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*Present address: Department of Geology, Portland State University, Portland, Oregon 97207
The continuous influx of solar radiation during the austral summer is an important driving force for hydrological and ecological systems in the McMurdo Dry Valleys, Antarctica. Spatial and temporal variations in radiative fluxes may influence glacier mass balance and stream discharge, as well as distribution and production of biological communities in the dry valleys. Because solar radiation plays an important role in these polar deserts, the primary goal of this study was to assess differences in solar radiation on three glaciers in Taylor Valley, Antarctica, over a 2-year period using data collected as part of the McMurdo Dry Valleys Long-Term Ecological Research (LTER) study.
Solar radiation data were continuously collected during 1994-1995 from the ablation zones on the surface of three glaciers in Taylor Valley (77°00'S 162°52'E):
For glacier locations, see figure 1 in Fountain, Lewis, and Dana (Antarctic Journal, in this issue). The elevations of the sites varied: 290 meters (m) on the Commonwealth Glacier, 327 m on the Taylor Glacier, and 437 m on the Howard Glacier. Although measurements were made over the entire 2-year period on the Commonwealth and Howard Glaciers, the site on the Taylor Glacier was not established until late in 1994. Incoming and outgoing solar radiation was measured with Eppley pyranometers (model PSP), and data were stored on a Campbell Scientific datalogger (model CR10). Further details on the three sites and coincident meteorological measurements can be found in Doran et al. (1995).
A comparison showed that the mean monthly amount of incoming solar radiation was different at each site. Taylor Glacier received up to 20 percent more solar radiation than the Commonwealth and Howard Glaciers, and the Commonwealth received up to 50 percent more radiation than the Howard (figure 1). These differences were even greater for net solar radiation (incoming minus reflected). For example, during November 1995 the Taylor Glacier on average absorbed 155 watts per square meter (Wm-2), 2.5-3.5 times more solar energy than the Commonwealth and Howard Glaciers, which absorbed on average, 65 and 44 Wm-2, respectively. Annual solar energy budgets bear out these trends as well as interannual differences existing between the Commonwealth and Howard Glaciers. Total amount of annual solar energy impinging on both of these glaciers increased between 1994 and 1995 ( figure 2). The annual energy increased from 3,422 to 3,474 megajoules per square meter per year (MJm-2yr-1) on the Commonwealth Glacier and from 2,990 to 3,115 MJm-2yr-1 on the Howard Glacier from 1994 to 1995. The annual net solar energy absorbed by the two glaciers, however, actually decreased from 1994 to 1995, from 1,061 to 780 MJm-2yr-1 on the Commonwealth Glacier and from 670 to 565 MJm-2yr-1 on the Howard Glacier.
The greater net solar radiation observed on the Taylor Glacier can largely be attributed to its much lower albedo and, to a smaller extent, the greater amount of incoming radiation it receives. Mean monthly albedos on the Taylor Glacier ranged between 49 and 66 percent whereas albedos on the Commonwealth and Howard Glaciers were usually much higher, between 61 and 90 percent. Snow cover present at the measurement sites of the Commonwealth and Howard Glaciers during most of 1994-1995 conferred the higher albedos measured on those two glaciers, whereas the mostly ice-covered (i.e., relatively snow-free) surface of the Taylor Glacier resulted in its lower albedo.
Albedo changes may also explain the small decrease in net solar radiation on the Commonwealth and Howard Glaciers from 1994 to 1995, despite the slight increase in incoming solar energy over the same time period (figure 2). In December and January 1994, a large expanse of bare ice was present at the measurement sites of these two glaciers, and mean monthly albedos ranged from 58 to 61 percent (figure 1, December 1993 not shown). New snow accumulation subsequent to January 1994 caused albedos to increase, and from 1994 to 1995, the mean annual albedo increased from 72 to 81 percent on the Commonwealth Glacier and from 76 to 83 percent on the Howard Glacier. The slightly higher annual averaged albedos on the Howard Glacier compared with the Commonwealth contribute to its lower net solar radiation.
Both the lower albedo and higher incoming solar radiation on the Taylor Glacier can be explained by the climatic regime in Taylor Valley. Precipitation is highest near McMurdo Sound, where low-pressure systems and easterly winds transfer moist air over the dry valleys (Bromley 1985). Precipitation, and presumably cloudiness, decreases westward with distance from the ocean (Bull 1966, pp. 177-194; Keys 1980). Of the three glaciers studied, the ablation zone of the Taylor Glacier is the farthest, 36 km, from the ocean and receives less snow (Fountain et al., Antarctic Journal, in this issue).
Given the observed climatic gradient in the dry valleys, it would be expected that the Commonwealth Glacier, which is closest to McMurdo Sound, would have the lowest solar energy budget of the three glaciers, but this is not the case. The greater flux of incoming and net solar radiation on the Commonwealth compared with the Howard Glacier requires other explanations. What is most different between these two glaciers that could influence the solar radiation budget is their aspects and elevations. The higher elevation of the Howard Glacier may result in a greater incidence of clouds, and its north-facing orientation in relation to the topographic relief of the surrounding Kukri Hills may result in more terrain shading than the Commonwealth Glacier. Solar modeling incorporating terrain features would be a useful tool in understanding the differences in solar radiation we have observed among the glaciers in the McMurdo Dry Valleys.
This research was supported by National Science Foundation grant OPP 92-11773, with additional support from a Desert Research Institute Nevada Medal Research Fellowship and a National Aeronautics and Space Administration Global Change Fellowship to G. Dana. We thank Karen Lewis, Paul Langevin, Peter Doran, and Paul Sullivan for assistance in the field and Robert Stone and Robert Davis for their invaluable advice on solar radiation measurements in cold regions.
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Fountain, A.G., K.J. Lewis, and G.L. Dana. 1996. McMurdo Dry Valleys LTER: Spatial variation of glacier mass balance in Taylor Valley, Antarctica. Antarctic Journal of the U.S., 31(2).
Keys, J.R. 1980. Air temperature, wind, precipitation and atmospheric humidity in the McMurdo region (Department of Geology publication No. 17, Antarctic Data Series No. 9). Wellington, New Zealand: Victoria University.