factor separation decomposition of the Great Lakes system indicates that various interactions among lake-scale processes contribute to the overall development of the regional-scale disturbance, which can modulate embedded lake-effect snowbands. Contributions from these interactions tend to offset the individual lake contributions, especially during the development of the collective lake disturbance, but vary spatially and temporally. As the regional-scale disturbance matures, lake–lake interactions then accentuate the individual lake contributions. Specifically, the modulation of lake-effect snowbands was translational, intensional, and in some instances morphological in nature. Near Lake Michigan, processes attributed to Lake Superior (upstream lake) were direct and synergistic (indirect) resulting in a time delay of maximum snowfall intensity, while processes attributed to the downstream lakes were primarily synergistic resulting in an overall decrease in snowfall intensity. Furthermore, as the collective lake disturbance matured, Lake Superior–induced processes contributed to a significant morphological change in the Lake Michigan lake-effect snowbands.

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Peter J. Sousounis
,
James Wallman
,
Greg E. Mann
, and
Todd J. Miner

Abstract

An intense cutoff low developed over the Great Lakes during the period 13–15 September 1996. The low developed as unseasonably cool air spread over the relatively warm water of the Great Lakes aggregate (i.e., all the Great Lakes). It eventually developed an eye, spiral rainbands, and a warm core, similar to those in a hurricane.

This event presented some forecast challenges for the Nested Grid Model (NGM) and Eta Model and hence for the National Weather Service. The NGM model forecasted a weaker low (999 vs 993 hPa) to be centered east of the observed location, over Lake Huron. The Eta Model forecasted a slightly stronger low (991 vs 993 hPa) to be centered even farther east than did the NGM, over southern Ontario. As a result of the sea level pressure errors, both models also forecasted much weaker winds than were observed over the lakes and much less precipitation around the lakeshores. The coarse resolution in both models likely contributed significantly to these errors.

With-lake (WL) and no-lake (NL) simulations were performed with the National Center for Atmospheric Research–Pennsylvania State University mesoscale model MM5 to determine the impacts of the Great Lakes on development of the low. The WL simulation agreed well with the observations. At the surface, the intensity and position of the WL low was within 1.7 hPa and 70 km at 30 h into the simulation (1800 UTC 14 September 1996), when the observed low was most intense. To the extent that the impact of the Great Lakes can be ascertained through comparison of the simulations, selected WL–NL differences at the surface revealed that the lakes deepened the WL low by ∼5–7 hPa and restricted its movement.

A comparison of WL and NL simulations at upper levels revealed equally impressive differences (e.g., lake-induced perturbations). Strong negative (positive) height and meso-α-scale cyclonic (anticyclonic) wind perturbations at 850 (300) hPa support the hypothesis that the Great Lakes were instrumental in generating a warm core and strong winds near the surface. A comparison of WL–NL differences for this case are compared with those from a more typical wintertime case to illustrate that the WL–NL perturbations can be more intense and can extend to considerably greater depths than in typical winter cases. Strong latent heat fluxes, low static stability, and slow movement (e.g., the cut-off nature) of the synoptic-scale low allowed the strong heating and moistening from the Great Lakes to extend to midtropospheric levels for an extended period of time.

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Peter J. Sousounis
,
Greg E. Mann
,
George S. Young
,
Richard B. Wagenmaker
,
Bradley D. Hoggatt
, and
William J. Badini

Abstract

Despite improvements in numerical weather prediction models, statistical models, forecast decision trees, and forecasting rules of thumb, human interpretation of meteorological information for a particular forecast situation can still yield a forecast that is superior to ones based solely on automated output. While such time-intensive activities may not be cost effective for routine operational forecasts, they may be crucial for the success of costly field experiments. The Lake-Induced Convection Experiment (Lake-ICE) and the Snowband Dynamics Experiment (SNOWBANDS) were conducted over the Great Lakes region during the 1997/98 winter. Project forecasters consisted of members of the academic as well as the operational forecast communities. The forecasters relied on traditional operationally available data as well as project-tailored information from special project soundings and locally run mesoscale models. The forecasting activities during Lake-ICE/SNOWBANDS are a prime example of how the man–machine mix of the forecast process can contribute significantly to forecast improvements over what is available from raw model output or even using traditional operational forecast techniques.

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David A. R. Kristovich
,
George S. Young
,
Johannes Verlinde
,
Peter J. Sousounis
,
Pierre Mourad
,
Donald Lenschow
,
Robert M. Rauber
,
Mohan K. Ramamurthy
,
Brian F. Jewett
,
Kenneth Beard
,
Elen Cutrim
,
Paul J. DeMott
,
Edwin W. Eloranta
,
Mark R. Hjelmfelt
,
Sonia M. Kreidenweis
,
Jon Martin
,
James Moore
,
Harry T. Ochs III
,
David C Rogers
,
John Scala
,
Gregory Tripoli
, and
John Young

A severe 5-day lake-effect storm resulted in eight deaths, hundreds of injuries, and over $3 million in damage to a small area of northeastern Ohio and northwestern Pennsylvania in November 1996. In 1999, a blizzard associated with an intense cyclone disabled Chicago and much of the U.S. Midwest with 30–90 cm of snow. Such winter weather conditions have many impacts on the lives and property of people throughout much of North America. Each of these events is the culmination of a complex interaction between synoptic-scale, mesoscale, and microscale processes.

An understanding of how the multiple size scales and timescales interact is critical to improving forecasting of these severe winter weather events. The Lake-Induced Convection Experiment (Lake-ICE) and the Snowband Dynamics Project (SNOWBAND) collected comprehensive datasets on processes involved in lake-effect snowstorms and snowbands associated with cyclones during the winter of 1997/98. This paper outlines the goals and operations of these collaborative projects. Preliminary findings are given with illustrative examples of new state-of-the-art research observations collected. Analyses associated with Lake-ICE and SNOWBAND hold the promise of greatly improving our scientific understanding of processes involved in these important wintertime phenomena.

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