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 synopticscale 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 lakeaggregate 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.

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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Abstract. Observations of lake-effect storms that occur over the Great Lakes region during late autumn and winter indicate a high sensitivity to ambient wind speed and direction. In this paper, a twodimensional version of the Penn State University/National Center for Atmospheric Research (PSU/ NCAR) model is used to investigate the wind speed effects on lake-effect snowstorms that occur over the Great Lakes region.Theoretical initial conditions for stability, relative humidity, wind velocity, and lake/land temperature distribution are specified. Nine different experiments are performed using wind speeds of U = 0, 2, 4,..., 16 m s -1. The perturbation wind, temperature, and moisture fields for each experiment after 36 h of simulation are compared.It is determined that moderate (4-6m s -1) wind speeds result in maximum precipitation (snowfall) on the lee shore of the model lake. Weak wind speeds (0 ~< U < 4 m s 1) yield significantly higher snowfall amounts over the lake along with a spatially concentrated and intense response. Strong wind speeds (6 < U ~< 16 m s -1) yield very little, if any, significant snowfall, although significant increases in cloudiness, temperature, and perturbation wind speed occur hundreds of kilometers downwind from the lake.

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