Effects of multiple reflection and albedo on the net radiation in the pack ice zones of Antarctica

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109,, doi: /2003jd003927, 2004 Effects of multiple reflection and albedo on the net radiation in the pack ice zones of Antarctica G. Wendler, B. Moore, B. Hartmann, M. Stuefer, and R. Flint Geophysical Institute, University of Alaska, Fairbanks, Alaska, USA Received 27 June 2003; revised 16 January 2004; accepted 10 February 2004; published 26 March [1] Radiative and meteorological measurements were collected continuously during a cruise from Australia to Antarctica in austral summer On the average, the amount of fractional cloud cover was high (81%), reducing the incoming solar radiation. The albedo varied widely from over 80% for snow-covered undisturbed sea ice to below 10% for open water. In general, sea ice concentration was the strongest determining factor for the reflectivity. However, different ice types and snow cover also had a substantial influence on the reflectivity. When a highly reflecting surface was present (total snow-covered undisturbed ice pack with an albedo of 81%), the incoming global radiation under overcast conditions was 85% higher than for a water surface due to multiple reflections. The net radiation was found to be a strong function of both fractional cloud cover and surface albedo. For low albedo values, the net radiation increases with decreasing cloudiness, e.g., for a water surface (albedo 8%) the mean daily value increased from 88 Wm 2 for total cloud cover to 226 Wm 2 for clear skies. For highly reflecting surfaces, the net radiation decreases with decreasing cloud amount for most of the day. The mean daily value was slightly negative for clear skies ( 7 Wm 2 ), but somewhat positive (23 Wm 2 )for overcast conditions. INDEX TERMS: 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; 1620 Global Change: Climate dynamics (3309); 3349 Meteorology and Atmospheric Dynamics: Polar meteorology; 3359 Meteorology and Atmospheric Dynamics: Radiative processes Citation: Wendler, G., B. Moore, B. Hartmann, M. Stuefer, and R. Flint (2004), Effects of multiple reflection and albedo on the net radiation in the pack ice zones of Antarctica, J. Geophys. Res., 109,, doi: /2003jd Introduction [2] On 14 December 2000, the USCGC Polar Sea left from Hobart, Tasmania, for a trip to McMurdo, Antarctica. Its primary mission was the opening of McMurdo Sound to tanker and ship traffic. In Figure 1 the route of the trip as well as the sea ice edge as of 22 December 2000 are shown. On 17 December 2000 we observed the first sea ice from the ship. During the whole trip we carried out continuous surface energy budget measurements from the ship, supported by visual hourly observations of cloudiness and sea ice concentration. In addition, a digital camera was mounted at the crow s nest looking forward, giving continuous observations of ice and clouds in this direction. [3] In a previous investigation [Wendler et al., 1997], we reported on the mean fluxes of the surface energy budget in the sea ice region surrounding Antarctica. In this research, we try to obtain a better understanding of the physical processes. More specifically, we investigate the effect of multiple reflection on the global radiation, which is especially pronounced during the presence of polar stratus clouds. Further, we examine the compensating effect of the longwave radiation in the presence of stratus clouds on the net radiation. Combined, these two effects can, for a Copyright 2004 by the American Geophysical Union /04/2003JD highly reflecting surface, result in a more positive net radiation in the presence of clouds than for clear skies, for which Ambach [1974] coined the term radiation paradox. [4] The southern sea ice extent varies widely throughout the year. The Antarctica continent has an area of some km 2, the maximum in sea ice extent is found in spring with km 2, which is reduced to km 2 in late summer. Hence, at its maximum ice extent, the area of Antarctica doubles as far as the radiation budget is concerned. 2. Instrumentation, Observations, and Mean Meteorological Conditions [5] Twenty-one meteorological, radiative, and support parameters were measured and recorded continuously. All instruments were sampled twice a second, and the data were averaged for 5-min intervals. The measurements were recorded on a Campbell Scientific 21X data logger. A notebook computer allowed access to the data in real or near-real time, so that the data quality could be checked any time, and instrumentation breakdown could be detected without large time delays. It should be pointed out that such a voyage is tough on instrumentation, as we are crossing one of the roughest seas (the Roaring Forties) with lots of spray in the air. Sea salt accumulates after the spray dries; therefore the domes of the radiation instruments must 1of8

2 Figure 1. The trip of the USCGC Polar Sea from Hobart, Tasmania, to McMurdo, Antarctica, during the 2000/2001 voyage. The position of the ship at noon is given, as well as the ice edge surrounding Antarctica as of 22 December be cleaned frequently. When within the ice pack the swell declines; however, the mechanical action of ice breaking can be severe, and the shocks are transmitted to the instrumentation. In Table 1, the instruments are listed. [6] The instrumentation was installed on the fly bridge, about 32 m above the ocean surface. At this height ocean spray is kept to a minimum. Furthermore, this high location was chosen to minimize the effects of shadowing from the upper superstructure of the ship, such as the crows nest. In Figure 2 some of the instrumentation is shown. As Table 1. Instrumentation Deployed on the Polar Sea Measured Variable Instrument Global radiation, incoming Eppley PSP Global radiation, reflected Eppley PSP IR radiation, incoming Eppley PIR IR radiation, outgoing Eppley PIR UV-A radiation Eppley UV-B radiation Yankee Environmental Systems Surface temperature Heitronics Air temperatures Vaisala HMP-35A Humidities Vaisala HMD-35D Atmospheric pressure Paroscientific 216b-101 Pitch Accu-Star Roll Accu-Star Wind speed Met One 014A Wind direction Met One 024A GPS Magellan Figure 2. Radiation equipment installed on the fly-bridge of the USCGC Polar Sea. Right to left: UV-B Yankee Environmental System, Eppley PIR (infrared), Eppley UV-A, Eppley PSP (visible). The sea ice concentration is 8/10. mentioned previously, all elements were measured twice per second and then averaged for 5 min. The only exception were pitch and roll, for which we measured the standard deviation for each 5-min interval, as the mean would have given values close to zero. We tested the radiation data against increasing standard deviation in pitch and roll, but the 5-min data appear to be independent of the wave actions. This might be in part due to the fact that increasing wave actions are normally occurring during stormy periods, during which high amount of cloudiness is observed. During overcast conditions only diffuse radiation is observed, which is less affected by deviation from the horizontal surface than direct radiation. [7] The reflected global and outgoing longwave radiation sensors, directed to the surface, were mounted on a boom extending 5 m over the side of the ship. These instruments were leveled for the nonmoving position of the ship in the Hobart harbor. The ship wall affects the measurements of these instruments. Direct spot measurements over the snowcovered sea ice showed that the ship-based reflectivity was on average 6% too low for 10/10 ice cover. For open water, errors were small and no corrections were necessary; for intervening ice concentrations, linear interpolation was used for the correction. [8] The sea surface or ice surface temperature was measured with a Heitronics infrared gun, which was placed in a waterproof housing with a window, and a dome shaped extension was attached to minimize the effect of salt water spray. When looking at open water areas within the pack, 2of8

3 trip, mostly consisting of polar stratus clouds. The lowest pressure was observed on 24 December, and the following day wind speeds in excess of 25 m/s were observed. For the first 4 days, no sea ice was observed, thereafter the ice concentration varied widely, until we reached the solidly frozen McMurdo Sound. Figure 3. Selected meteorological conditions during the cruise through the ice pack to McMurdo, Antarctica, summer normally values within 0.2 C of the expected 1.8 C were found. Further, for open water, good agreement was found between direct hourly measurement and the measurements from our instrument. One of the precision pyranometers had been recalibrated by the manufacturer (Eppley Corporation) and was intercompared with the second instrument of this type. The cosine error is 1% for solar elevations above 20, but increases with decreasing solar elevations. For a more detailed description of all the instrumentation and their limitations, see Hauser et al. [1999]. [9] Besides these measurements, a networked digital camera was installed at the crow s nest looking forward, giving continuous information of the ice conditions. The crows nest was also a minor obstruction for the upward looking radiation instrumentation, however, the digital camera made it possible to judge if shading occurred on these instruments. One such case was observed in the McMurdo Sound and corrections were carried out. Further, hourly data of all standard meteorological conditions, cloud amount and type, and the sea ice conditions, were carried out by the Marine Science Technicians on board the Polar Sea. [10] When leaving Hobart, Tasmania, on 14 December, the temperature was around 15 C, and during the next 3 days, the temperature decreased steadily (see Figure 3). On 17 December the first ice became visible and the temperature hovered around the melting point of sea ice (about 1.8 C) until we reached McMurdo Sound on 28 December, where due to the absence of the moderating effect of open water, the diurnal temperature variation increased, and night temperatures dropped to 9 C. The humidity was high during the whole trip with values of about 80% relative humidity, typical for a marine environment. Overcast conditions were dominant during the whole 3. Discussion and Results [11] The cloud amount during the voyage was very high. Throughout the whole cruise not one totally clear day was observed. The global radiation (G) is, of course, strongly influence by the amount of fractional cloud cover. Further, the global radiation is a function of the solar elevation, which varied even at solar noon due to the ship s location, as the latitude of the ship changed substantially even while in the ice pack. To a lesser extent, the date of the observations in respect to the summer solstice played a role. To minimize this effect, we normalized the radiation data using the total transmittance of the atmosphere (KT), also called the clearness index. The total transmittance is defined as the ratio of the global radiation (G) to the extraterrestrial radiation incident on a horizontal surface. Even with unchanging atmospheric conditions, the path length through the atmosphere affects the clearness index. Hence, a comparison makes sense only for similar air mass values. In Table 2, clearness indices are presented for an air mass of around 1.5 (close to local noon), for clear and overcast conditions, both for a highly reflecting surface (10/10 ice which is snow-covered mean albedo 71%) and an absorbing surface (open water, mean albedo 8%). It should be pointed out that even for closed icepack, the albedo varied widely. The sea ice surrounding Antarctica is thin in comparison to the Arctic, and hardly any multiyear ice was observed. With the exception of the McMurdo Sound, the ice has gone though multiple surface melting/refreezing cycles. The snow depth on the ice varied widely, and sometimes flooded ice floes with very low albedo values were observed. However, melt ponds were not observed, perhaps due to the small average thickness of the ice. Only after entering McMurdo Sound, where the ice was much thicker with typical values of cm, covered by undisturbed snow, were fairly uniform high albedo values around 80% measured. It should be also mentioned, that the albedo of water was not constant either. During a major storm, which caused many white caps, substantial increases were observed. However, this happened in the open sea and is of limited interest here. Each transmittance value in the table consists of the mean of 10 observations; the standard deviation is also presented. [12] The extraterrestrial radiation incident on a horizontal surface (ET h ) for the air mass of 1.5 (solar elevation 43 ) is 934 Wm 2, hence by multiplying this value with the clearness index, one obtains the global radiation at the surface. The table shows that the standard deviation is small Table 2. Total Atmospheric Transmittance (KT) as a Function of Cloudiness and Surface Albedo 10/10 Ice Open Water Clear ± ± Overcast ± ± of8

4 for all four data sets. Further, for clear conditions, KT is fairly independent of the surface albedo, while for 10/10 stratus cloud cover, KT nearly doubles when going from water to snow-covered sea ice. When under partial cloud cover, large variations are experienced, depending on whether or not the radiometer is exposed to direct sunshine. On average, KT shows a strong dependence on cloud amount. As one can also deduce from the table, the decrease in transmittance with increasing fractional cloud cover is much more pronounced for open water than for snowcovered sea ice. [13] The effect of the surface albedo on the incoming radiation has been known for more than a century and was of great practical importance for navigation in ice-covered waters. Nansen [1897] and other early polar explorers utilized the dark lower part of the stratus clouds to navigate to open water, as the open water areas reflect less visible light upward than the surrounding sea ice or snowcovered surfaces. Angström [1933] investigated this phenomenon and developed an empirical formula, including the effects of the surface albedo on shortwave radiation received at the ground. Later, Vowinckel and Orvig [1962] reported the importance of examining cloud type and surface albedo for estimating the incoming solar radiation. Möller [1965] showed the increase in global radiation over snow-covered ground for both clear and overcast skies. The importance of multiple reflection between a highly reflecting ground surface and the lower portion of polar stratus clouds was shown by Catchpole and Moodie [1971] and Wendler [1972]. Wiscombe [1973] was the first to develop models to estimate the solar radiation in the presence of stratus clouds with cloud-ground reflection approximations included in the model. Warren [1982] wrote a comprehensive review article on the optical properties of snow, and Allison et al. [1993] carried out albedo measurements of Antarctic sea ice as function of sea ice thickness and snow cover. Further, Stone [1997] investigated the temperature response to cloud-radiative processes in the Arctic. In the present investigation, high quality radiative data add information for drastically changing surface albedo, namely water, broken sea ice and closed, snow-covered pack. The moving platform allowed such measurements for close to midsummer within a short time period without much change in the solar radiation on top of the atmosphere. [14] If one assumes that a cloud scatters light in all directions equally, which is approximately true for a cloud of sufficient thickness, then the schematic simplified diagram given in Figure 4 is applicable. It can be seen that the global radiation (G), which is received at the surface, is G ¼ I 0 ð1 aþð1 bþþi 0 ð1 aþ 2 ð1 bþab þ... ð1þ which can be also written as G ¼ I 0 ð1 aþð1 b Þ X1 where z is (1 a)ab, I 0 is radiation on top of the cloud, a is absorption within the cloud, b is albedo of cloud, and a is albedo of surface. n¼0 z n ; ð2þ Figure 4. A sketch of multiple reflection between a cloud cover and the surface. [15] One can see from the equations that multiple reflection becomes increasingly important with increasing cloud and surface albedo and with decreasing absorption within the cloud. Middleton [1954] mathematically developed a theory of the spectral irradiance received from an overcast sky by examining the effects of droplet size and concentration, cloud thickness, global radiation above the clouds, ground reflection and absorption by cloud water. He found the radiation absorbed by the droplets to be small and neglected this effect in his development. More specifically he stated that k/b is less than 10 4 for all wavelengths in the range 0.3 to 0.8 microns, with k being the observed fraction of light on a drop and b being the coefficient of attenuation by scattering. However, Wiscombe et al. [1984] showed the importance of cloud droplets for the absorption in the near infrared of the solar spectrum. The absorption coefficient will depend on the cloud thickness and the composition of stratus clouds. We assumed a value of 7% [Wendler, 1972]. The stratus cloud albedo was originally studied by Neiburger [1949] and depends on several factors, such as thickness of the cloud layer, single or multiple cloud layers, and number and size distribution of the droplets. We assumed an albedo value of 60% over a nonreflecting surface. Using these input data, and the measured surface albedo, we calculate a reduction in radiation of 61% and 39%, respectively, for open water and unbroken pack due to the presence of stratus clouds. This compares well with our measurements; reductions of 66% and 40%, respectively, were found (Table 2). Our measurements and calculation are also in general agreement with Shine [1984]. [16] Summarizing, it has been shown that multiple reflection is important in polar regions. It is not possible to give typical values of global radiation for specific cloud types without knowing the albedo of the underlying surface. Global radiation measurements obtained from an ice floe or ice island tend to overestimate the radiative flux and are not typical for the whole region Global Radiation [17] We would like to compare the global radiation (G)for a totally clear and totally cloudy (10/10 St) day, both for a highly reflecting surface (10/10 snow-covered ice) and for an absorbing surface (open water). We did not, however, expe- 4of8

5 with a mean daily rate of 311 Wm 2. For cloudy conditions over water (107 Wm 2 ) and clear conditions over snow (77 Wm 2 ) somewhat similar mean daily rates were found, while the fluxes for cloudy conditions over snow are indeed low, with a maximum at noon of less than 100 Wm 2, and a mean daily value of 44 Wm Net Radiation [19] The net radiation (R n ) is the sum of the shortwave (SWB) and longwave radiation balance (LWB) and can be written as R n ¼ Gð1 aþþðlw in LW out Þ: ð3þ Figure 5. Global radiation on top of the atmosphere (curve 0), for clear conditions over a highly reflecting surface (10/10 ice) (curve 1) and an absorbing surface (water) (curve 2) and for cloudy conditions over ice (curve 3) and water (curve 4), midsummer, 70 S. rience totally clear days, and in addition the ship moved, sometimes rapidly, changing day length and solar elevations. Hence, we normalized our radiation data to 70 S and summer solstice. First, we calculated from our measurements the clearness index as a function of air mass, cloudiness and surface reflectivity. We found the clearness index for unchanging sky conditions (e.g., clear skies) and unchanging ice conditions (e.g., 10/10 ice cover) to be a function of air mass. The relationship we found was semi-logarithmic, and using the extraterrestrial radiation incident to a horizontal surface (curve 0 of Figure 5), we were able to compare the global radiation under different conditions. Curves 1 and 2 of Figure 5 are similar and represent the global radiation for clear conditions over snow and water, while there is a large difference between curve 3 and 4, the radiation for overcast skies. As stratus cloud cover is the most common type, we use clearness indices for this cloud type. It can be seen that the global radiation for snow (curve 3) is about twice as high as the value observed for water (curve 4), and about 1/2 of the value for clear sky conditions (curve 1 and 2). Differently expressed, the global radiation is reduced during the presence of 10/10 Stratus cloud cover by about 50% to 75%, depending on the surface reflectivity. [18] The shortwave radiation balance [G (1 a)] is the amount of radiation absorbed at the surface, consisting of the difference between the global incoming and the global reflected radiation. A mean albedo (a) of 8% was determined for water without white caps, typical for areas within the outer extent of the Antarctic sea ice area. The albedo for sea ice displayed much more variability, depending on the sea ice type and snow cover. The highest values of around 80% were found for sea ice covered with dry snow. We use these two values to calculate the shortwave radiation balance (Figure 6). There is further a slight increase in the albedo for lower solar elevations [Carroll and Fitch, 1981; Wendler et al., 2000], which is well pronounced for clear sky conditions. As for low solar elevations the global radiation is weak, this effect has limited influence on the daily mean values and was ignored. It can be seen from the figure that the shortwave radiation budget is high over water under clear skies. The flux at noon is above 600 Wm 2, The shortwave radiation balance is always positive or zero, as the global radiation is always positive. Further, it is strongly dependent on the surface albedo, as was demonstrated in Figure 6. In addition, the global radiation is affected by the surface albedo due to multiple reflection, the effect becoming stronger with increasing amount of fractional cloudiness. In contrast to this, the longwave radiation balance is normally negative. The outgoing values are a function of the surface temperature according to the Stefan-Boltzmann law, and to a lesser extent, dependent upon the emissivity of the surface. For water, sea ice and snow, emissivity values close to unity are found [e.g., Geiger, 1975]. A number of atmospheric gases contribute to the downwelling long wave radiation, water vapor being the one having the most variable concentration. Even more significant contributors are low clouds, when present. As the atmosphere and the clouds are normally colder than the surface, the longwave incoming radiation is normally less than the outgoing, making the balance negative. Only during times of strong advection of warm clouds from lower latitudes can the balance become positive. In Figure 7 we plotted hourly values of the net longwave radiation against fractional cloud cover. A large amount of scatter is observed. This is believed to be caused by different cloud thicknesses, multilayer clouds, and observed changes in ceiling heights. Further, the optical and physical characteristic will change even for the same cloud type, as, for Figure 6. Shortwave radiation balance for clear (curve 1) and cloudy (curve 2) conditions over an absorbing surface (water) and for clear (curve 3) and cloudy (curve 4) conditions over a highly reflecting surface (10/10 ice), midsummer, 70 S. 5of8

6 Figure 7. The longwave radiation balance as function of fractional cloudiness. The solid line represents the best exponential fit. A correlation coefficient of 0.84 was found. example, in the McMurdo Sound with no or little open water the moisture fluxes from the surface will be reduced. Additionally, sometimes different cloud types were observed. The quality of the observations by the crew on the ship is believed to be good. We also made twice daily weather observations, and the agreement was good. Stratus is easy to distinguish, especially as there was always daylight. The net longwave radiation for clear sky conditions has a mean daily rate of 84 Wm 2, and for overcast conditions of 21 Wm 2. On average a systematic diurnal variation in the longwave radiation balance was observed, with maximum losses in the afternoon around h, and a minimum in the early morning hours (not shown). The diurnal variation had an amplitude of 11 Wm 2. This is mainly caused by the diurnal temperature variation and represents about 2.5 C, which is in agreement with the mean diurnal temperature measurements. Open water has a fairly constant surface temperature, however sea ice normally went through a daily cycle with freezing at night and melting around noon. [20] In Figure 8 the net radiation is presented, factoring in the diurnal variation of the longwave radiation. For a water surface under clear skies, the net radiation is strongly positive for most of the day (17 hours). A maximum flux of 589 Wm 2 is found at noon; at night the fluxes become negative with losses of up to 78 Wm 2. For overcast skies, the maximum in irradiation is reduced substantially (207 Wm 2 ), but the losses at night are as well reduced ( 16 Wm 2 ). The time period with positive net radiation is slightly enhanced when compared to clear sky conditions. Over a snow-covered ice surface, the net radiation is much lower and nearly identical at noon for clear and overcast conditions, however, at night the losses for clear sky conditions are more substantial. Specific values can be seen from Table 3. [21] Perhaps the most interesting result of the table is the fact that the mean daily net radiation is slightly negative on a clear day over a highly reflecting surface even in midsummer, when the solar radiation on top of the atmosphere for 70 S has a mean daily value of 514 Wm 2, more than in the tropics at that time of the year. With the Sun continuously above the horizon, only 11 hours show positive values, at which melting can occur due to a positive net radiation. For cloudy conditions, the mean daily flux becomes slightly positive (23 Wm 2 ) and the time period with a positive radiation balance is extended to 16 hours. A more positive net radiation under cloudy conditions was already found by Ambach [1974], carrying out measurements on the Greenland ice cap; he coined the term Figure 8. Net radiation for clear (curve 1) and cloudy (curve 2) conditions over an absorbing surface (water) and clear (curve 3) and cloudy (curve 4) conditions over a highly reflecting surface (10/10 ice), midsummer, 70 S. Table 3. Characteristics of the Net Radiation Fluxes for Midsummer at 70 S for Snow-Covered Sea Ice and Open Water Under Clear and Cloudy Conditions a Clear Snow Clear Water Cloudy Snow Cloudy Water Maximum, Wm Minimum, Wm Average, Wm Time, hours a Further, the duration of time with a positive net radiation is given. 6of8

7 [24] It should be noted that these extreme conditions are not common. Exceptions are areas that are protected, normally found close to the Antarctic Continent like the McMurdo Sound. Here the ice was unbroken and uniformly covered with snow. The other side of the spectrum, open water, was more common. One large open water area within the pack ice zone was observed when entering the Ross Sea. However, broken pack of varying concentration was much more frequent. We observed mean values for the ice concentration of 65%, mean albedo value of 0.45 and a mean cloudiness of 81%. For these values we obtain a mean daily net radiation flux of 89 Wm 2. Under the assumption that the whole amount is used solely for the melting of ice, 25 mm of ice can be melted daily. Once the ablation process has started, the fairly high melting rate accounts for the fast disintegration of the sea ice in midsummer, Figure 9. Modeled mean net radiation for 70 S and midsummer as function of ice concentration (0/10 = open water, 10/10 = closed pack) and cloud amount (0/10 = totally clear sky, 10/10 = total overcast). radiation paradox for the enhanced net radiation under cloudy skies. It is well known that in polar regions cloudy conditions are normally warmer in winter than clear ones. However, the fact that this can hold true even in midsummer as long as the surface reflectivity is high is rather surprising. [22] Applying the values of Table 3, we can now calculate the daily mean net radiation (R n ):. R n ¼ SWB 0=0 SWB 0=0 SWB i 0=10 10 SWB 0=0 SWB ċ 10=0 10 SWB 0=0 SWB 0=10 ðswb 10=0 SWB 10=10 ÞŠ ċ 10 þ LWB 0 þ ðlwb 10 LWB 0 Þ ċ 10; ð4þ where SWB is shortwave radiation balance: first subscript cloudiness in tenths, second subscript ice concentration in tenths; i is ice concentration in tenths: 0 = open water, 10 = closed ice pack; c is cloudiness in tenths: 0 = clear sky, 10 = total overcast; a is surface albedo; LWB 0 is longwave radiation balance for clear skies; and LWB 10 is longwave radiation balance for total overcast. [23] The first term of the equation represents the short wave radiation balance for clear sky and open water, the second term modifies this for changing ice cover (surface albedo), the third term takes the cloudiness into consideration, and the fourth one corrects for nonlinearity. The last two terms of the equation consider the longwave radiation, the first one gives the longwave net radiation for clear sky, the second term represents a correction for increasing cloudiness. In Figure 9 this equation is displayed in graphical format, the net radiation being presented as isopletes. The highest values are found for clear skies and low albedo values. However, with increasing ice concentration, the relationship changes and for closed pack, the net radiation increases with increasing cloudiness. 4. Conclusion [25] It has been demonstrated that the underlying surface has a great influence on the surface radiation balance both due to albedo changes and multiple reflection. As long as the sea ice is continuous and covered by snow, little ablation occurs. We found, for both clear and cloudy conditions, a diurnal cycle, even in mid-summer, with melting around noon with high solar elevations and refreezing at night during times of low solar elevations. This fact is not only of academic interest, but also of great importance for climatic change issues. Besides the effect on the surface radiation balance, we found the sea ice to insulate the warm ocean from the cooler atmosphere, reducing in this way the sensible and latent heat fluxes. While most trend studies carried out until recently found a decrease in ice extent, a new study [Parkinson, 2002] using in part new algorithms to determine the ice amount came to the opposite conclusion. Hence, no definite conclusion can be reached at this time, and more studies with longer and improved data sets are essential. [26] Acknowledgments. The research was supported by NSF grant OPP Captain Keith Johnson, XO Steve Wheeler, and the whole crew of the Polar Sea supported us wonderfully. Special thanks go also to the MST s and the helicopter detachment. Gunter Weller and Martha Shulski read the manuscript and made valuable comments; to both our thanks. References Allison, I., R. Brandt, and S. Warren (1993), East Antarctic sea ice: Albedo, thickness distribution, and snow cover, J. Geophys. Res., 98(C7), 12,417 12,429. Ambach, W. (1974), The influence of fractional cloud cover on the net radiation balance of a snow surface with high albedo, J. Glaciol., 13(67), Angström, A. (1933), On the total radiation from sun and sky at Sveanor ( N, E), Geogr. Ann., 15(2/3), Carroll, J. J., and B. Fitch (1981), Effects of solar elevation and cloudiness of snow albedo at the South Pole, J. Geophys. Res., 86(C6), Catchpole, A. J., and D. W. Moodie (1971), Multiple reflection in arctic regions, Weather, 26, Geiger, R. (1975), The Climate Near the Ground, 611 pp., Harvard Univ. Press, Cambridge, Mass. Hauser, H., G. Wendler, U. Adolphs, and M. O. Jeffries (1999), Energy exchange in early spring over sea ice in the Pacific sector of the Southern Ocean, J. Geophys. Res., 104(D4), Middleton, W. E. K. (1954), The color of overcast sky, J. Opt. Soc. Am., 44, Möller, F. (1965), On the backscattering of global radiation by the sky, Tellus, 17, of8

8 Nansen, F. (1897), In Nacht und Eis.In: Die Norwegische Polar-Expedition, , vol. 1, Brockhaus, Leipzig. Neiburger, M. (1949), Reflection, absorption, and transmission of insolation by stratus clouds, J. Meteorol., 6, Parkinson, C. (2002), Trends in the length of the Southern Ocean sea ice season , Ann. Glaciol., 34, Shine, K. P. (1984), Shortwave flux over high albedo surfaces, Q. J. R. Meteorol. Soc., 110, Stone, R. S. (1997), Variations in western Arctic temperatures in response to cloud radiative and synoptic scale influences, J. Geophys. Res., 102(D18), 21,769 21,776. Vowinckel, E., and S. Orvig (1962), Relation between solar radiation income and cloud type in the Arctic, J. Appl. Meteorol., 1, Warren, S. (1982), Optical properties of snow, Rev. Geophys., 20, Wendler, G. (1972), Effect of arctic stratus clouds on the radiation regime, in Studies of the Solar and Terrestrial Radiation Fluxes Over Arctic Pack Ice, Rep. 1783, Adv. Res. Proj. Agency, Washington, D. C. Wendler, G., U. Adolphs, A. Hauser, and B. Moore (1997), On the surface energy budget of sea ice, J. Glaciol., 43(143), Wendler, G., B. Moore, D. Dissing, and J. Kelley (2000), On the radiation characteristics of Antarctic sea ice, Atmos. Ocean, 38(2), Wiscombe, W. (1973), Solar radiation calculations for arctic summer stratus conditions, in Climate of the Arctic, edited by S. A. Bowling and G. Weller, pp , Geophys. Inst., Univ. of Alaska, Fairbanks. Wiscombe, W., R. Welch, and W. Hall (1984), The effect of very large drops on cloud absorption, part I: Parcel models, J. Atmos. Sci., 41, R. Flint, B. Hartmann, B. Moore, M. Stuefer, and G. Wendler, Geophysical Institute, University of Alaska, Fairbanks, AK , USA. 8of8