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    Features of Hurricane Huron. (a) Visible satellite image valid 1745 UTC 14 Sep 1996. (b) Surface observations valid 1800 UTC 14 Sep 1996. Observations identified by ×s over Lake Huron in (b) correspond to those at buoys 45003 (northern) and 45008 (southern)

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    NGM SLP (solid, 2 hPa), 1000-hPa temperatures (dashed, 2°C), and 1000-hPa winds (full barb = 5 m s−1) from a run initialized at 1200 UTC 13 Sep 1996 valid at (a) 0 h and (b) 30 h

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    Eta Model SLP (solid, 2 hPa), 2-m temperatures (dashed, 2°C), and 10-m winds (full barb = 5 m s−1) from a run initialized at 1200 UTC 13 Sep 1996 valid at (a) 0 h and (b) 30 h

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    Precipitation totals from 1200 UTC 13–15 Sep 1996 for (a) observations, (b) 48-h NGM forecast, and (c) 48-h Eta Model forecast

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    MM5 FGM model SLP (solid, 2 hPa), ground temperatures (dashed, 2°C), and surface winds (full barb = 5 m s−1) from a run initialized at 1200 UTC 13 Sep 1996. (a) CGM representation valid at 0 h. (b) FGM representation valid at 30 h. Shaded regions indicate ground temperatures below 12°C

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    MM5 FGM model forecast of 48-h precipitation totals from a run initialized at 1200 UTC 13 Sep 1996 valid at 48 h to 1200 UTC 15 Sep 1996

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    Heights (solid, 30 m), temperatures (dashed, 1°C) and winds (full barb = 5 m s−1) at 500 hPa valid at 1200 UTC 15 Sep 1996. (a) NGM analysis, (b) MM5 48-h forecast, (c) NGM 48-h forecast, (d) Eta Model 48-h forecast

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    Soundings for Buffalo, NY, valid at 1200 UTC 15 Sep 1996 from (a) observations and (b) MM5 FGM

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    Features of WL, NL, and observed low pressure systems every 6 h beginning at 1200 UTC 13 Sep 1996 (0 h). (a) Central SLP in hPa, (b) positions

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    WL–NL wind and equivalent potential temperature differences at (a) 850, (b) 500, and (c) 300 hPa valid at 48 h. Full barbs indicate 5 m s−1. Contour interval is 2 K in (a) and 1 K in (b) and (c). Heavy contour is 2 K in (a), 1 K in (b), and −1 K in (c). Shading indicates regions where perturbation is >8 K in (a), >4 K in (b), and <−4 K in (c)

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    Heights (solid, 30 m), temperature (dashed, 1°C), and winds (full barb = 5 m s−1) at 300 hPa for (a) WL and (b) NL simulations valid at 27 h

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    Vertical cross section of WL (dashed) and NL (solid) temperatures (2°C interval) from CGM model domain valid at 48 h. Domain extends east–west approximately from North Dakota to a point located several hundred kilometers east of Cape Cod, MA. Variable inner shading indicates approximate location and strength (e.g., dark is strong and light is weak) of warm core in WL simulation and outer shading indicates approximate location of cold core in NL simulation

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    Cross sections of WL–NL differences. (a) Geopotential height (dashed, 10 dm) and equivalent potential temperature (solid, 2 K) differences from Madison, WI (MSN) to Augusta, ME (AUG), from FGM model domain valid at 1200 UTC 15 Sep 1996 (48 h). Heavy contour indicates ±2 K isentrope. Shading indicates equivalent potential temperature differences greater than 2 K. Cross section spans a distance of roughly 1550 km. (b) Equivalent potential temperature (solid, 2 K) differences from latitude and longitude points shown for NOV82 case examined by Sousounis (1997) valid at 0000 UTC 15 Nov 1982 (48 h). Heavy contour indicates ±2 K isentrope. Shading indicates equivalent potential temperature differences greater than 2 K. Cross section spans a distance of roughly 900 km. Line segments at bottom of both panels indicate portions of Great Lakes (projected onto cross section)

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    WL–NL surface heat flux differences valid at 24 h. (a) Latent heat flux difference. (b) Sensible heat flux difference. Contour interval is 100 W m−2

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    Model and climatological temperature soundings at 45°N, 80°W, (a) WL and NL soundings for SEP96 case valid at 0300 UTC 15 Sep 1996 (39 h) and climatological sounding for Sep (1980–89). (b) WL and NL soundings for NOV82 case valid at 0000 UTC 15 Nov 1982 (48 h) and climatological sounding for Dec (1980–89)

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“Hurricane Huron”: An Example of an Extreme Lake-Aggregate Effect in Autumn

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  • 1 University of Michigan, Ann Arbor, Michigan
  • | 2 The Pennsylvania State University, University Park, Pennsylvania
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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.

Corresponding author address: Dr. Peter J. Sousounis, Atmospheric, Oceanic, and Spaces Sciences Dept., University of Michigan, Ann Arbor, MI 48109-2143.

Email: sousou@umich.edu

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.

Corresponding author address: Dr. Peter J. Sousounis, Atmospheric, Oceanic, and Spaces Sciences Dept., University of Michigan, Ann Arbor, MI 48109-2143.

Email: sousou@umich.edu

1. Introduction

Cox (1917) may have been among the first meteorologists to demonstrate that the Great Lakes attract and strengthen lows in winter because heat and moisture is transferred from the lake aggregate (i.e., all the Great Lakes) to the air. From a simple hydrostatic standpoint, this transfer of heat and moisture and its distribution over a broad area reduce the sea level pressure (SLP), which can deepen and alter significantly the tracks of lows passing near the region. Very few studies since Cox (1917) have addressed thoroughly how the Great Lakes influence synoptic-scale systems. A more complete survey of these studies is presented by Sousounis and Fritsch (1994).

All of these lake-aggregate studies have focused on late fall or early winter situations, when the lakes are climatologically warmer than the air so the lower troposphere is “unstable.” The unstable season extends typically from mid-October through late February, when maximum climatological lake–air temperature differences are around 7°–8°C (Eichenlaub 1979). However, it is not uncommon for early season outbreaks of Canadian air to find their way across the lakes to generate unstable conditions and lake-effect weather on a variety of spatial and temporal scales. For example, lake-effect rainstorms (Miner and Fritsch 1997) are common during September and October. These individual lake–scale storms are generated by the same mechanisms that cause lake-effect snowstorms. They are convective in nature, and they are sometimes accompanied by lightning, high winds, and copious rainfall.

The existence of lake-effect rainstorms suggests that the lakes as an aggregate can influence synoptic-scale systems during early fall in a way that is similar to that during winter. Moreover, this aggregate effect may be strong, particularly if the system is dynamically weak (e.g., weak positive vorticity advection aloft) or if it is moving slowly across the lakes so that the heat and moisture from the lakes have a greater period of influence. Large lake–air temperature differences, relatively high saturation vapor pressures associated with (climatological) lake surface temperatures (LSTs) near 20°C, and moderate wind speeds can generate combined sensible and latent heat fluxes near 1000 W m−2, which are comparable to those for polar lows and tropical cyclones. These intense surface fluxes can result in significant vertical mixing over deep layers and very windy conditions over the lakes. The strong winds can then respond in a positive way to strengthen the surface fluxes of sensible and latent heat in a manner similar to that described by wind induced surface heat exchange instability (Craig and Gray 1996).

While it is reasonable to assume that the lakes can have a significant impact on synoptic-scale systems in early autumn, they have never before been explored. An ideal candidate for exploration is a case that occurred during the period 11–15 September 1996 (hereafter SEP96). While this period is still considered to be part of the summer season from an astronomical sense, it is usually considered to be part of the early fall season for most meteorological purposes. The meso-α-scale cut-off low that developed was considerably larger and dynamically different than the meso-β-scale and meso-γ-scale vortices that can develop over the lakes during cold air outbreaks in winter (Forbes and Merritt 1984; Pease et al. 1988; Laird 1999). The low moved slowly eastward, then westward, then eastward again across the relatively warm Great Lakes. Lake temperatures were as high as 20°C and 850-hPa temperatures were as low as −5°C, which meant that the 13°C lake–850-hPa air temperature difference necessary for dry neutral stability between 1000 and 850 hPa and for lake-effect snow (Rothrock 1969) was satisfied nearly twofold. Sustained winds at the surface reached 15–18 m s−1 in response to the low that was deepening over Lake Huron and caught many people, including forecasters, by surprise because operational model forecasts did not simulate adequately the strong and gusty winds. Numerous sailors who were participating in a race on Lake St. Claire reported wind gusts to 23 m s−1 that resulted in damage to sailboats and injuries to crew members that required emergency hospital visits. Visible satellite imagery (Fig. 1a) showed that this system resembled a hurricane for several hours because of the eyelike structure and spiral rainbands that it exhibited while it was over Lake Huron. Miner et al. (2000 hereafter referred to as HH) present a more complete description of this unique case. The fact that this system was a considerable challenge to the operational forecasting community and the fact that it likely was influenced significantly by the Great Lakes for an extended period of time make this case an ideal one to study.

The objectives of this paper are 1) to evaluate the performances of some of the operational models at that time and to compare their forecasts of the SEP96 low with that from a high-resolution mesoscale model, 2) to quantify the extent and the magnitude to which the Great Lakes aggregate influenced the SEP96 low, and 3) to compare these aggregate effects in the current case with those from a more typical wintertime case. Section 2 presents model output from the operational numerical weather prediction models as well as excerpts from some of the forecasts that were issued by the U.S. and Canadian weather services in order to demonstrate the challenging nature of the forecast. Section 3 provides an evaluation of a 48-h simulation of the case that was performed with the National Center for Atmospheric Research–Pennsylvania State University (NCAR–PSU) mesoscale model version 5 (MM5). Section 4 presents a comparison of with-lake and no-lake numerical simulations that were run with that model, explanations for some of the lake aggregate effects, and a comparison of those effects with those from a more typical wintertime case. Section 5 provides a summary and conclusions.

2. Operational forecasts of the SEP96 case

Output from the operational models as well as forecasts from the National Weather Service for this event are presented to demonstrate the difficulty of the forecast situation, to help motivate the choice of the case, and to provide a basis for comparison to results that were obtained using the MM5 mesoscale model with higher resolution. The Nested Grid Model (NGM) and Eta Model forecasts are discussed because they typically guide the textual forecasts that are released to the public. The Aviation Model forecast is not described because it closely resembled that of the NGM.

a. NGM and Eta Model forecast assessments

Figure 1b shows the observed low at 1800 UTC 14 September 1996, when the system was near its peak intensity. It is appropriate to assess the model runs that were initialized 30 h earlier, at 1200 UTC 13 September 1996, because participants in the sailboat race that began the next morning were still deciding early that Friday evening whether or not to participate. Additionally, these model runs were consistent with the MM5 runs that will be described in the next section that had to be initialized at this time. A later initialization time would have been within the period of maximum deepening and hence would have resulted in the no-lake run being contaminated by lake-aggregate heating that would have already modified the atmosphere.

A careful surface analysis at 1200 UTC 13 September 1996 (e.g., Fig. 5a in HH) revealed a complicated situation with an occluded front separating two 1004 hPa lows: one over Lake Huron and another one over eastern Pennsylvania. A third weak low was located just off the southern New Jersey coast. Figure 2a shows that the NGM initialization included all three lows: one low over Lake Huron with a central SLP of 1004 hPa, one low in Virginia with a central SLP of 1004 hPa, and one low farther offshore. During the first 12 h of the NGM forecast, the low over Lake Huron weakened as the one over Virginia retrograded northwestward to a position north of Lake Ontario. As a result, the NGM forecasted position of the primary surface low was close to the observed position at 12 h (0000 UTC 14 September 1996) over Lake Huron, but then the NGM low proceeded to slow its westward progression relative to the observed low over the next 30 h. The forecasted center remained east of Lake Huron throughout the 48-h period, which is significant given the storm’s observed rapid intensification once it moved over water. For example, Fig. 2b shows that the 30-h NGM forecast, valid 1800 UTC 14 September 1996, had the center over Georgian Bay, at the base of the Bruce peninsula, while the observed center was located over southern Lake Huron (Fig. 1b). Additionally, the central pressure was about 6 hPa too high (999 vs 993 hPa) during the most rapid intensification and peak intensity of the storm (1800 UTC 14 September 1996). Weakening also took longer in the NGM. By 1200 UTC 15 September 1996, the observed central pressure had risen to 1005 hPa, while the NGM kept the central pressure at 999 hPa (not shown).

Figure 3a shows the Eta Model initialization, which included a 1004-hPa surface low just east of Buffalo and another one just off the southern New Jersey coast. A weak trough was located over Virginia. During the first 24 h of the Eta forecast, the two lows merged over central Pennsylvania and then slowly retrograded northwestward as a single low to a position between Lakes Huron and Ontario. This position was as far west as the Eta forecasted the low to move, this is farther east than was observed and even farther east than it was in the NGM (Fig. 3b). During the 24–48-h forecast period, the low was forecasted to move eastward too quickly. Despite the position errors, the forecast intensity of the surface low was closer to the observed than that from the NGM. At 1800 UTC 14 September 1996, when the observed low was most intense, the Eta Model low was 300 km too far east and the central pressure was 2 hPa too deep (e.g., 991 hPa). The fact that the low was too deep was a characteristic that developed immediately, once the two lows in the initialization merged over central Pennsylvania. The performance of the two models in terms of the observed low over Lake Huron may be summarized as follows: the NGM low position was 200 km too far east and the intensity was 6 hPa too weak; the Eta low position was 300 km too far east and the intensity was 2 hPa too strong.

The NGM and Eta Model forecast errors in the position and intensity of the surface low contributed to (near) surface wind direction and speed errors. Wind speed errors over land were generally less than 3 m s−1 in both models but more significant over water. For example, at 1800 UTC 14 September 1996, winds predicted by the NGM were only about 5–10 m s−1 over Lakes Huron and Erie. Winds predicted by the Eta Model were 8–10 m s−1 over Lake Huron and 10–13 m s−1 over Lake Erie. Observed winds were 15–18 m s−1 with gusts to 23 m s−1 over both lakes (Fig. 1b and Fig. 7b in HH).

An analysis of observed (station measured not grid averaged) 48-h cumulative rainfall ending 1200 UTC 15 September 1996 indicated that both the NGM and Eta forecasts underestimated precipitation totals near Lakes Huron and Erie (Fig. 4). For example, reports of more than 5 cm (2 in.) of rain in the thumb of Michigan (e.g., along the southwestern shores of Lake Huron) and in western New York were common with some isolated reports of 10 cm (4 in.). In contrast, the NGM forecasted amounts for the same time period in these regions were about 2.5 cm (1 in.). The Eta forecasted amounts were better, having forecasted maximum total precipitation amounts over 5 cm over Lake Ontario and southern Ontario. The Eta Model did seem to simulate to some degree the precipitation enhancement along the downwind lakeshores, but the heaviest amounts were too far east, along the north shore of Lake Ontario.

A likely cause for the NGM and Eta Model errors is the coarse grid spacing, which was insufficient to resolve the meso-β-scale precipitation signatures in the observed precipitation distribution along the lakeshores. The spacing may have been too coarse to simulate properly the convergence/divergence patterns over the individual lakes that resulted from the surface fluxes of heat and moisture if not the surface fluxes themselves. The 80-km grid spacing in the NGM meant that only two to three grid points were located over Lake Huron in the east–west direction. This resolution is insufficient to simulate properly the heating (effects) from the individual lakes, as was evidenced by individual lake-scale troughs that were barely discernable over the lakes during the forecast period. The 48-km grid spacing in the Eta Model meant that three to four grid points were located over Lake Huron. This resolution is slightly better than that of the NGM at resolving the heating (effects) from the individual lakes, so effects in terms of generating individual lake-scale troughs were only slightly more evident.

Another potential source for errors is the parameterization schemes. Gallus (1999) evaluated the performance of the Eta Model using different resolutions and different convective schemes. He found that one feature of the Betts–Miller–Janjic convective scheme in that model is that it can be prematurely triggered, which can result in too much (convective) precipitation, too much gridscale drying, and too little subsequent gridscale precipitation. The release of too much latent heat early on may have been the case with the Eta Model. This hypothesis is supported by the facts that the Eta low developed more rapidly than observed, after it merged with another low over central Pennsylvania, and that the model generated too much convective precipitation along the north shore of Lake Ontario. The premature (convective) development may have been fueled by strong surface fluxes from Lakes Erie and Ontario, the subsequent convective release of latent heat, and the associated sea level pressure falls, which may have prevented the low from developing farther west, over Lake Huron.

Other possible causes for errors in both models may have been strong horizontal diffusion that can force synoptic-looking precipitation distributions, improper surface low initialization, incorrectly initialized LSTs, or failure to account for the observed changes in LSTs during the period. Buoy 45008 in southern Lake Huron reported nearly a 7°C decrease over a 36-h period of the event. About half of that occurred in one 4-h period (see HH for details). The impact of this temperature decrease on surface fluxes and the evolution of the surface low will be described in more detail in section 3. The interested reader is referred to Hoke et al. (1989) and Black (1994) for more complete descriptions of the NGM and Eta Models, respectively.

b. Textual forecast assessment

The textual forecasts issued by the National Weather Service Forecast Office (NWSFO) in White Lake, Michigan (DTX), and the Canadian Meteorological Centre in Toronto, Ontario, closely paralleled the operational model forecasts. The forecasts from these two offices were similar, so only the ones from DTX are described.

The marine forecast that was issued from DTX for Lake Huron and Lake St. Claire was certainly an improvement over that from the raw model output, but it still underestimated the observed wind speeds. For example, a small craft advisory was issued on Friday afternoon (13 September 1996) for the coastal waters of Lake Huron. Northwest winds between 8 and 13 m s−1 and wave heights between 1 and 2 m were expected throughout the ensuing 24–36-h forecast period. Recall that observed wind speeds around the center of the storm were near 15 m s−1 with gusts to 23 m s−1 and wave heights were near 3 m as reported by the two buoys on Lake Huron (Fig. 7b in HH). Numerous sailboats that were participating in the Bayview “Night Race,” which is a 50 nautical mile race on Lake St. Claire, also reported sustained winds throughout the afternoon between 15 and 18 m s−1 with gusts to 23 m s−1. Several boats were demasted, and at least one person suffered a concussion as a result of the high winds and very choppy lake surface. Gale warnings and increased wave height forecasts were not issued by DTX until 2100 UTC 14 September 1996, when the system had already passed its peak intensity. In contrast, the NWSFO at Buffalo, New York, had called for these strong winds and waves earlier in the period for eastern Lake Erie, starting with Friday morning’s forecasts, primarily due to an intense lake-effect situation and intense vertical mixing that was anticipated over that region. However, the strong winds that were forecasted for that region did not occur because the system was farther west than anticipated.

In terms of precipitation, the forecast issued by DTX for the Great Lakes region called for mostly showery precipitation with some areas of steadier rain closer to the low center. Recall that many places in the region received 0.635–1.25 cm (0.25–0.5 in.) of rain, except for portions of the thumb and Ontario along the southeastern Lake Huron shoreline, where heavier amounts occurred. Localized flooding occurred in these areas as more than 5 cm (2 in.) of rain fell. Flood warnings were later issued for this area upon receiving these reports (advisories not shown). In contrast, the NWSFO in Buffalo, New York, had called for frequent and significant lake-enhanced rain over western New York where it was thought a significant early season lake-effect event would take place. Rain amounts in excess of 5 cm (2 in.) were reported by spotters, which prompted flood warnings to be issued for areas surrounding Lake Erie.

3. MM5 forecast of the SEP96 case

A 48-h forecast using the NCAR–PSU mesoscale model MM5 was performed to assess the utility of running a high-resolution model on forecast accuracy for an unstable situation over the Great Lakes in autumn. The forecast also provided a control run that was used to assess the impacts of the lake aggregate on development of this system that will be discussed in the next section. The term forecast is used here because no special preprocessing was performed to initialize the model and for convenience, despite the fact that the model simulation was performed after the event occurred.

a. Model information and initialization

A fine grid mesh (FGM) with 20-km grid spacing was surrounded by a coarse grid mesh (CGM) with 60-km grid spacing. The grids were run interactively; information from the FGM was fed to the CGM and the CGM provided boundary conditions for the FGM. The 20-km spacing in the FGM provided 9–10 grid points across Lakes Superior and Huron from north to south and six to seven grid points across Lake Michigan from east to west. The model used 35 vertical (sigma) levels with a model top pressure of pt = 100 hPa. Most of the levels were chosen to be below 700 hPa in order to capture important boundary layer processes. The Blackadar (Blackadar 1979) high-resolution planetary boundary layer scheme was used to simulate conditions near the surface, the Kuo scheme was used to model convection in the CGM, and the Kain–Fritsch (Kain and Fritsch 1990, 1993) scheme was used to model convection in the FGM. Sousounis and Fritsch (1994) used the Kain–Fritsch scheme to simulate a cold air outbreak in November 1982. The choices for the two different convective schemes for the CGM and FGM were appropriate for the resolutions of these domains (Kain and Fritsch 1998).

The model was initialized using surface and upper air data on mandatory levels valid at 1200 UTC 13 September 1996 from the National Centers for Environmental Prediction (NCEP) Historical Archives. The initialization procedure was similar to that described in Sousounis and Fritsch (1994) for their MM4 simulations of a cold air outbreak case in November 1982 (hereafter NOV82). Lake surface temperatures were initialized using satellite-derived data from the Great Lakes Environmental Research Laboratory and buoy data from the National Data Buoy Center. No special or subjective preprocessing (e.g., bogusing), data assimilation, or changes in parameterizations were implemented for this MM5 simulation. More information about the MM5 model is available in Dudhia (1993) and Grell et al. (1994).

b. Model simulation and verification

Figure 5a shows that the MM5 FGM initialization of the SLP field at 1200 UTC 13 September 1996 revealed two 1005-hPa low centers: one over southern Lake Huron and one over northwestern Pennsylvania. The CGM initialization additionally revealed two troughs outside the FGM: one extending off the coast of southern New Jersey and the other extending into Virginia. The MM5 initialization was very close to those in the NGM and Eta Model. The largest difference may have been the weak center of low pressure in northwestern Pennsylvania, which did, however, exist as a weak center of low pressure in south-central Pennsylvania in the surface observations (see Fig. 5 in HH). Other features in the MM5 initialization such as the troughs over Lake Superior and central Ohio also compared well with those in the observations. Their existence in the MM5 initialization was probably a result of the finer grid spacing than that which existed in the other two models. By 12 h (0000 UTC 14 September 1996), the MM5 SLP field showed a more coherent (single) low structure north of Lake Ontario, a behavior similar to that found in the operational models. Over the next 18 h, the MM5 forecasted low retrograded slowly, to a position near Lake Huron. By 1800 UTC 14 September 1996, the low was located along the eastern shore of Lake Huron (Fig. 5b), about 70 km to the east of the observed low for that time (see Fig. 1b). The MM5 SLP minimum was within 1.7 hPa of the observed minimum. During the remainder of the 48-h period, the MM5 low moved more slowly eastward and weakened more slowly than the observed one.

The MM5 forecasted winds in the lowest sigma layer of the model, which was about 20 m above ground level, show close agreement with observed surface winds. Specifically, winds over both land and water were generally within 2–3 m s−1 of the observed values. Winds over land were weaker because of increased friction and reduced surface heating. Winds over the lakes were considerably stronger because of reduced friction and increased surface heating. It is particularly noteworthy that the MM5 forecasted accurately the observed wind speed maximum of 15 m s−1 over southern Lake Huron near buoy 45008 during the time of peak intensity at 1800 UTC 14 September 1996 (Fig. 5b). It is important to mention that the buoy winds on the Great Lakes are measured at 5 m above the lake surface and that observed winds at a height of 10–20 m could have easily been 10%–20% higher. That is, a 16 m s−1 wind at 5 m could have been a 19 m s−1 wind at 20 m, and could have accounted for some of the differences between the MM5 forecasted and observed wind speeds over the lakes. Recall that some of the sailboats on Lake St. Claire, just to the south, had reported winds near 18 m s−1 around that time. These winds were measured for the most part at mast height between 10 and 30 m.

Recall that both the NGM and Eta Model forecasted considerably weaker winds over southern Lake Huron. As a result of good agreement with observations regarding not only surface winds but also surface temperatures and moisture, the surface fluxes that were generated by MM5 compared well with those determined from observations. Table 1 shows surface fluxes from observational data at buoy 45008 and from NGM, Eta Model, and MM5 output taken at the corresponding grid point. Because no moisture information was available from the buoy, a climatological value for relative humidity of 80% (Saulesleja 1986) was assumed at anemometer level. Also, for consistency, the model fluxes in Table 1 were determined using bulk aerodynamic formulas. Two features are important to mention. First, the observed fluxes at the beginning of the time period were significantly higher than those from any of the models. This aspect was likely the result of a cold front that had just passed the buoy (45008) so that winds had a significant ageostrophic component associated with them (directed across the front) and hence a higher speed that was not captured in the model initializations. Second, all of the models generated fluxes that were significantly higher than those observed at the end of the period. The higher fluxes were probably a result of the fact that none of the models accounted for changes in LSTs but could also be simply a result of the missed location of the low center with respect to the buoy. Because lake–air temperature differences during the period of strongest development were 10°–12°C, the observed 5°–6°C drop in lake surface temperatures at buoy 45008 alone could have accounted for a 50% reduction in combined sensible and latent heat fluxes (∼150 W m−2) at the surface (see Fig. 6 in HH). The fact that the surface air became less unstable as a result of the reduced lake–air temperature difference and that stronger winds could not mix down to the surface could account for the 50% reduction in wind speeds and an additional 50% reduction in fluxes. It is also likely that, as the surface winds and fluxes weakened, the central pressure rose and weakened the pressure gradient, further weakening the winds and fluxes. These likelihoods are supported by the fact that surface winds did decrease abruptly over buoy 45008. These two effects likely reduced the observed fluxes significantly at the end of the period.

An analysis of the precipitation forecast shown in Fig. 6 indicates that the MM5 was superior to both the Eta Model and the NGM. For example, the MM5 generated storm total precipitation amounts between 0.635 and 2.5 cm (0.25–1.0 in.) over east-central lower Michigan, and amounts over 10 cm (4 in.) along the southeast shore of Lake Huron in Ontario and northeast of Lake Erie. Model signatures of lake-effect rainbands were also produced along the shores of Lakes Michigan and Erie, in which the Lake Erie band is confirmed by the greater than 13 cm (5 in.) of rain reported in Buffalo, New York (Fig. 4). Lighter amounts were observed on the southeast shores of Lake Michigan.

The MM5 forecast of the position and intensity of the 500-hPa low at 48 h (1200 UTC 15 September 1996) was within 100 km and 1 dam, respectively, of the NGM analyzed (observed) 500-hPa low (Fig. 7).1 The NGM forecasted low was approximately 200 km farther to the northeast and 2 dam deeper than observed. The Eta forecasted low was better located but 4 dam deeper than observed. The relative position and intensity errors in the NGM and Eta Model mirror those that existed at the surface. The higher, more accurately forecasted, 500-hPa heights by the MM5 were likely the result of the heat and moisture from the lake aggregate being distributed correctly (and more deeply) through the atmosphere. Other features at 500 hPa demonstrate the greater accuracy of the MM5 forecast. For example, the northwestward extension of the 500-hPa trough across the western lakes region that existed in the analysis was captured for the most part by MM5. The NGM had only a weak version of this extension, while the Eta Model did not show it at all. Additionally, the 10 m s−1 northwesterly winds over southern Lake Huron were also captured well by MM5. Both the NGM and the Eta Model had forecasted winds to be nearly 15 m s−1 at this time. The weaker winds in the analysis and MM5 forecast may have been the result of deep convective mixing, which may have resulted because of the relatively warm lake surface. This deep mixing not only distributed the heat over a great depth and caused a weaker height gradient and hence weaker geostrophic winds, but it also likely reduced the vertical wind shear. Some evidence that the MM5 simulated the deep mixing better than the other models can be seen by comparing the temperature distributions from these models with the observed one. For example, the NGM had −20°C air surrounding the 500-hPa low and the Eta had −16°C air surrounding the 500-hPa low, while the MM5 and the analysis both had −18°C air surrounding the 500-hPa low at 48 h. In fact, the analysis and the MM5 temperature distributions show a very weak thermal gradient surrounding the low center. While the suggestion of a warm core is evident, it is not visible from the contours. A warm core is more evident in the MM5 temperature distributions at lower levels (e.g., 700 hPa). In contrast, the other two models have very different temperature distributions. The NGM model appears to have a cold core surrounding the low center and the Eta Model appears to have a very baroclinic structure with cold and warm advection on either side of the low center.

Other aspects of the vertical structure of the low were also forecasted well, even at 48 h, despite the higher than observed surface fluxes at buoy 45008. Figure 8 shows that the 48-h model forecast sounding for Buffalo, New York, which is valid at 1200 UTC 15 September 1996, compares very well to the observed sounding. Small differences exist; for example, the intricate structure near the top of the planetary boundary layer (750–700 hPa) is not precisely replicated in the model. However, the MM5 temperatures are within 1°C of the observed temperatures everywhere else below 400 hPa. At 300 hPa, the observed sounding is 2°C cooler than the modeled sounding, which is likely a result of the MM5 inversion being 25 hPa lower than observed. The lower than observed model tropopause suggests that the lake-aggregate heating and moistening was not extending as high into the atmosphere as observed. The accuracy of the temperature structure over Buffalo at 48 h, as well as many other aspects of the forecast, are especially important to demonstrate the efficacy of this model (forecast) because it is over this (eastern Great Lakes) region where the lake-aggregate heat and moisture was accumulating and hence where the impacts of the lakes were likely greatest at the end of the forecast period.

The MM5 forecast more closely resembled the observations than did either the Eta Model or the NGM in many variables, perhaps because of its finer grid spacing. In order to demonstrate further the importance of resolution, an additional MM5 simulation was performed using just one CGM domain with 60-km grid spacing. This simulation showed a weaker surface low, with a minimum SLP of 1001 hPa, within 50 km of the one in the two-way interactive FGM version. Maximum surface wind speeds over Lake Huron were 5–10 m s−1. The results from this stand-alone CGM simulation suggest that the higher resolution in the coupled MM5 run was necessary for capturing the minimum in the central mean sea level pressure and the higher surface winds that were observed, but not necessarily the location. The fact that the stand-alone CGM MM5 simulation forecasted the location of the low very accurately adds credibility to the hypothesis that the errors in the Eta Model forecast may have been compounded by other features in the model besides the 48-km grid spacing.

Warner and Seaman (1990) and Ballentine et al. (1998) have demonstrated the improvement that a high-resolution mesoscale model can provide during lake-effect snow situations over the lakes. It is likely that high resolution was beneficial in this case as well because surface fluxes over the lakes were important. This benefit may have stemmed from two sources. First, while higher resolution by itself does not necessarily lead to a better gridpoint representation of surface fluxes of heat, moisture, and momentum, it does provide for more water points and it does likely create the potential for the model to generate more realistic boundary layer heating, more realistic convergence zones, more realistic vertical motion, and more realistic precipitation in situations when cold air flows across warm lakes. Second, this more realistic representation of individual lake effects means a better representation of aggregate effects, which may include the development or enhancement of a meso-α-scale cyclonic circulation at the surface, and associated meso-α-scale convergence, vertical motion, and precipitation. The meso-α-scale circulation may then contribute to the strength of the fluxes over the individual lakes in a positive feedback loop as mentioned earlier. In short, a high-resolution numerical weather prediction model may perform better over the lakes in unstable situations because it simulates more accurately the multiscale impacts of the Great Lakes.

It is certainly a valuable exercise to explore even further and understand more conceptually the causes for the better performance by MM5 by performing a series of MM5 and/or Eta Model simulations with various resolutions and parameterizations. However, because that exercise is not really within the scope of this paper, the next section will examine some specific aggregate impacts of the lakes and describe possible mechanisms that influenced the characteristics of the low.

4. Effects of the Great Lakes

To isolate the impacts of the Great Lakes on the development of this system, the with-lake (WL) simulation that was described in the previous section was compared to a corresponding no-lake (NL) simulation. The NL simulation involved replacing all of the Great Lakes with surrounding land types (e.g., coniferous forest) and initializing it in a manner similar to that described in Sousounis and Fritsch (1994). It was run for the identical 48-h period: 1200 UTC 13–15 September 1996.

a. WL–NL sea level pressure and surface wind differences

Figure 9 shows the intensities and positions of the WL, NL, and observed surface lows at selected times. By 1800 UTC 14 September 1996 (30 h), the WL low was 5 hPa deeper than the NL low. Additionally, the WL low executed a tighter trajectory, remained to the east of, and moved more slowly than the NL low. Casual inspection of Fig. 9b may suggest that the NL simulation was a better match to the observed conditions than the WL simulation because the NL low was closer to the observed one than was the WL low at 30 h. This feature, however, does not invalidate the simulations. Rather, it suggests that a combination of strong surface fluxes, frictional convergence, rising motion, and latent heat release in the WL simulation may have caused the low to remain closer to the lakeshore for a period of about 6 h instead of moving farther westward over the open water before (stronger) dynamical processes began to effect an eastward movement. Inadequately specified LST gradients may have further contributed to WL track errors. Importantly, the tighter trajectory in the WL simulation suggests that the aggregate heating (and frictional convergence at the surface near the Lake Huron shoreline) provided a greater focusing mechanism for the low, while in the NL simulation the low was steered more by dynamic processes aloft such as vorticity advection from a short wave at 500 hPa (see Fig. 5 in HH). More careful inspection of Fig. 9a reveals the fact that the central SLP of the WL low was less than 2 hPa higher than that in the observed (995.2 vs 993.5 hPa), while the central SLP of the NL low was nearly 7 hPa higher (for the same times). Furthermore, considering the fact that the NL simulation was initialized at 1200 UTC on 13 September, when cold air was already overspreading the lakes, it is likely that the NL low had some residual lake-induced heating at the start of the simulation, so that the WL–NL differences (for many fields) were likely underrepresented. The initialization time of 1200 UTC 13 September 1996 in fact was considered to be a good compromise between starting the simulation at an earlier time, which would have eliminated heat contamination but increased the likelihood of other sources of error, and starting the simulation at a later time, which would have exacerbated the heat contamination problem. The weaker surface low and the absence of (smooth warm) water in the NL simulation were the primary reasons for the lower NL wind speeds, which were nearly 10 m s−1 less than the WL wind speeds over Lake Huron.

Strong easterly flow during the last 12 h, from 0000 to 1200 UTC 15 September 1996, likely resulted in upwelling across eastern Lake Huron and reduced LSTs (see Fig. 8c in HH). These lower LSTs led to reduced surface fluxes and hence to a reduced tendency for the lakes to constrain the movement of the low, which by this time was eastward once again as indicated by observations and by the NL simulation. Because the MM5 model currently does not account for changes in LSTs during a forecast/simulation, the surface fluxes were overforecast during this period, which likely resulted in the low continuing to remain close to the lakes rather than to begin moving eastward.

b. Vertical structure of the WL–NL perturbation

An examination of, and even an explanation for, the WL–NL SLP differences provides only a partial description of how the lake aggregate affected the synoptic-scale low. More insight to the impacts of the lakes on this system can be obtained by examining WL–NL differences (e.g., perturbations) at selected upper levels. For example, at 850 hPa, the heat and moisture from the lake aggregate spread upward and outward quickly. By 24 h, perturbations in equivalent potential temperature (θe) and wind extended across the Great Lakes. By 48 h, θe perturbations exceeding 2 K covered most of the Great Lakes region; strongest θe perturbations ranged from 8 to 10 K over certain locations of western Ontario (see shading in Fig. 10a). A closed cyclonic perturbation circulation, with a width near 700 km, was centered over eastern Lake Huron where the strongest θe perturbations existed and demonstrated the lake-aggregate-scale nature of the perturbation. Perturbation winds just north and south of the center exceeded 10 m s−1.

The perturbation took a little longer to reach 500 hPa, but by 24 h, θe perturbations of 2 K covered small areas near Lakes Michigan, Huron, Erie, and Ontario. In the ensuing 24 h, however, the perturbation spread quickly over a large region. By 48 h, θe perturbations exceeding 1 K covered most of the eastern Great Lakes, extending southeastward from eastern Lake Superior to northwestern Pennsylvania. Strongest θe perturbations ranged from 4 to 6 K over Lake Ontario (see shading in Fig. 10b). The region of positive θe perturbation coincided with a well-defined anticyclonic perturbation circulation with a width near 700 km. Perturbation wind speeds exceeded 10 m s−1 near the center.

Perturbations did not take much longer to reach 300 than 500 hPa. By 24 h, negative θe perturbations exceeding −1 K spanned a region that covered southern Lake Huron, Lake Erie, Lake Ontario, and western Ontario. The negative θe perturbations that existed over eastern Lake Huron were stronger than the positive ones that existed at 500 hPa for the same time. By 48 h, negative θe perturbations exceeding 1 K extended southeastward from a region just north of Lake Superior to western New York and negative θe perturbations from −4 to −7 K existed over southeastern Ontario (see shading in Fig. 10c). The region of negative θe perturbations coincided with a well-defined anticyclonic perturbation circulation with a width near 700 km. Perturbation wind speeds near 8 m s−1 existed near the center, which was located just north of Lake Ontario.

The WL–NL perturbations at 48 h were primarily the result of magnitude (e.g., Lagrangian) differences. For example, recall from Fig. 9a that the magnitude difference at the surface was approximately 5 hPa (or equivalently, 40 m at 1000 hPa) that had developed by 24 h. The magnitude differences that developed at upper levels were slightly less and occurred later. For example, Fig. 11 shows WL and NL 300-hPa heights, winds, and temperatures valid at 27 h (1500 UTC 14 September 1996), just before the surface low reached peak intensity. A comparison between the two illustrates how the WL heights were only just beginning to differ from the NL ones at 300 hPa. Specifically, heights were slightly (e.g., 20 m) higher, more (geostrophic) diffluence was clearly evident over southern Lake Huron, weaker winds covered that region, a warm core was developing over southern Lake Huron, and a ridge, extending from southeastern lower Michigan to the center of the 300-hPa low, was beginning to appear in the WL simulation. The higher heights and the ridge in the WL simulation at 300 hPa shown in Fig. 11 (and at 500 hPa; not shown) were likely the result of deep diabatic heating from the lakes, similar to what happens with a hurricane or any warm core vortex where a deep warm layer near the center of the circulation causes an upward deformation of height surfaces above the level of maximum warm perturbation or heating and anticyclonic wind shear results in reduced cyclonic or (increased) anticyclonic winds at upper levels.

The developing magnitude differences also contributed to location differences. By 48 h, the 300-hPa WL low was centered over eastern Lake Huron with a height of 916 dm while the 300-hPa NL low was centered only 150 km to the east-southeast, over Lake Ontario, with a height of 911 dm. In fact, the NL 300-hPa heights fell from 913 to 911 dm while the WL 300-hPa heights rose from 913 to 916 dm over the 48-h period. The higher heights (and deeper vertical mixing) in the WL simulation also meant that slightly weaker winds, vorticity, and vorticity and temperature advections existed at mid- and upper levels. It is likely that the weaker dynamic forcing at upper levels and the continuous thermal forcing from the lakes over a fixed region in the WL simulation combined to cause a slower and more confined movement of the WL low at all levels at the surface for most of the 48-h period.

The heating and moistening from the Great Lakes was so extensive that it did not just warm the center of an extratropical system, but rather converted a cold core system into a warm core system. Figure 12 is a CGM vertical cross section of WL and NL temperature structures across a region extending eastward from North Dakota to a point located several hundred kilometers east of Cape Cod, Massachusetts, valid at 48 h. The deep synoptic-scale cold core (outer shaded region) of the air mass is clearly evident in the NL simulation. Going from west to east along constant-pressure surfaces across the region, temperatures drop abruptly over Minnesota and then rise abruptly over Massachusetts. The WL temperature structure, however, reveals a warm core (embedded within the cold core)—a sharp temperature rise over the western edge of the lakes, a peak over western New York, and then a gradual decline toward the east—especially at low levels. The width of the warm core (inner shaded region) at any given pressure level is determined by the locations where the horizontal WL temperature gradient is zero. The strength (darkness of inner shading) is determined by the temperature difference between the maximum WL temperature (in the middle) and the average WL temperature at the edges. The warm core weakens near 600 hPa, becomes stronger at 450 hPa, and then weakens again around 350 hPa. Nowhere is the core much stronger than ∼2°C. The weakening around 600 hPa is likely a result of the warming being spread over a larger area so the horizontal temperature distribution is flat rather than exhibiting a localized maximum. The cooler conditions in the WL simulation above 350 hPa are likely the result of 1) the tropopause having been lifted and stable air having been adiabatically cooled, 2) mesoscale upward motion, or 3) convective overshooting.

c. Differences between this case and a wintertime case

A cold air outbreak that occurred over the Great Lakes during 13–15 November 1982 (NOV82) has been studied extensively (Sousounis and Fritsch 1994; Sousounis 1997, 1998). It provides an opportunity, through comparison with the present case, to understand more about the sensitivity of aggregate effects to synoptic conditions. The NOV82 case began with a strongly baroclinic low moving northeastward through the Great Lakes region, bringing cold air southward across the lakes. The cold air persisted, even as a synoptic-scale high moved east across the region and the surface flow changed from northwesterly to southwesterly to southeasterly ahead of an approaching trough. The flow then became northwesterly once again as the weak trough developed into a closed low and moved northeastward across Lake Huron. Combined surface sensible and latent heat fluxes were 200–500 W m−2 for much of the 48-h period (see Fig. 3 in Sousounis 1998). Lake–850-hPa air temperature differences were −20°C (e.g., lake surface temperature for Lake Michigan was +7°C and 850-hPa temperature was −13°C at Green Bay, Wisconsin, at 1200 UTC 13 November; see Fig. 4a in Sousounis and Fritsch 1994). Snow showers were widespread across the region for much of the time (Sousounis and Mann 2000). The NOV82 case was one studied by Weiss and Sousounis (1999) in their climatological evaluation of wintertime effects of the Great Lakes aggregate. The WL–NL SLP perturbation for the NOV82 case was around the median value of 6 hPa, which they found from their study that included more than 200 cases over a 10-year period.

A comparison of the WL–NL perturbations for the SEP96 and NOV82 cases shows distinct differences. First, the WL–NL potential temperature and wind perturbations were significantly stronger in the SEP96 case than those in the NOV82 case. Perturbation θe values in the current case were as high as 12°C at 850 hPa at 36 h. Perturbation θe values in the NOV82 case were only 6°–7°C at 850 hPa. Second, WL–NL perturbations extended to depths that were considerably greater in the SEP96 case than those in the NOV82 case. Perturbation θe values exceeding −2°C in magnitude and perturbation wind speeds exceeding 5 m s−1 in the current case extended to 200 hPa. Such perturbations were basically nonexistent above 600 hPa in the NOV82 case. Third, the anticyclonic perturbation flow collocated with positive isentropic differences at 500 hPa were opposite to those in the NOV82 case at any level, where either cyclonic perturbation flow typically accompanies positive isentropic perturbations (near the surface) or anticyclonic perturbation flow typically accompanies negative isentropic perturbations (near 700 hPa).

The unique features in the present case are illustrated from a different perspective in Fig. 13, which shows at 48 h a vertical cross section of the FGM WL–NL height and θe perturbations extending from Madison, Wisconsin, to Augusta, Maine. Three aspects are noteworthy because they are distinctly different from those in the NOV82 case, a cross section of which is included in Fig. 13 for comparison. First, the fact that negative height perturbations extended above 700 hPa and that positive height perturbations extended as high as 200 hPa in the current case indicates that not only was the WL–NL response deeper than that in the NOV82 case, but that the level of maximum lake-aggregate heating was above 700 hPa. Second, the amplitude of the positive height perturbations at 400 hPa was greater than the amplitude of the negative height perturbations at the surface. Again, this feature is different than what was found in the NOV82 case, which may be related to the lower stability and to the fact that height displacements are easier to achieve at higher altitudes than at lower altitudes because of lower pressure. Third, the fact that a warm core in the perturbation θe field extended to 400 hPa indicates the high altitude to which the perturbation extended. Note in the NOV82 case how the warm plume only extended to 700 hPa and how it was tilted severely downwind because of the strong winds above the surface that were associated with low-level baroclinicity and the eastward advancement of an open wave.

The slow movement of the system, the strong surface heating, and the low static stability likely contributed to the unique WL–NL differences in this case. Additionally, surface frictional differences between the WL and NL simulations likely contributed to differences in characteristics of the WL and NL lows directly above the lakes themselves. The slow movement underscores two important differences between this system and typical wintertime systems (or any other systems for that matter) that move through the Great Lakes Region. First, the generally slow movement of this system was related to a large degree to the cut-off nature of the system and to a lesser degree to the influence of the lakes. Wintertime systems are typically associated with more (background) baroclinicity, have a more open wave structure, and hence move more rapidly across the region. The fact that the residence time over the lakes can be considerably greater for cut-off (nearly barotropic) systems than for open wave (baroclinic) systems means that the time-integrated lake-aggregate effect can be significant for cut-off lows, even if the other conditions such as lake–air temperature difference are not as favorable. Second, because this system was associated with very little wind shear, the heat and moisture from the lakes remained concentrated rather than spread over a large region, as is commonly the case with typical wintertime systems.

Strong surface heating also contributed to the WL–NL differences. For example, the WL simulation exhibited LSTs as high as ∼20°C, surface air temperatures as low as 10°C, and strong (gusty) surface winds exceeding 17 m s−1 over southern Lake Michigan and southern Lake Huron. The NL simulation exhibited ground and surface air temperatures that were nearly identical and surface winds around 7 m s−1 over the same region. The WL–NL differences for the combined surface sensible and latent heat fluxes approached 700 W m−2 at times for the region. While heat fluxes of this magnitude are typical for winter during cold air outbreaks across the Great Lakes, the ratio of latent to sensible heat flux is much different. Figure 14 shows that latent heat flux differences exceeded sensible heat flux differences in the SEP96 case by a factor of 2 at 24 h over all of the lakes except for Lake Superior. The Bowen ratio B ∼0.5 was typical throughout much of the 48-h period. Bowen ratios B ∼ 0.1 are characteristic of tropical cyclones (Anthes 1982). In contrast, during typical winter conditions when the air and lakes are colder, much lower saturation vapor pressures contribute to much lower latent heat fluxes so that the sensible and latent heat fluxes are more nearly equal (B ∼ 1). Such high Bowen ratios are characteristic of polar lows (Shapiro et al. 1987). Because latent heat is released upon condensation (in clouds), the deeper clouds and higher saturation vapor pressures in the SEP96 case contributed to a deeper lake-aggregate response than for typical winter cases. Additionally, the deeper heating and higher level of maximum heating likely contributed more effectively to SLP falls (Hirschberg and Fritsch 1993).

Low static stability in the current WL simulation also likely contributed to the strong surface heat fluxes warming a deep layer of the atmosphere. A comparison of WL and NL soundings for a representative time and location2 over Lake Huron (Fig. 15) illustrates that the temperatures in the WL sounding were higher than those in the NL sounding by several degrees from the surface to ∼400 hPa. The fact that the (warmer) WL temperature sounding nearly follows a moist adiabat over a considerable depth suggests that deep convection was occurring over the region. The WL–NL temperature differences reverse above 400 hPa because the WL tropopause is ∼100 hPa higher than the NL one. Importantly, the NL sounding is less stable relative to that for typical wintertime cases (cf. Figs. 15a and 15b). The low stability and the strong surface fluxes explain the great depth of the WL–NL perturbations. Table 2 summarizes some of the synoptic-scale environmental differences as well as the perturbation differences between this case and the NOV82 case. The environment values are obtained from NL output averaged over the center of the perturbation and the perturbation values are obtained from WL–NL differences near the time of maximum WL–NL surface differences.

5. Summary and conclusions

A case that occurred in early September 1996 that is described in HH is examined to demonstrate the impacts of the Great Lakes aggregate on synoptic-scale systems in the fall. The case was selected because of the forecast challenges that it presented, the considerable lake–air temperature differences that existed at the time, and the spectacular satellite imagery that it provided. The NGM did not sufficiently deepen the storm, bring it back far enough west to keep it in the Lake Huron region, or forecast the winds over the lakes very well. The Eta Model did better with the SLP intensity but had problems with the location and the winds over the water. As a result, the forecasts that were issued from the National Weather Service did not mention the gale force winds over Lakes Huron or Erie until Saturday evening, after they had occurred for most of that day. The winds caught many sailors that were participating in a sailboat race on Lake St. Claire by surprise; several boats were demasted and at least one crew member suffered a concussion. The surface sensible and latent heat fluxes, which exceeded 700 W m−2 at times, suggest that the lakes played a considerable role in generating the gale force winds.

A pair of simulations was performed with the NCAR–PSU mesoscale model MM5 for the period 0000 UTC 13–15 September 1996 to determine the impact of the Great Lakes on some of the characteristics of the low. A with-lake (WL) simulation performed better than the operational models and showed good agreement with observations. The simulated central SLP was only 1.7 hPa too high and the low center was only 70 km too far east at 30 h, when the system was observed to be most intense. The better agreement was in part likely a result of the higher resolution (e.g., 20 vs 48 km in the Eta Model and 80 km in the NGM), which allowed more grid points to be located over the lakes. A no-lake (NL) simulation was performed in a manner similar to that used by Sousounis and Fritsch (1994).

A comparison of the WL and NL simulations at the surface revealed that the lake aggregate deepened the WL surface low by ∼5 hPa and restricted its movement over the eastern lakes region. Specifically, the WL low deepened to 995 hPa and executed a small circular track as it remained near the eastern shore of Lake Huron. A combination of strong surface fluxes, frictional convergence, rising motion, and latent heat release likely caused the low to remain near the shore rather than to move slightly offshore as observed. The NL low remained near 1000 hPa and executed a larger circular track, closer to that which was observed. A comparison of the simulations above the surface revealed lake-aggregate-warmed air and the development of a meso-α-scale cyclonic perturbation circulation at 850 hPa and lake-aggregate-cooled air and the development of a meso-α-scale anticyclonic perturbation circulation at 300 hPa.

These perturbations were qualitatively different than those which have been identified for another case that occurred in November 1982, which is more typical of a wintertime situation. For example, for the NOV82 case that has been documented by Sousounis (1997), the WL–NL θe perturbations were only near 7 K and cyclonic wind perturbations were near 6–7 m s−1 at 850 hPa; perturbations did not extend above 600 hPa, let alone to 300 hPa, and at no level were positive θe perturbations collocated with anticyclonic perturbation winds. The vertical structure of the WL–NL perturbation in this case was the result of strong surface (latent) heating from the lake aggregate, low static stability, and slow synoptic-scale movement (e.g., the cutoff nature) of the low. The latent heating was stronger than that which occurs during wintertime cold air outbreaks with comparable lake–air temperature differences and wind speeds because the high (e.g., 20°C) LSTs accentuated the lake–air specific humidity differences. These characteristics allowed the lake-aggregate heat and moisture to extend to midtropospheric levels and to convert the cold core of the system to a warm core. The warming at upper levels caused heights to rise rather than fall. The height rises weakened the height gradient and the winds, and hence the advections of temperature and vorticity, and so the effectiveness of the dynamical forcing over the surface low.

It is concluded that the lake aggregate can have an impact on early fall synoptic-scale low pressure systems because of lake–air temperature differences that are similar to those that exist in winter over the region. However, it is also concluded that aspects of the synoptic-scale setting and LSTs, which may be more characteristic of fall than winter, can generate a qualitatively different lake-aggregate response.

Acknowledgments

The first author wishes to thank Jon Shefferly of the Bayview Yacht Club in Detroit, Michigan, for the opportunity to experience firsthand the wrath of Hurricane Huron aboard a 35-ft sailboat. This study and an earlier one of Hurricane Huron were motivated by the rapidity with which weather conditions can deteriorate over the Great Lakes in mid-September. This study was funded in part by NSF Grant ATM-9502009.

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Fig. 1.
Fig. 1.

Features of Hurricane Huron. (a) Visible satellite image valid 1745 UTC 14 Sep 1996. (b) Surface observations valid 1800 UTC 14 Sep 1996. Observations identified by ×s over Lake Huron in (b) correspond to those at buoys 45003 (northern) and 45008 (southern)

Citation: Monthly Weather Review 129, 3; 10.1175/1520-0493(2001)129<0401:HHAEOA>2.0.CO;2

Fig. 2.
Fig. 2.

NGM SLP (solid, 2 hPa), 1000-hPa temperatures (dashed, 2°C), and 1000-hPa winds (full barb = 5 m s−1) from a run initialized at 1200 UTC 13 Sep 1996 valid at (a) 0 h and (b) 30 h

Citation: Monthly Weather Review 129, 3; 10.1175/1520-0493(2001)129<0401:HHAEOA>2.0.CO;2

Fig. 3.
Fig. 3.

Eta Model SLP (solid, 2 hPa), 2-m temperatures (dashed, 2°C), and 10-m winds (full barb = 5 m s−1) from a run initialized at 1200 UTC 13 Sep 1996 valid at (a) 0 h and (b) 30 h

Citation: Monthly Weather Review 129, 3; 10.1175/1520-0493(2001)129<0401:HHAEOA>2.0.CO;2

Fig. 4.
Fig. 4.

Precipitation totals from 1200 UTC 13–15 Sep 1996 for (a) observations, (b) 48-h NGM forecast, and (c) 48-h Eta Model forecast

Citation: Monthly Weather Review 129, 3; 10.1175/1520-0493(2001)129<0401:HHAEOA>2.0.CO;2

Fig. 5.
Fig. 5.

MM5 FGM model SLP (solid, 2 hPa), ground temperatures (dashed, 2°C), and surface winds (full barb = 5 m s−1) from a run initialized at 1200 UTC 13 Sep 1996. (a) CGM representation valid at 0 h. (b) FGM representation valid at 30 h. Shaded regions indicate ground temperatures below 12°C

Citation: Monthly Weather Review 129, 3; 10.1175/1520-0493(2001)129<0401:HHAEOA>2.0.CO;2

Fig. 6.
Fig. 6.

MM5 FGM model forecast of 48-h precipitation totals from a run initialized at 1200 UTC 13 Sep 1996 valid at 48 h to 1200 UTC 15 Sep 1996

Citation: Monthly Weather Review 129, 3; 10.1175/1520-0493(2001)129<0401:HHAEOA>2.0.CO;2

Fig. 7.
Fig. 7.

Heights (solid, 30 m), temperatures (dashed, 1°C) and winds (full barb = 5 m s−1) at 500 hPa valid at 1200 UTC 15 Sep 1996. (a) NGM analysis, (b) MM5 48-h forecast, (c) NGM 48-h forecast, (d) Eta Model 48-h forecast

Citation: Monthly Weather Review 129, 3; 10.1175/1520-0493(2001)129<0401:HHAEOA>2.0.CO;2

Fig. 8.
Fig. 8.

Soundings for Buffalo, NY, valid at 1200 UTC 15 Sep 1996 from (a) observations and (b) MM5 FGM

Citation: Monthly Weather Review 129, 3; 10.1175/1520-0493(2001)129<0401:HHAEOA>2.0.CO;2

Fig. 9.
Fig. 9.

Features of WL, NL, and observed low pressure systems every 6 h beginning at 1200 UTC 13 Sep 1996 (0 h). (a) Central SLP in hPa, (b) positions

Citation: Monthly Weather Review 129, 3; 10.1175/1520-0493(2001)129<0401:HHAEOA>2.0.CO;2

Fig. 10.
Fig. 10.

WL–NL wind and equivalent potential temperature differences at (a) 850, (b) 500, and (c) 300 hPa valid at 48 h. Full barbs indicate 5 m s−1. Contour interval is 2 K in (a) and 1 K in (b) and (c). Heavy contour is 2 K in (a), 1 K in (b), and −1 K in (c). Shading indicates regions where perturbation is >8 K in (a), >4 K in (b), and <−4 K in (c)

Citation: Monthly Weather Review 129, 3; 10.1175/1520-0493(2001)129<0401:HHAEOA>2.0.CO;2

Fig. 11.
Fig. 11.

Heights (solid, 30 m), temperature (dashed, 1°C), and winds (full barb = 5 m s−1) at 300 hPa for (a) WL and (b) NL simulations valid at 27 h

Citation: Monthly Weather Review 129, 3; 10.1175/1520-0493(2001)129<0401:HHAEOA>2.0.CO;2

Fig. 12.
Fig. 12.

Vertical cross section of WL (dashed) and NL (solid) temperatures (2°C interval) from CGM model domain valid at 48 h. Domain extends east–west approximately from North Dakota to a point located several hundred kilometers east of Cape Cod, MA. Variable inner shading indicates approximate location and strength (e.g., dark is strong and light is weak) of warm core in WL simulation and outer shading indicates approximate location of cold core in NL simulation

Citation: Monthly Weather Review 129, 3; 10.1175/1520-0493(2001)129<0401:HHAEOA>2.0.CO;2

Fig. 13.
Fig. 13.

Cross sections of WL–NL differences. (a) Geopotential height (dashed, 10 dm) and equivalent potential temperature (solid, 2 K) differences from Madison, WI (MSN) to Augusta, ME (AUG), from FGM model domain valid at 1200 UTC 15 Sep 1996 (48 h). Heavy contour indicates ±2 K isentrope. Shading indicates equivalent potential temperature differences greater than 2 K. Cross section spans a distance of roughly 1550 km. (b) Equivalent potential temperature (solid, 2 K) differences from latitude and longitude points shown for NOV82 case examined by Sousounis (1997) valid at 0000 UTC 15 Nov 1982 (48 h). Heavy contour indicates ±2 K isentrope. Shading indicates equivalent potential temperature differences greater than 2 K. Cross section spans a distance of roughly 900 km. Line segments at bottom of both panels indicate portions of Great Lakes (projected onto cross section)

Citation: Monthly Weather Review 129, 3; 10.1175/1520-0493(2001)129<0401:HHAEOA>2.0.CO;2

Fig. 14.
Fig. 14.

WL–NL surface heat flux differences valid at 24 h. (a) Latent heat flux difference. (b) Sensible heat flux difference. Contour interval is 100 W m−2

Citation: Monthly Weather Review 129, 3; 10.1175/1520-0493(2001)129<0401:HHAEOA>2.0.CO;2

Fig. 15.
Fig. 15.

Model and climatological temperature soundings at 45°N, 80°W, (a) WL and NL soundings for SEP96 case valid at 0300 UTC 15 Sep 1996 (39 h) and climatological sounding for Sep (1980–89). (b) WL and NL soundings for NOV82 case valid at 0000 UTC 15 Nov 1982 (48 h) and climatological sounding for Dec (1980–89)

Citation: Monthly Weather Review 129, 3; 10.1175/1520-0493(2001)129<0401:HHAEOA>2.0.CO;2

Table 1.

Surface fluxes (W m−2) of sensible (bold, italic) and latent heat at buoy 45008 obtained from observations or model output for times shown

Table 1.
Table 2.

Representative values of selected features associated with synoptic scale situation and WL–NL differences for present case and for case examined by Sousounis (1997)

Table 2.

1

The accuracy at upper levels at 48 h is important to demonstrate because the accumulated effects from the lake aggregate as deduced by with-lake–no-lake comparisons are largest at this time. Note also that the NGM analyzed 500-hPa low is 2 dam lower than that from the NCEP reanalysis. This discrepancy is likely a result of the NGM analysis practice to use the previous run’s 12-h forecast as a first guess.

2

Although the soundings in Fig. 15 are for the same point and hence represent an Eulerian perspective, similar magnitude tropopause height differences also existed from a Lagrangian perspective.

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