Potential Future Weather Patterns over the Great Lakes Region

J. Great Lakes Res. 28(4):496–520 Internat. Assoc. Great Lakes Res., 2002

Potential Future Weather Patterns over the Great Lakes Region Peter J. Sousounis* and Emily K. Grover Atmospheric, Oceanic, and Space Sciences The University of Michigan Ann Arbor, Michigan ABSTRACT. Evaluating changes in synoptic patterns is tantamount to understanding regional climate change. To date, the synoptic evaluations that have been done regarding climate change output from General Circulation Models have been restricted mainly to examining changes in storm tracks across large areas (the Atlantic Ocean). In this study, output from the Canadian Coupled Climate Model (CGCM1) and the Hadley Coupled Climate Model (HadCM2) are examined relative to present conditions to evaluate potential changes in synoptic patterns over the Great Lakes region towards the end of this century. These models were used as part of the U.S. National Assessment of Climate Change. Both models show a decrease in the number of extremely cold days, an increase in the number of extremely hot days, and an increase in precipitation for the future—particularly for heavy precipitation (> 12.5 mm) events. The Canadian Model shows more of a precipitation increase from December to July. The Hadley Model shows more of a precipitation increase from July to December. Both models show a decrease in surface windspeed and an increase in the number of days with an easterly wind component. Both models exhibit decreases in cyclone numbers for the future. The Canadian Model shows a general decrease in the number of moderately strong cyclones and decreases in each month. The Hadley Model shows a slight increase in the number of strong cyclones but a greater decrease in the number of weak cyclones—especially during the spring. The Canadian Model exhibits significant decreases in the number of anticyclones in summer and significant increases occur in fall but does not exhibit any systematic changes in terms of intensity. The Hadley Model shows a slight increase in the number of weak anticyclones but a greater decrease in the number of strong anticyclones. Most of the decreases occur during the summer—so that the seasonal distribution is more uniform. All of the changes are consistent with changes in the general large scale flow patterns. An understanding of all these synoptic changes provides richness and a more conceptual understanding of how climate change may affect the Great Lakes region. INDEX WORDS:

Climate change, synoptic weather, Great Lakes.

INTRODUCTION Climate impact assessments typically bring together people with a wide range of interests and skills. A comprehensive assessment requires some people with expertise in hydrology, others with expertise in agriculture, and still others with expertise in ecology, to name a few. One skill however, is almost uniformly required. The ability to interpret climate model output from General Circulation Models (GCMs), at least to some degree, is almost a prerequisite for assessing any aspect of climate change. All of the researchers involved in the Great Lakes Regional Assessment, as well as in other re-

gional assessments, were asked to consider altered climate scenarios as defined for the most part only by surface temperature and precipitation changes— with little guidance from other accompanying fields. Without an understanding of some of the physical mechanisms that are responsible for the temperature and precipitation changes that are generated by these models, the task of an impacts assessment can become an exercise in blind faith. Besides providing only a portion of the climate change picture, surface temperature and precipitation, from a numerical weather prediction standpoint, are two of the most difficult variables to forecast—even with a high resolution mesoscale model. Most forecasters will usually consider additional model output (and current observations) to

*Corresponding author. Present address: Dept. of Geography, Michigan State University, East Lansing, MI 48824. E-mail: [email protected]

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Potential Future Weather Patterns over the Great Lakes Region develop a conceptual model of what will be happening in the future and then make a subjective surface temperature forecast. Precipitation is perhaps handled even more poorly than surface temperature by numerical weather prediction models. Forecasters typically consider the model-generated precipitation for guidance, but usually rely more heavily on understanding larger scale variables such as winds and moisture at 850 hPa, geopotential heights and vorticity advection patterns at 500 hPa, and jet stream/streak configurations at 200 to 250 hPa, in which the forecaster typically has more confidence. Using this larger scale information, some theoretically/empirically derived decision trees, and their personal experience, a forecaster will typically make a more accurate precipitation forecast than any one model does. The purpose of this study is to evaluate from a synoptic standpoint, some of the model output from the Canadian (CGCM1) and Hadley (HadCM2) Climate Models, which are two GCMs that participants involved in the Great Lakes Regional Assessment were asked to use for future climate scenarios. The evaluation provides information regarding characteristics of future heat and cold waves, heavy precipitation events, winds, and cyclone and anticyclone frequency and intensity. MODEL INFORMATION In order to understand more clearly some of the differences in the output between the two models in the next section, it is useful to understand and compare some basic aspects about the two models and the (output from their) simulations in this section. More information about the first version of the Canadian Global Coupled Model, CGCM1, and the control and climate change simulations are described in McFarlane et al. (1992); Flato et al. (2000); and Boer et al. (2000a,b). The HadCM2 is described more fully in Mitchell et al. (1995a,b) and Johns (1996). The CGCM1 has a spatial grid resolution of roughly 3.75° × 3.75° latitude by longitude and 10 vertical levels. The HadCM2 has a spatial resolution of 2.50° × 3.75° latitude by longitude and 19 vertical levels. The coarse resolution in the CGCM1 precludes the existense of the Great Lakes in the model in terms of moisture, heat, and momentum effects. The HadCM2 represents the Great Lakes in terms of three grid cells spanning western Lake Superior, northern Lake Michigan, and Lake Huron. The land surface model in the CGCM1 is a modi-

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fied bucket model. It uses a single soil layer with spatially varying field capacity and soil properties. Runoff from the land surface is transferred immediately to the ocean at the outflow point of each prescribed drainage basin. There is no mechanism for subsurface flow or storage and some enclosed drainage basins are arbitrarily connected to the ocean to avoid the possibility of continued accumulation of water on land. The land surface model in the HadCM2 has four soil layers and includes canopy processes and stomatal resistance. Both models account for sea-ice. The convective scheme in the CGCM1 is based on moist convective adjustment when the atmosphere is conditionally unstable. The convective scheme in HadCM2 is based on a mass flux penetrative scheme that includes entrainment, detrainment, evaporative cooling, and downdrafts (Gregory and Rowntree 1990, Gregory and Allen 1991). Both models have oceanic components. The CGCM1 ocean component has a spatial resolution of 1.80° × 1.80° and 29 vertical levels. The HadCM2 ocean component has a spatial resolution of 2.50° × 3.75° and 20 vertical levels. Both models use a flux adjustment procedure, which means that the coupled model uses heat and water flux adjustments obtained from uncoupled ocean and atmosphere model runs (of 10 years and 4,000 years duration, respectively), followed by an adaption procedure in which the flux adjustment fields are modified by a multi-year integration of the coupled model. Multi-century control simulations with both coupled models were performed using the presentday carbon dioxide (CO2) concentration to evaluate the stability and accuracy of the modeled climates as well as their variability to that observed. The modeled control climates show a negligible long term trend in surface air temperature. The climate drift trend was about +0.15°C per century in the CGCM1 and +0.04°C per century in the HadCM2, which are comparable to other such experiments (Stouffer et al. 1994). Both models were used to perform an ensemble of experiments to simulate the response of the coupled-ocean atmosphere system to a gradual increase in equivalent CO2 concentration. Unlike previous simulations with earlier versions of GCMs, these simulations also included the negative radiative forcing effects from sulphate aerosols. The addition of the negative forcing effects of sulphate aerosols represents the direct radiative forcing due to anthropogenic sulphate aerosols by means of an increase in clear-sky surface albedo proportional to the local

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sulphate loading (Mitchell et al. 1995a). The indirect effects of aerosols were not simulated. The simulations were initiated with a warm start—e.g., forcing from the middle industrial era, (1860), when radiative forcing was relatively small compared to the present—and a forcing change corresponding to an increase of CO2 at a rate of 1% per year (compounded) thereafter until year 2100. The direct forcing effect of sulphate aerosols is also included by increasing the surface albedo (Reader and Boer 1998) based on loadings from the sulphur cycle model of Langner and Rodhe (1991). The prescription described above is similar to the IPCC “business as usual” scenario. The global-mean temperature response by 2090 is around 4.5°C for CGCM1 and nearly 3°C for HadCM2. The output from the CGCM1 examined in this study is an average from three simulations. The daily output from the HadCM2 is only available from one member of an ensemble of four. (Global average temperatures from the other three members were within a 0.25°C envelope.) Although the output is daily from both models, there are slight differences in the interpretation of daily between corresponding fields. For example, the sea-level pressure is an average of 00 and 12 UTC distributions in CGCM1, while it is simply the 00 UTC snapshot of SLP in the HadCM2. The geopotential height information used is from 12 UTC in the CGCM1 and from 00 UTC in the HadCM2. The precipitation in both models is an average 24 h rate. FUTURE SYNOPTIC WEATHER Daily output from the Canadian and Hadley Models is evaluated to identify possible changes in heat and cold waves, precipitation, wind, surface pressure systems, and large scale flow patterns. Evaluating potential changes in smaller scale features, such as thunderstorms and tornadoes is important and possible with the available model output in terms of examining the synoptic scale environment, but is beyond the scope of this study. A study by Kunkel et al. (2002) in this issue describes potential impacts on lake effect storms. Evaluation of the models for the current climate period is limited to 1975 to 1994 in the Canadian Model and 1960 to 1979 in the Hadley Model. The current climate model performances are compared to daily NCAR/NCEP Reanalysis Data (Kalnay et al. 1996) for the period 1960 to 1979. The future climate period corresponds to 2080 to 1999 in the Canadian Model and 2070 to 1989 in the Hadley Model.

Cold Waves and Heat Waves In the U.S. alone, over 700 people die each year from exposure to cold (Kunkel et al. 1999). That number is twice the national average that die from heat. The ratio of cold-related deaths to heat-related deaths is even higher for the Great Lakes region. The ratio means that when heat waves do occur, many people and places can be caught unprepared. One of the deadliest heat waves occurred during July 1995, when more than 400 people died in just a 5-day period in Chicago alone. A real measure of heat and cold waves involves more than just surface temperature. For cold waves, it involves wind and snow for example. For heat, it involves humidity and sunshine. Additionally, the surface temperature output from the models themselves may not be as good of an indicator as some of the other large scale parameters that are more accurately simulated. Here, the 1,000 to 500 hPa geopotential thickness is used to identify the presence of heat wave and cold wave days with acknowledgment that the above mentioned parameters also play a significant role. Thickness correlates highly with surface temperature. For example, hot air masses occupy a thick layer of air between the 1,000 and 500 hPa pressure surfaces and cold air masses occupy a thinner layer. From a synoptic standpoint, an air mass with thickness below 510 decameters (dam) constitutes an arctic air mass and air with thickness above 570 dam constitutes a tropical air mass. In winter, arctic air masses correspond to high temperatures below –12°C (10°F) while in summer tropical air masses correspond roughly to high temperatures over 26°C (80°F) in mid latitudes. Both models underrepresent the number of arctic air mass days that influence the region (Fig. 1—left column). For example, the NCEP/NCAR Reanalysis Data show that over 40 days per year with thicknesses below 510 dam occur along the northern fringe of the region; around 14 days in Minneapolis, Minnesota, and around 8 days in Detroit, Michigan. The Hadley Model shows only 14, 5, and 2 days for the same regions. The Canadian Model shows even fewer: 9, 2, and 1. For polar air masses (with thicknesses between 510 and 540 dam—not shown), there is excellent agreement between the Hadley Model and observations, while the Canadian Model shows 10 to 30 fewer days than observations from north to south across the region. For tropical air masses, both models agree well with observations, although the Canadian Model

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FIG. 1. Regional distribution of annual number of arctic and tropical days for NCEP/NCAR Reanalysis Data, Canadian Model, and Hadley Model for current climate (numbers) and future climate (shaded regions—models only).

yields about 10 more days per year along the southern fringe of the region (Fig. 1—right column). In general, the thicknesses in the Canadian Model are better correlated with observations from an intensity distribution standpoint than those in the Hadley Model (Fig. 2), although the average thickness at 550 dam does not compare as favorably with that from the Hadley Model at 546 dam to that from observations at 545 dam. From an intensity distribution standpoint, the Canadian Model agrees better

with observations than the Hadley Model (corr = 0.98 Canadian vs. corr = 0.94 Hadley). From a monthly distribution standpoint, both models agree well with observations (corr = 0.98 Canadian vs. corr = 0.99 Hadley). Both models considerably underestimate arctic air mass thicknesses and overestimate polar air mass thicknesses (Fig. 2). By the end of the 21st century, both models suggest that there will be fewer cold air outbreaks in winter. For example, the Canadian Model all but

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FIG. 2. Distributions of number of days by thickness (upper) and month (lower) and annual average thickness for NCEP/NCAR Reanalysis Data, Canadian Model, and Hadley Model for current climate. eliminates arctic air masses from the region (Fig. 1) and reduces the number of polar air masses by 40 to 80% from north to south over the region (not shown). Average thicknesses in the Canadian Model increase from 534 dam to 546 dam during winter (Fig. 3), which suggests an increase of ~6°C in the high temperatures across the region. The Hadley Model has more modest increases in thicknesses. It suggests that arctic air masses will continue to move as far south as northern Minnesota, the Upper Peninsula of Michigan, and southern On-

tario during a typical winter (Fig. 1)—albeit barely. Average thicknesses in the Hadley Model increase from 530 to 536 dam during winter (Fig. 3), which suggests an increase of ~3°C in the high temperatures across the region. Both models also suggest big changes in summer by the end of the 21 st century. In the Canadian Model, average thicknesses in summer generally increase from 567 to 577 dam across the region (Fig. 3) and the number of tropical air mass days increases monotonically from ~50 days per year over

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FIG. 3. Distributions of number of days by thickness and month for Canadian and Hadley Models for current and future climates. the northeast corner of the region to ~190 days per year across the southwestern corner (Fig. 1—right column). Warming in the Hadley Model is more modest. Average thicknesses in summer generally increase from 565 to 570 dam across the region (Fig. 3) and the number of tropical air mass days increases monotonically from ~10 days per year over the northeast corner of the region to ~130 days per year across the southwestern corner (Fig. 1—right column). Figure 4 illustrates from a synoptic pattern perspective how winter cold waves and summer heat waves may change in the future. Note that Figure 4 allows a comparison between future model output and current observations, but the relative changes suggested by the models are also valid when compared to the current model projections. For cold waves, days with thicknesses less than 510 dam over southeastern lower Michigan currently occur 7 to 8 days per year (winter)—or less than 10% of the time during the season. By the end of this century, the Canadian and Hadley Models suggest that such rare events will be characterized by 528 and 520

dam thicknesses over the region respectively. Moreover, both models suggest that winds will be weaker, so that wind chills may be even more tolerable and that these cold events will likely be shorter in duration—especially in the Canadian Model. The absence of a surface pressure trough over the Great Lakes region in the Canadian Model is likely the result of the absence of the Great Lakes in that model. For heat waves, days with thicknesses greater than 576 dam over southeastern lower Michigan occur 7 to 8 days per year (summer). By the end of this century, both models suggest that such rare events will be characterized by 586 dam thicknesses over the region, which translates to high temperatures in the low 30s °C (mid to upper 90s °F). Both models show the continued influence of the Bermuda High to the east. However, both models suggest the existence of stronger surface winds and a more prominent trough through the region. The Hadley Model suggests more southerly (moister) flow, a stronger baroclinic zone just to the north of the Great Lakes, and heavy precipitation over the eastern part of the region. The Canadian

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FIG. 4. Synoptic patterns during coldwaves and heatwaves of sea level pressure (solid, 2 hPa contour interval), and thickness (dashed, 6 dam contour interval) and daily precipitation rate (shaded: light 6 to 12 mm/day; medium 12 to 25 mm/day; dark > 25 mm/day) with a reoccurrence frequency of ~7 to 8 per year for NCEP/NCAR Reanalysis Data, and Canadian and Hadley Models (future climates).

Model shows more westerly (drier) flow, a weaker baroclinic zone to the north, and heavy precipitation over the western part of the region. A more well-defined trough to the north suggests more precipitation from more frequent cold frontal passages and hence shorter duration heat waves. The discrepancies between the two models in terms of thicknesses underscore one problem that researchers involved with the National Assessment had to address—especially because temperature is a key concern to researchers and stakeholders. The

discrepancies in precipitation, as will be shown in the next subsection, underscore another problem. An evaluation of flow patterns in the “large scale flow patterns” below will provide some insight into these discrepancies. Heavy Precipitation Events Precipitation has been increasing over the Great Lakes region over the last 100 years. Karl and Knight (1998) showed that most of these increases

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FIG. 5. Distributions of precipitation by month and annual totals for NCEP/NCAR Reanalysis Data, Canadian Model, and Hadley Model for current climate. have come in the form of increased extreme precipitation events. Grover and Sousounis (2002) have shown that the increases have come most dramatically during autumn—since the mid 1960s. Precipitation from both models was evaluated over the region. Both models generate more monthly (Fig. 5) and annual (Fig. 6—first column) precipitation than observed1 for the current climate. The Canadian Model is wetter than observed for all seasons, with largest differences occurring from late spring to early fall and smallest differences occurring in mid-winter. The Hadley Model is wetter than observed during fall, winter, and spring, and drier during summer. The differences in the Canadian Model suggest a link to convective instability. That is, when the model atmosphere is climatologically least stable, model precipitation is greatest, and when the atmosphere is convectively most stable, model precipitation is least. This is further evidenced by the fact that the Canadian (and Hadley) Models have peaks in June, when solar insolation is also at a peak. 1

Observed daily precipitation rates were obtained from NCEP/NCAR Reanalysis Data on T63 Gaussian Grids and converted to 2.5° latitude × 2.5° longitude Grids (to be consistent with other Reanalysis Data) using spherical harmonics. The daily Reanalyses of precipitation actually comes from four 6-hour forecasts. The accuracy of this dataset is very good in the Great Lakes region (Janowiak 1998, Higgins et al. 1996).

The total number of precipitation days per year (Fig. 6—second column) and heavy precipitation days per year (> 12.5 mm per day; Fig. 6—third column) for each model are comparable to those from observations. The high numbers of precipitation days in the NCEP/NCAR Reanalysis Data and model output reflect the sizes of the grid cells to some degree. The heavy precipitation days increase almost uniformly from northwest to southeast in the Canadian Model. The greater spatial variability in the Hadley Model reflects the higher resolution of terrain and land cover type. The amounts of precipitation per event are higher than those from observations, which suggests that the higher annual precipitation totals in the models are a result of too much precipitation per event. In general both models show increases in precipitation over the region for the future. The Canadian Model shows most of it coming during the first half of the year (Fig. 7) and over the west-central portion of the region (Fig. 6—shading in middle row). The large increases during spring are the combined result of an increase in the number of days with precipitation and an increase in the amount of precipitation per precipitation day. In general, four more precipitation days per year occur at the end of this century. Most of the precipitation increase comes from an increase in heavy precipitation events (> 25 mm per

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FIG. 6. Regional distributions of current climate precipitation characteristics for NCEP/NCAR Reanalysis Data, Canadian Model, and Hadley Model as indicated by numbers: annual precipitation in mm (first column), days per year with precipitation (second column), days per year with precipitation over 12.5 mm (third column), and precipitation amount per event in hundredths of mm/day (fourth column). Shaded regions indicate changes in quantities over relevant 20-year periods between future and current climates for model scenarios. day). The Hadley Model shows more of an overall increase, with most of it coming during the second half of the year (Fig. 7) over the southern part of the region (Fig. 6—shading in bottom row). Most of the increase comes from an increase in moderate to heavy (> 12.5 mm per day) events. In general 4 fewer precipitation days per year occur over the region at the end of this century. The decreases in total precipitation days are countered by comparable, if not greater, increases in amounts of precipitation per precipitation day to result in the increased precipitation in both models. The projected increases for the heavier precipitation categories in both models are consistent with increases that have occurred over the region during the past century (Grover and Sousounis 2002). Figure 8 shows the corresponding synoptic situations for extreme precipitation events in both models. An extreme precipitation event is defined here as one that occurs fewer than 7 to 10 times per year.

The current synoptic pattern for extreme precipitation events over southeastern Michigan from NCEP/NCAR Reanalysis Data exhibits a precipitation maximum of 22 mm over Detroit, Michigan with a 1,006 hPa low centered 200 km to the west (not shown). This pattern suggests that these events are associated with warm fronts. Both models show similar patterns for the current and future climate scenarios but with lows farther to the southwest. The Hadley Model suggests that precipitation from these extreme events will increase from 23 to 29 mm per event (Fig. 8 third column) by the end of the 21st century. The Canadian Model also shows increases—but from 40 to 47 mm for these events. The heavier precipitation will likely result from slightly more intense lows and sharper warm fronts. Surface Winds Surface winds are important for several reasons. First, windspeed affects evaporation and therefore

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FIG. 7. Distributions of precipitation by month for Canadian and Hadley Models for current and future climates. influences lake levels. Second, windspeed is also a better measure than central low pressure for intensity of synoptic-scale storms because low pressure storms may be associated with a weak pressure gradient and low windspeeds. Finally, one-third of all registered U.S. boaters reside in the (eight state) Great Lakes region (Allardice and Testa 1991). Boaters, and particularly sailboaters, have a considerable interest in wind—especially during the summer. The NCEP/NCAR Reanalysis Data show that observed geostrophic windspeeds 2 over the eastern

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Surface geostrophic winds from the Canadian and Hadley Models are compared against those from Reanalysis Data. Actual surface winds depend on surface friction and stability but generally they are 50% of the geostrophic value and oriented 45 degrees to the left over flat land and 80% of the geostrophic value and oriented 20 degrees to the left over smooth water. Geostrophic winds were computed using sea level pressure (gradients), the hydrostatic relationship, and latitude information.

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Great Lakes (Lake Huron) exhibit a maximum in January and a minimum in August (Fig. 9). A secondary maximum occurs in November. The observed maximum in January is likely related to the strength and proximity of the baroclinic zone. The one in November may be related to the strong cyclones that develop over the region at that time. Both models capture the seasonal variation in windspeed (corr = 0.98 Hadley; corr = 0.88 Canadian). The NCEP/NCAR Reanalysis Data show that the prevailing wind speed over Lake Huron is 5 to 8 m per s (10 to 15 knots). Figure 10 shows that this average is slightly lower in summer and slightly higher in winter. Winds in excess of 12.5 m per s (25 knots) occur more than 10% of the time overall—more than 20% of the time in winter but less than 2% of the time in summer. Figure 10 also shows that the prevailing wind direction is westsouthwesterly in summer and west-northwesterly in winter. A secondary maximum in northwesterly prevailing flow occurs in July—but extends from June through August (not shown). This secondary maximum may be the influence of the lake aggregate during summer. Because the lakes remain relatively cooler than the surface air during summer, they hydrostatically induce high pressure over the entire region—causing anticyclonic (northwesterly) flow over the eastern part of the region. The peak in northeasterly flow in April may be a result of the baroclinic zone being situated just to the south of the region (not shown). The frequent cyclones that develop along the baroclinic zone at this time of year and track just south of the region before curving northeastward may account for the prevailing northeasterly flow. Both models capture the seasonal distribution of windspeeds well but overestimate the windspeeds (Fig. 9). The Canadian Model overestimates the windspeeds more than the Hadley Model in all seasons except fall. This may be related to the fact that the Hadley Model includes the Great Lakes and may generate stronger cyclones during this unstable season than the Canadian Model. Interestingly, the Canadian Model indicates a prevailing wind direction of southwesterly during all months of the year. In winter, the increased number of days with southwesterly winds comes at the expense of fewer days with northeasterly and southeasterly flow (Fig. 10). This increase in southwesterly flow days may suggest that more lows in the Canadian Model pass by the location (Lake Huron) to the north and west instead of to the south and east. This result is certainly consistent with the extreme precipitation

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FIG. 8. Synoptic patterns from Canadian and Hadley Models for extreme-precipitation events of sea level pressure (solid, 2 hPa contour interval), thickness (dashed, 6 dam contour interval) and daily precipitation rate (shaded, light = 6 to 12 mm/day, medium = 12 to 25 mm/day, and dark = > 25 mm/day) with a reoccurrence frequency of ~7 per year for current and future climates. scenario in the Canadian Model. The Hadley Model exhibits a broader distribution of wind directions and is more like the observations in this sense during both summer and winter. The prevailing windspeed in both models is 5 to 8 m per s (10 to 15 knots), although the frequencies of occurrence for this range are slightly less than observed. Both models overestimate the number of days with strong windspeeds (> 12.5 m per s or 25 knots). By the end of the 21 st century, the Canadian Model shows a decrease in windspeeds for all months (Fig. 11), but especially in December, making January the windiest month. August rather than July is the calmest month. The windspeed intensity distribution also shifts so that higher windspeeds will be less likely and weaker windspeeds will be more likely (Fig. 12). The prevailing windspeed in summer will still be 5 to 8 m per s (10 to 15 knots)—but with a greater frequency. The biggest change in the distribution of windspeeds occurs in winter for winds over 12.5 m per s (25 knots), which become considerably less frequent. The Canadian Model also suggests that southwesterly

winds throughout the year will be less likely, while northeasterly winds will be more likely (Fig. 12). The Hadley Model also shows a decrease in windspeeds for the future, but not as dramatically as in the Canadian Model. The Hadley Model also suggests more variable windspeeds in summer— with a more frequent northwesterly component and a less frequent northeasterly or easterly component. The most likely windspeed range will still be 5 to 8 m per s (10 to 15 knots), but with a broader and flatter distribution. In winter, the Hadley Model suggests weaker winds with a less frequent (south)westerly component and a more frequent (south)easterly component. The weaker winds in winter are related to the weaker baroclinic zone. The shift from more westerly component days to more easterly component days may be partially a result of a weaker westerly component from the weaker baroclinicity but may also be a result of a shift in weather patterns and tracks of highs and lows. February rather than January will be the windiest month. The decreased windspeeds would contribute to less evaporation from the lakes.

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FIG. 9. Distributions of surface geostrophic windspeeds over Lake Huron by month for NCEP/NCAR Reanalysis Data, Canadian Model, and Hadley Model for current climate. Surface Pressure Features Cyclones and anticyclones are fundamental aspects of any synoptic climatology study. Cyclones can account for heavy precipitation and strong winds in a region. Anticyclones can account for fair weather and droughts. In this study, cyclones and anticyclones were counted if a center of low or high pressure within the region was surrounded by at least one closed contour at a 4 hPa contour interval at standard values (1,000 ± n4 hPa; n = 0, 1, 2, 3 . . .). If more than one low or high center was in the region on any given day (very rare), then only the strongest low or high was counted. Cyclones Figures 13 and 14 (first column) show that both models capture the total number of cyclones fairly well for the current climate (1,142 Canadian cyclones vs. 1,047 Hadley cyclones vs. 980 Reanalysis cyclones)—especially given the slight differences in regional area and number of days per year. Both models simulate more cyclones than observed in spring, (especially over the lakes them-

selves—not shown). The Canadian Model simulates a considerably larger number of cyclones than observed from late spring through summer (May to August). This bias is consistent with the considerably higher than observed precipitation for the same time of year. The larger number of cyclones in the Canadian Model during late spring and summer accounts for the greater number of cyclones generated by the Canadian Model than are observed. The lower than observed number of cyclones in the Canadian Model for October and November may be due to the absence of the Great Lakes. In general, the Hadley Model compares better with observed than does the Canadian Model in terms of monthly distribution (corr = 0.94 for Hadley; corr = 0.40 for Canadian) (Fig. 13). Both models capture fairly well the mean sea level pressure (SLP) distribution although the Hadley Model has a negative SLP bias so that the Canadian Model compares slightly better with observed than does the Hadley Model in terms of SLP intensity distribution (corr = 0.86 for Hadley; corr = 0.97 for Canadian). Precipitation associated with cyclones is a combined result of cyclone intensity, speed of propaga-

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FIG. 10. Frequencies of occurrence (%) for different surface geostrophic wind speed (m per s) and compass direction categories over Lake Huron for current climate for summer and winter for Reanalysis Data, Canadian Model, and Hadley Model. Numbers at the centers represent percentage of time that calm conditions (e.g., < 1 m per s) exist.

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FIG. 11. Distributions of windspeeds by month for Canadian and Hadley Models for current and future climates. tion, and cyclone track. The southward increase in precipitation per cyclone at the (cyclone) center exhibited in the NCEP/NCAR Reanalysis Data (Fig. 14—second column) is likely a result of increasing saturation vapor pressure with temperature and is reproduced well in the Canadian Model despite the slightly higher than observed thicknesses. The Hadley Model generates less precipitation per cyclone in general, and does not exhibit the north to south gradient. The lower amounts at the center may be a result of excessive warm frontal precipitation as suggested during heavy precipitation events. The absence of the latitudinal gradient may be a result of the strong storm track that runs through the middle of the domain. The precipitation from the stronger and more frequent storms that move through this part of the domain dominates the latitudinal signal. Wind is a key feature of cyclones in the Great Lakes region. This feature more than any other sig-

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nificantly impacts shipping in the region, particularly in late fall when gale force winds can accompany storms. Figure 14 (third column) shows that both models overestimate the number of intense cyclones (surface geostrophic winds at the center > 12.5 m per s)—the Hadley Model more so than the Canadian Model. These results reflect the overall windy bias in both models. However, both models agree with the NCEP/NCAR Reanalysis Data in that the highest occurrences of intense storms are near the lakes themselves. With one exception in each model, no double-digit occurrences are more than one grid point away from a lake. This feature in the Canadian Model reflects a tendency for intense cyclones to develop independently of the lake-induced thermal instability that can generate strong winds over the lakes in fall. Snow is another key aspect of cyclones in the Great Lakes region. Cyclones associated with 1,000 to 500 hPa thicknesses < 540 dam at the center have the potential to generate significant snow just to the north and west of the center. Figure 14 (fourth column) shows that both models underestimate slightly the number of cold cyclones—the Canadian Model more so than the Hadley Model. The low numbers in the Canadian Model reflect its warm bias. The Hadley Model agrees better with the NCEP/NCAR Reanalysis Data in that these storms occur most frequently over Lake Superior and just north of Lake Huron. Both models exhibit decreases in cyclone numbers (14% for the Canadian Model and 18% for the Hadley Model) by the end of this century (shaded regions in Fig. 14—left column). The overall decreases in both models are consistent with earlier results from previous generation GCMs (Lambert 1995, Carnell et al. 1996). The Canadian Model shows a general decrease in the number of cyclones with central SLP < 1,010 hPa and more or less general decreases in each month. The greatest decreases occur in early winter and early summer. The Hadley Model shows a slight increase in the number of cyclones with central SLP < 1,000 hPa but a greater decrease in the number of cyclones with central SLP > 1,005 hPa (Fig. 15). Additionally, most of the decreases occur during the spring—so much so that spring and fall are nearly tied for season with the most cyclones. Spatially, the Hadley Model shows biggest decreases over Lake Huron (shaded regions in Fig. 14—left column). Most of these decreases occur in winter (not shown). Climatologically, this region is where the lake aggregate has the strongest influence

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FIG. 12. Frequencies of occurrence (%) for different surface geostrophic wind speed (small numbers in/ms) and compass direction categories over Lake Huron for future climates for summer and winter for Canadian and Hadley Models. Numbers at the centers represent percentage of time that calm conditions (e.g., < 1 m per s) exist.

(Weiss and Sousounis 1999). The decrease suggests that the lake aggregate will have less of an influence in winter. In general, the biggest decreases occur over the Great Lakes. In fact, 7 of the 12 most central grid points (out of 30 total) account for nearly two-thirds of the total reduction. Seasonal and spatial changes in the Canadian Model suggest the presence of a more zonal cyclone track in winter through the central portion of the region: Wisconsin, Michigan, southern Ontario, and New York. In spring, greatest decreases occur in the northeast portion of the region and along the southern edge of the region. In summer, decreases occur everywhere except over the northern edge of the region. In fall, decreases occur over most of the domain and reflect no particular change in cyclone track.

An interesting point is the amount of precipitation from cyclones relative to that from all mechanisms. While the actual numbers from cyclone precipitation and total precipitation have different interpretations, the Canadian Model suggests that a considerably higher fraction of the precipitation comes from cyclones (Figs. 10 and 20). This higher fraction from cyclones may be the reason for the higher amounts of precipitation from the Canadian Model. The higher fraction correlates with the higher number of cyclones from the Canadian Model in summer but it is difficult to tell whether too much precipitation is generating too much latent heat release and too many cyclones or whether too many cyclones are developing and therefore generating too much precipitation.

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FIG. 13. Distributions of cyclones by central pressure (upper) and month (lower) and annual totals for NCEP/NCAR Reanalysis Data, Canadian Model, and Hadley Model for current climate. Anticyclones Figure 16 shows that overall, both models simulate too many anticyclones for the current climate (852 Canadian anticyclones vs. 775 Hadley anticyclones vs. 531 Observed anticyclones). On a

monthly basis, the Hadley Model performs better than the Canadian Model (corr = 0.70 for Hadley; corr = 0.10 for Canadian). Only the Hadley Model captures the early fall season maximum. The late winter/early spring minimum is not captured well

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FIG. 14. Regional distributions of current climate cyclone characteristics for NCEP/NCAR Reanalysis Data, Canadian Model, and Hadley Model as indicated by numbers for respective 20-year periods: number of cyclones (first column), precipitation per cyclone (mm) at center of cyclones (second column), cyclones with winds over 12.5 m per s (third column), and cyclones with thicknesses less than 540 dam (fourth column). Shaded regions indicate changes in quantities over relevant 20-year periods between future and current climates for model scenarios. by either model. The Canadian Model simulates too many anticyclones in spring through early summer and late fall. The Hadley Model simulates too many anticyclones in late spring through summer. Both models capture fairly well the mean SLP distribution although the Hadley Model again has a negative SLP bias—it simulates too many anticyclones with central SLP < 1,025 hPa. As a result, the Canadian Model compares slightly better with observed than does the Hadley Model in terms of SLP intensity distribution (corr = 0.96 for Hadley; corr = 0.97 for Canadian) even though it generates considerably more anticyclones (at all intensities). Figure 17 (first column) indicates that the observed anticyclones exhibit a slight bias for being located near the lakes, which may suggest that the lakes are influencing development of the anticyclones. The Canadian and Hadley Models also show this bias. The Hadley Model exhibits very exaggerated behavior in this sense for a couple of grid points. For example, the Hadley Model exhibits

considerably more anticyclones than observed in the area just northeast of Lake Huron and then no anticyclones just to the east of there. Aside from this bias, the Hadley Model seems to have reasonably good agreement spatially with observed conditions for all seasons. Curiously, the model (as well as the observed) anticyclones (at the centers) are associated with precipitation (Fig. 17—second column). These light and relatively infrequent precipitation amounts may be the result of weak upper level shortwaves and/or isolated convection that can develop in the presence of surface highs. The Hadley Model agrees more closely with the observed percentage of dry anticyclones (~70%), while the Canadian Model shows fewer (~60%). The Hadley Model also agrees more with observations in terms of weak anticyclones (winds < 5 per ms; Fig. 17—third column), which is likely a result of the higher windspeed bias in the Canadian Model. Finally, the number of anticyclones with 1,000 to 500 hPa thickness over 570 dam at the

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FIG. 15. Distributions of cyclones by central pressure and month for Canadian and Hadley Models for current and future climates. center exhibited in the Hadley Model are considerably higher than observed. The fact that these anticyclones are not very latitude-dependent and the fact that the mean warm season thickness in the Hadley Model is less than that in the Canadian Model suggests that the Hadley Model exhibits greater variability. Both models exhibit slight changes in anticyclones by the end of this century (3% increase for the Canadian Model and 3% decrease for the Hadley Model). The Canadian Model does not exhibit changes that are as systematic as those in the Hadley Model in terms of intensity—but significant decreases occur in summer and significant increases occur in fall so that the double peak structure that the Canadian Model exhibited for the current climate (but which is not observed) is amplified in the future climate. The Hadley Model shows a slight increase in the number of anticyclones with central SLP < 1,020 hPa but a greater decrease in the number of anticyclones with central SLP > 1,025 hPa. Additionally, most of the decreases occur during the

summer—so much so that the monthly distribution is more uniform (Fig. 18). Spatially, the Canadian Model shows some of the biggest decreases along the southern border and general increases over the western Great Lakes region. The Hadley Model shows less significant changes in the spatial distribution of anticyclones (Fig. 17). From a precipitation perspective, the fraction of dry anticyclones increases slightly in the Canadian Model and remains unchanged in the Hadley Model (Fig. 17) although both models show an increase in dry anticyclones over the western and central Great Lakes. The number of tranquil anticyclones increases by ~5% in both models with both showing increases extending from the south central through the northeast part of the region (Fig. 17). These increases are a result of an overall reduction in windspeed in both models. Finally, the number of hot anticyclones increases by nearly an order of magnitude in the Canadian Model with the largest increases over the eastern Great Lakes and more modestly in the Hadley Model with the largest in-

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FIG. 16. Distributions of anticyclones by central pressure (upper) and month (lower) and annual totals for NCEP/NCAR Reanalysis Data, Canadian Model, and Hadley Model for current climate. creases over the western Great Lakes. These increases are a result of a general thickness increase in both models. Large-scale Flow Patterns It is useful to examine the large-scale flow changes to help understand the regional changes

that both models generate. The more important current climate differences are shown in Figures 19 and 20. In winter, they include an Aleutian Low that is slightly stronger in both models and a Rocky Mountain High that is stronger than observed in the Canadian Model and weaker than observed in the Hadley Model. In general, the large-scale flow is more meridional than observed in the Canadian

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FIG. 17. Regional distributions of current climate anticyclone characteristics for NCEP/NCAR Reanalysis Data, Canadian Model, and Hadley Model as indicated by numbers for respective 20-year periods: number of anticyclones (first column), number of anticyclones with no precipitation at center (second column), anticyclones with winds under 5 m per s at center (third column), and anticyclones with thicknesses over 570 dam at center (fourth column) for the respective 20-year periods. Shaded regions indicate changes in quantities over relevant 20-year periods between future and current climates for model scenarModel and more zonal than observed in the Hadley Model. In summer, the Canadian Model has too strong of a low in the desert southwest U.S. and too strong of a Bermuda High off the East Coast. The Hadley Model has slightly more anticyclonic flow over the eastern half of the U.S. than observed. A more detailed evaluation of the large-scale model performance for the current climate is provided by Doherty and Mearns (1999). By the end of this century, both models show an even stronger Aleutian Low (Fig. 19) in winter. In this sense, the future model climate becomes more El Niño-like than at present, which suggests more frequent if not more intense Pacific lows (Felzer and Heard 2000). Farther east, both models also show a stronger surface ridge over central Canada and a weaker surface low over Hudson Bay. The Canadian Model (Fig. 19) shows more zonal flow everywhere except over Alaska, where the Aleutian Low is more pronounced. Cyclonic flow associated with the stronger Aleutian Low extends farther

south and east toward southern California. Despite the weaker 500 hPa trough and surface low over Hudson Bay, positive vorticity advection and warm advection over the Great Lakes region are slightly stronger, and jet-induced subsidence is weaker. These features are consistent with the increased precipitation and reduced number and intensity of cyclones over the Great Lakes region in the Canadian Model. The increased zonal flow implies fewer cold air outbreaks and fewer Alberta Clippers but more Pacific systems over the northern tier states (Rodionov 1994). Alberta Clippers are a primary source for reinforcing cold air over the Great Lakes in winter. Fewer outbreaks likely means less lakeeffect snow (Kunkel et al. 2002—this issue). Many of the other corresponding kinematic features associated with the stronger Aleutian Low in the Canadian Model are present in the Hadley Model, and even more pronounced. For example, the jet-maximum at 250 hPa increases from 50 to 72 m per s in the Hadley Model as opposed to 200 hPa

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FIG. 18. Distributions of anticyclones by central pressure and month for Canadian and Hadley Models for current and future climates. jet-maximum increases from 60 to 67 m per s in the Canadian Model over the Pacific Ocean. As another example, the storm track (250 hPa jet axis and strong baroclinic zone) along the Eastern North American coast is shifted farther south by about 1,000 km in the Hadley Model as opposed to only 500 km in the Canadian Model. A stronger surface trough over the Great Plains, enhanced cyclonic flow at 250 hPa, and a less extensive high with warmer and moister flow over the Gulf states all support the increased precipitation over that region. A more pronounced surface trough over the Great Lakes, despite the weaker trough over Hudson Bay, is consistent with the deeper lows that are generated by the Hadley Model over the region. Despite the similar changes in both models, the most obvious difference is that the Hadley Model reveals a more meridional flow while the Canadian Model exhibits a more zonal flow—especially over the western U.S. This fundamental difference may explain why the wintertime precipitation increase in the Canadian Model is greater than that in the Hadley Model.

Large-scale flow changes in summer are also significant albeit less obvious (Fig. 20). For example, the upper air flow in the Canadian Model becomes slightly more meridional. The thermal low over the southwestern U.S. and the southerly surface flow from the back side of the Bermuda High over the southeastern U.S. and Gulf states become weaker. The surface flow over the Great Lakes region becomes more anticyclonic. All of these changes help explain the lack of significant increase in summer precipitation over the Great Lakes region in the Canadian Model (Fig. 7). In the Hadley Model, the upper air flow becomes more zonal. At the surface, the southerly flow from the back side of the Bermuda High over the southeastern U.S. and Gulf states becomes stronger in the Hadley Model and the northwestward extension of the Bermuda High over the Great Lakes becomes weaker. These changes, along with the higher thicknesses, help explain the significant increase in precipitation over the Great Lakes region in the Hadley Model. Additionally, the slightly more zonal pattern

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FIG. 19. Selected model fields in winter for 1961 to 1990 (left) and 2090 to 2099 (right). First Row: Canadian 200 hPa heights (solid, dam) and isotachs (dashed, m per s; shading—light = 40–50 m per s, medium = 50–60 m per s, dark = > 60 m per s. Second Row: Canadian Mean sea level pressure (solid, hPa—referenced to 1,000 hPa), 1,000 to 500 hPa thickness (dashed, dam), and precipitation intensity (shaded light = 2 to 5 mm/day, medium = 5 to 8 mm/day, dark = > 8 mm/day). Third Row: Hadley 250 hPa heights (solid, dam) and isotachs (dashed, m per s; shading—light = 40 to 50 m per s, medium = 50 to 60 m per s, dark = > 60 m per s. Fourth Row: Hadley Mean sea level pressure (solid, hPa—referenced to 1,000 hPa), 1,000 to 500 hPa thickness (dam), and precipitation intensity (shaded light = 2 to 5 mm/day, medium = 5 to 8 mm/day, dark = > 8 mm/day).

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FIG. 20. Selected model fields in summer or 1961 to 1990 (left) and 2090 to 2099 (right). First Row: Canadian 200 hPa heights (solid, dam) and isotachs (dashed, m per s; shading—light = 40 to 50 m per s, medium = 50 to 60 m per s, dark = > 60 m per s. Second Row: Canadian Mean sea level pressure (solid, hPa—referenced to 1,000 hPa), 1,000 to 500 hPa thickness (dashed, dam), and precipitation intensity (shaded light = 2 to 5 mm/day, medium = 5 to 8 mm/day, dark = > 8 mm/day). Third Row: Hadley 250 hPa heights (solid, dam) and isotachs (dashed, m per s; shading—light = 40 to 50 m per s, medium = 50 to 60 m per s, dark = > 60 m per s. Fourth Row: Hadley Mean sea level pressure (solid, hPa—referenced to 1,000 hPa), 1,000 to 500 hPa thickness (dam), and precipitation intensity (shaded light = 2 to 5 mm/day, medium = 5 to 8 mm/day, dark = > 8 mm/day).

Potential Future Weather Patterns over the Great Lakes Region in the Hadley Model suggests a tendency for more transient rather than stationary features, which may account for the increased number of precipitation days (in summer) as well as the higher thickness values associated with extremely hot days—despite the overall lower increase in thickness relative to the Canadian Model. The increase in transient features may have a significant impact on air quality (Sousounis et al. 2002—this issue). SUMMARY Outputs from the Canadian Coupled Climate Model (CGCM1) and the Hadley Coupled Climate Model (HadCM2) are examined to evaluate potential changes in synoptic patterns over the Great Lakes region toward the end of this century. By the end of the 21st century, both models indicate that there will be fewer arctic air outbreaks in winter and more heat waves in summer. The Canadian Model suggests a warmer scenario than the Hadley Model. From a synoptic standpoint, extremely cold winter days will be characterized by thicknesses 10 to 20 dam higher than current values and slightly weaker winds. Extremely hot summer days will be characterized by 1,000 to 500 hPa geopotential thicknesses 10 dam higher and stronger winds. Both models show the influence of the Bermuda High to the east and a trough oriented from southwest to northeast through the region. The Hadley Model shows more southerly (moister) flow at the surface while the Canadian Model shows more westerly (drier) flow and suggests shorter duration heat waves than the Hadley Model. Both models show future climate increases in precipitation, which are primarily a result of increases in heavy precipitation events. The Canadian Model shows a small increase with most of it coming during the first half of the year. The Hadley Model shows more of a precipitation increase in general with most of the increase coming during the second half of the year. In both models these heavier precipitation events will likely result from an increase in the frequency and intensity of warm fronts. Both models show future decreases in surface windspeeds and increased frequency of days with an easterly wind component. The Canadian Model shows a significant increase in days with northeasterly winds and a significant decrease in days with southwesterly winds, especially during winter. The Hadley Model shows a significant increase in days with southeasterly winds and a significant decrease

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in days with southwesterly winds, especially during the winter. Both models exhibit decreases in cyclone numbers for the future. The Hadley Model shows a slight increase in the number of strong cyclones but a greater decrease in the number of weak cyclones—especially during the spring. The Canadian Model shows a general decrease in the number of moderately strong cyclones and decreases in each month. The Hadley Model shows a slight increase in the number of weak anticyclones but a greater decrease in the number of strong anticyclones. Most of the decreases occur during the summer—so that the seasonal distribution is more uniform. The Canadian Model does not exhibit changes that are as systematic as those in the Hadley Model in terms of intensity, but significant decreases in frequency occur in summer and significant increases occur in fall. All of the changes are consistent with changes in the general large-scale flow patterns. The increased zonal flow exhibited by the Canadian Model in winter and the Hadley Model in summer are consistent with precipitation increases in those models for those seasons. The increased waviness in the flow exhibited by the Canadian Model in summer and the Hadley Model in winter are also consistent with precipitation decreases in those models for those seasons. More large-scale analyses are needed to understand the similarities and differences in the model synoptic scale features. More importantly, more sensitivity studies (simulations) with the models themselves are needed to understand some of the large scale differences between the two models. ACKNOWLEDGMENTS We gratefully acknowledge the Hadley Centre for providing the HadCM2 daily output and the Canadian Climate Centre for providing the CGCM1 daily output used in this study. We also gratefully acknowledge the NOAA-CIRES Climate Diagnostics Center in Boulder, Colorado, for providing the NCEP/NCAR Reanalysis Data via their website at http://www.cdc.noaa.gov. This research was supported by EPA Cooperative Agreement CR-82723601-0 to the University of Michigan.

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