G. WENDLER*, B. MOORE, D. DISSING, and J. KELLEY, University of Alaska-Fairbanks, Fairbanks, Alaska 99775
*Present address: International Centre for Antarctic Information and Research, Christchurch, New Zealand
During the course of the year, the areal extent of antarctic sea ice varies a great deal. The normal maximum amount in late winter is 19x106 square kilometers; the minimum in late summer is only 3.5x106 square kilometers. The presence or absence of sea ice influences strongly the energy transfer between ocean and atmosphere and is of great importance in understanding climate (Radok, Streten, and Weller 1975). In the coastal region, the interaction of the sea ice with the semi-permanent katabatic winds is of special importance (Wendler, Gilmore, and Curtis 1997).
Most of the studies on the radiative characteristics of sea ice have been carried out in the Arctic (e.g., Hansen 1961). Ice conditions around Antarctica, however, are not similar to those in the Arctic, a fact known for a long time. In March 1840, Charles Wilkes wrote in a letter to Sir James Clark Ross: "The ice of the Antarctic is of totally different character from that of the Arctic." (cited from Kearns and Britton 1955). The majority of the sea ice around Antarctica melts in summer, so that unlike the Arctic, where a large amount of multiyear ice is present, most ice encountered is first-year ice and thin (mostly <1 meter). Comparable radiative studies of sea ice using a ship as a platform were carried out by Allison, Brandt, and Warren (1993) and Wendler et al. (1997).
In 1997, we analyzed the reflectivity and the radiation budget of sea ice, data obtained from the 1995-1996 USCGC Polar Star cruise, which went from Hobart, Tasmania, to McMurdo, Antarctica, to open up the sound for tanker and other support ship traffic. During this cruise, radiative and meteorological measurements were carried out continuously. Short-wave incoming and reflected radiation, ultraviolet radiation, long-wave incoming and outgoing fluxes as well as numerous meteorological and support data were obtained. Some of the results follow.
The incoming solar radiation had a mean value of 217 watts per square meter (W m-2); this was a relatively weak value due to the large amount of fractional cloud cover observed. For a large part of the trip, the Sun was above the horizon for 24 hours a day. Looking at the mean diurnal variation, a maximum of 461 W m-2 was observed around local noon, whereas the minimum was 40 W m-2 at midnight. Daily courses for individual days varied widely, depending mostly on the amount and type of clouds and to a lesser degree on the position of the ship and atmospheric turbidity. A maximum mean daily global value of 305 W m-2 was observed on 10 January 1996, a partly cloudy day; the minimum (139 W m-2) was observed on 25 December 1995, a day with a thick layer of stratus overcast and occasional snowfall.
The albedo varied widely and was found to be a function not only of ice concentration but also of ice type. If the sea ice was snow covered, especially high values were observed. Two-minute values for an hour close to solar noon are presented in figure 1. For calm sea conditions (figure 1 A ) values around 6 percent were found for water; they increased with both decreasing solar elevations and sea stage. On 22 December 1995, a very stormy day with wind speeds in excess of 20 meters per second and a large amount of white caps, values up to 23 percent were observed (figure 1 B ). In figure 1 C , the transition from 10/10 sea ice to open-water conditions is depicted. Although the albedo of the snow-covered sea ice was 67 percent, it dropped to 18 percent in less than an hour, indicating a relatively well-pronounced ice edge, which was observed on 26 December 1996. Ice conditions, however, can be much more variable as can be seen in figure 1 D . Albedo values between 11 and 69 percent were observed during this hour, indicating a large variation in ice concentration over a relatively small area. Although the albedo is fairly constant for each ice type, it varies considerably from type to type. For thin ice (figure 1 E ), the values varied little during the hour. A mean value of 34 percent was observed with a maximum deviation of 2 percent. Such low values are in agreement with Kukla and Robinson (1980). Flooded sea ice (figure 1 F ), which we observed on 27 December 1995, showed similar values to thin ice (figure 1 E ); however a much larger variation in albedo than with the thin ice was observed. The values varied between 22 and 43 percent. In contrast to broken ice, which showed a similar or larger magnitude in the total value, the variation in space is much smoother for flooded ice. Finally, snow-covered pack ice (figure 1 G ) showed high, fairly constant values. For an hour on 13 January 1996, a mean value of 74 percent was observed.
Although the albedo, and with it the short-wave radiation budget, is strongly influenced by the presence of sea ice, the long-wave radiation budget is affected to a much lesser degree. The outgoing radiation is a function of the surface temperature; hence, the infrared losses from an ice-covered surface are always equal to or less than those for sea water. There is only a very small diurnal variation of about 10 W m-2 following roughly the diurnal course of the air temperature. Typical values of the outgoing long-wave radiation vary between 250 W m-2 and 300 W m-2. Outgoing radiation tended to decrease with increasing ice concentration, which is, of course, an effect of the cooler surface temperature.
The long-wave incoming radiation is mostly a function of cloudiness and, to a lesser degree, dependent on the water vapor content and turbidity of the atmosphere. These values do not show a systematic diurnal variation for our cruise. The values are normally smaller than the outgoing long-wave radiation, resulting in mean negative net long-wave radiation values. The mean value, -27 W m-2, represents only about 10 percent of the outgoing flux. In other words, about 90 percent of the long-wave outgoing surface radiation is radiated back from the clouds and the atmosphere. This value is high and a result of the large amount of the fractional cloud cover. The net long-wave radiation shows the smallest losses during the early morning hours (-23 W m-2), when the surface temperature is at a minimum, and the greatest losses in the early afternoon (-33 W m-2), when the surface temperature has its maximum.
For the observed albedo and long-wave radiation values, modeling results show (figure 2) that the net radiation was always positive when averaged over a day. The magnitude and diurnal variation, however, depended strongly on the surface albedo. Integrating over a day (figure 2 B ), one sees that an albedo in excess of 85 percent is necessary to obtain a negative balance. During our trip, mean hourly albedo values were seen to range from 6 percent to about 70 percent, an indication that the net radiation was positive. This result is the expected one because the ice around Antarctica melts during the summer months.
This research was supported by National Science Foundation grant OPP 94-13879. We thank the captain and crew of the Polar Star as well as the helicopter detachment, which supported us wonderfully. Science Officer Matt Smith, who helped us a great deal, deserves special mention. C. Stearns and J. Cassano from the University of Wisconsin serviced/installed the automatic weather station. A. Hauser read and improved this manuscript. To all of them our sincere thanks.
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