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  • Author or Editor: Peter J. Sousounis x
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Peter J. Sousounis
and
J. Michael Fritsch

A high-resolution numerical model is employed to examine effects of the Great Lakes aggregate, defined to be the five major Great Lakes, on regional and synoptic-scale weather. Simulations wherein the effects of the lakes are included and then excluded are performed on a selected cold air outbreak episode during late autumn when the lakes are still ice-free.

Examination of the differences between the model simulations reveals that several dynamical effects result from heating and moistening by the lake aggregate. These effects are manifested primarily in the form of a 4-km-deep, 2000-km-wide, lake-aggregate mesoscaie disturbance (circulation) that develops slowly over the region. The simulated lake-aggregate circulation splits a synoptic-scale high into two distinct centers and redirects and intensifies a weak synoptic-scale low, as verified by existing observations. These modifications of the synoptic-scale environment result in additional precipitation over, downstream, and upwind from the lakes.

The model simulations also reveal that the developing lake-aggregate circulation influences significantly the lake shore surface winds. In some locations, the surface winds switch from onshore to offshore or vice versa. Because it is well known from observations that the location and orientation of lake-induced snow bands are very sensitive to the low-level wind direction over the lakes, it is concluded that the exact locations of heavy snowfall are the result of a complex multiscale interaction among circulations on three different scales: synoptic, individual lake, and lake aggregate.

In addition to the developing primary lake-aggregate circulation, a secondary dynamic response appears at a distant location, adjacent to the eastern seaboard. The organization of this secondary circulation suggests that the lakes may play a direct role in some cases of East Coast cyclogenesis.

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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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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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