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The Use of Thermal AVHRR Imagery To Construct An Estimator of Seasonal Heat Budgets for Large Lakes in North America
RESEARCH SUMMARY
Investigators: S. Taylor Jarnagin and E. Terrence Slonecker
U.S. Environmental Protection Agency, Office of Research and Development, Reston, Virginia 20192.
The consequence of climate change and global warming on lakes has been the subject of much concern and research (see DeStasio et al., 1996 for a partial review and listing). Globally, the impacts of climate change are predicted to be most noticeable at higher latitudes (e.g., Mitchell et al., 1990; Chapman and Walsh, 1993; Groisman et al., 1994). The North American Great Lakes are a major economic and ecological resource and form the world's largest reservoir of freshwater. Recent research has focused upon how the biotic and abiotic environments of these lakes and their surroundings may be affected by climate change (Croley, 1991; Hobbs et al., 1994; Croley et al., 1995; Mortsch and Quinn, 1996; Changnon, 1997; Mortsch et al., 1997; and Quinn, 1998).
Lakes, particularly large lakes, have significant impacts upon the local climate of the land adjacent to the lakes due to the differences in heat capacities between the water and land surface and the moisture supplied to the lower atmosphere by the lakes. The Great Lakes of North America moderate maximum and minimum temperatures of the region in all seasons, increase cloud cover and precipitation over and just downwind of the lakes during winter, and decrease summertime convective clouds and rainfall over the lakes (Phillips, 1978; Scott and Huff, 1996). Seasonal changes in the heat content of large lakes have direct ecological and economic impact if those changes result in changes in fish and invertebrate communities or algae and aquatic vegetation. Changes in ice cover extent and duration affect the length of the shipping season and ease of navigation on the Great Lakes. Changes in the heat content of large lakes could alter the local climatic effects of those lakes on their surroundings (such as seasonal lake-effect snowfall amounts). Changes in the seasonal heat content of the epilimnion of large lakes tracked over this time may reveal trends due to climate change. Seasonal changes in the heat content of large lakes may be correlated with changes in other ecological phenomena that are currently being suggested as indicators of climate change. These indirect climate change indicators include such phenological measurements as the duration of ice cover on lakes, terrestrial snow cover duration, spring snowmelt runoff characteristics, and blooming dates for flowering plants.
Perhaps the most dramatic examples of local climatic impact of large lakes are the snow-belts extending downwind of the North American Great Lakes. Locally, up to 100 - 300% more precipitation falls downwind from the Great Lakes in winter than would be expected if not for the influence of the lakes (Rothrock, 1969; Scott and Huff, 1996). The increased snowfall in lake-effect areas is the result of a complex interaction of variables. Lake-effect snow is in part a function of the heat and moisture lost from the relatively warm lake to the colder air mass moving over it, the temperature of the air at the surface and at an altitude, the temperature difference between the lake and adjacent land, orographic precipitation due to air mass lift with the difference in terrain elevation between lake and the adjacent land, and pressure gradient effects due to movements of air masses (Jiusto, 1973; Dockus, 1985; Niziol 1987, 1989; Niziol et al., 1995). This research will be focused upon the relationship between the surface temperature of Lake Superior and lake effect snowfall to the lee of Lake Superior.
The goal of this research is to use thermal AVHRR imagery to construct an estimator of the seasonal heat budgets of large lakes. If this research is successful, a single numeric estimator derived from thermal AVHRR imagery can be used as an indicator of the heat content of large lakes. Changes in the seasonal heat content of lakes over time can be tracked remotely with this indicator. The AVHRR database covers 1979 to the present and this process could be applied to a wide range of lakes large enough to be sampled at the AVHRR pixel size. Archival AVHRR data are available through the NOAA National Operational Hydrological Remote Sensing Center (NOHRSC), the GLERL CoastWatch Program, and the National Environmental Satellite, Data, and Information Service (NESDIS). AVHRR products for the period from 1985 to the present utilize Local Area Coverage (LAC, 1 km spatial resolution) AVHRR data, but NOAA only archived Global Area Coverage (GAC, 4 km spatial resolution) data prior to 1985.
A seasonal time series of images of Lake Superior will be obtained via the NOAA CoastWatch Archive and Access System (NCAAS). The time series of images will be processed to replace cloud-impacted or missing thermal values with temporally interpolated values. The surface temperature of the lake recorded at each thermal image pixel will be summed for each date to provide an analog estimator of the epilimnetic heat content of the entire lake on the image date. The seasonal series of heat content estimates will be integrated over the season to compute an analog estimator of the seasonal heat budget of Lake Superior. The seasonal estimate will be compared with seasonal and annual values computed from the average temperatures calculated with the GLERL Great Lakes Surface Environmental Analysis (GLSEA) and with NOAA National Data Buoy Center (NDBC) moored and C-MAN buoys when those records are available. Other large lakes in North America will be selected based upon their area, morphology, and availability of AVHRR imagery and ground truth to act as targets for this research.
The objectives of this project are to:
- evaluate the technique of temporal interpolation of thermal values
as a method of estimating surface temperatures in comparison with the
sequential addition technique used in the GLSEA data set;
- compare imagery-based estimates of lake thermal values with ground-based
values and evaluate the use of remote sensing to estimate changes in
lake thermal values for lakes of differing area and morphology; and
- compare temporal changes of lake thermal values with environmental impacts, specifically, lake effect snowfall in the lee of Lake Superior.
This study is driven by the hypotheses that:
- There is a relationship between the sum of the seasonal surface temperature
that can be remotely sensed and the heat budget of a large lake; and
- There is a relationship between the thermal indicator values to be
generated and regional climatology; and
- There are relationships between the thermal indicator values to be generated and other phenological measurements such as ice cover duration and lake-effect snowfall.
The criteria for success of this study will be the ability to statistically detect the above relationships.
CoastWatch AVHRR imagery of Lake Superior has been obtained for the April - October period of calendar year 1994. The imagery has been georectified, land-masked, and processed to allow the determination of surface temperature to the nearest 0.1 °C. Initial analyses has identified cloud-impacted areas for masking and thermal interpolation.
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Niziol, T. A. 1989. Some synoptic and mesoscale interactions in a lake-effect snowstorm. Postscripts, 2nd National Winter Weather Workshop, Raleigh, NC, NOAA, p. 260-269.
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Phillips, D. W. 1978. Environmental climatology of Lake Superior. Journal of Great Lakes Research 4: 288-309.
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Rothrock, H. J. 1969. An aid in forecasting significant lake effect snows. ESSA Technical Memorandum WBTM CR-30, National Weather Service Central Region, Kansas City, MO. 12 pp.
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