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Some Aspects of the Thermal Energy Exchange on the South Polar Snow Field and Arctic Ice Pack'

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Some Aspects of the Thermal Energy Exchange on the South Polar Snow Field and Arctic Ice Pack' MAY 1961 MONTHLY WEATHER REVIEW 173 SOME ASPECTS OF THE THERMAL ENERGY EXCHANGE ON THE SOUTH POLAR SNOW FIELD AND ARCTIC ICE PACK' KIRBY J. HANSON Po!ar Meteorology Research Project, US. Weather Bureau, Washington, D.C. [Manuscript received September 13, 1960; revised February 21, 1961 ] ABSTRACT Solar and terrestrial radiation measurements that were obtained at Amundsen-Scot>t (South Pole) Station and on Ice lsland (Bravo) T-3 are presented for representative summer and winter months. Of the South Polar net radiation loss during April 1958, approximately20 percent of the energy came from the snow and80 percent from the air. The actual atmospheric cooling rate during that period was only about lj6 of the suggested radiative cooling rate. The annual net radiation at various places in Antarctica is presented. During 1058, the South Polar atmos- phere transmitted about 73 percent of the annual extraterrestrial radiation, while at T-3 the Arctic atmosphere transmitted about 56 percent. The albedo of melting sea ice is discussed. Measurements on T-3 during July 1958 indicate that the net radiation is positive on both clear and overcast days but greatest on overcast days. Refreezing of the surface with clear skies, as observed by Untersteiner and Badglep, is discussed. 1. INTRODUCTION Ocean.ThisIsland wasabout by 5 11 miles in size and The elliptical orbit of the earthbrings it about 3 million about 52 meters thick (Crary et al. [2]) in 1953 when it miles farther from the sun at aphelion than at perihelion; drifted near 88' N., looo W. In the years that followed, consequently,during midsummer, about 7 percent less this it drifted southward and in July 1958 was located solar radiation impinges on the top of the Arctic atrnos- 79.5' N., 118' W. phere than on the Antarct'ic atmosphere during a corn- Solar radiationmeasurements at bot,h stations were parableperiod. This difference is enhancedas solar obtainedwith Eppley pyranometers. The data are energy penetrates into both polar atmospheres. Absorp- corrected for the temperature response of the instrument tion, scattering, nnd reflection of the solar rays gives each (MacDonald [Ill) and are presented in the International polarregion its own particularradiation environment. Pyrheliometric Scale of 1956. At both stations, Beckman There are also other notable digerences between the heat andWhitley (Gier andDunkle type) radiometers were budgets of these two areas; forexample, the conduction of used tomeasure the combinedsolar andterrestrial heatthrough the ice is distinctlydifferent. Annually, radiation streams. heat from the Arctic Ocean is conducted upward through 2. WINTER MONTH AT THE SOUTH POLE the thin ice pack to the relatively cold surface where the temperatureaverages about "20' C. In contrast,the With the exception of a few weeks of twilight, sunset flux of heat through the ice layers of central Antarctica at the time of the March equinox marks the beginning of is quite small. Because of the heat budget differences, the 6 months of continuousdarkness atthe South Pole. annual temperature near the North Pole is about 30' C. During the first of the dark months, April, the tempera- warmer than that at the South Pole. It is the purpose of ture a few metersabove the snowaveraged "58' C. this paper to discuss some aspects of the thermal energy (1957-59)"the same as the average temperature during budgets of these regions. the entire dark period. The datawhich are presented were obtained during the In April 1958 the long-wave radiation from the snow International Geophysical Year and later yearsat Amund- surface averaged 229 ly. day" (table l),while the atmos- sen-Scott Station, located within a mile of the geographic pheric (back) radiation returned 76 percent of this energy South Pole, and at Ice Island T-3, drifting in the Arctic (175 ly. day") tothe surface. The net radiation aver- aged "54 ly. day". 1 Paper presented at the International Antarctic Symposium at Buenos Aires, No- vember 1959. 2 Terrestrial radiation, with mostof the energy between wavelengthsof 3 to 30 microns. Unauthenticated | Downloaded 09/26/21 02:19 AM UTC 174 REVIEWWEATHERMONTHLY MAY 1961 TABLE1.-Thermal energy exchange at the snow surface, Amundsen- Scott (South Pole) Station, Antarctica AT (DEG c) TEMPERATURE (DEG C) -6 -2 +2 +6 I I -65 -60 -55 -50 -45 -40 January 1 1958 0 Incident solar radiation (ly. day-I)* ..................... 770 0 266 Reflected solar radiation (ly. day-1) ..................... 674 0 218 2 Albedo (percent) ....................................... nn .......... 82 Snow surface radiation (ly. day"). ..................... 440 229 301 Atmospheric (back) radiation (ly. day-1)". ............ 309 175 218 Return (percent) ....................................... 70 76 72 ................................ -4 Net radiation (ly. day-1) -35 "54 -35 07 Thermal energy fromsnow (ly. day-1) ...................... "". 11 .......... Thermal energy frommr (ly. day-]). ....................... ."" 43 .......... -65 *One Langley (ly.) equals one cal. cm.3 E 38 The total thermal energy in the top 12 meters of snow at the beginning and end of April 1958 wascalculated, 10 using the equation 12 Q=S" cpTdz FIGURE1.-Snow temperature profiles of April 1 and 30, 1958, and 0 monthlytemperature-change profile, Amundsen-Scott(South Pole) Station, Antarctica. where c is the specific heat in cal. gm" deg." (List [9]), p is thedensity in gm. ~m.-~(Giovinetto [4]), and T is thetemperature (fig. 1) in OK. The calculations indicate thatduring the month heat wasconducted to thesurface atthe rate of 11 ly. day". This suggest,s averaged 770 ly. day". Measurementsindicate that, of that, of the net radiation loss during April, about 20 per- this amount', 88 percent (674 ly. day") was reflected from cent of the energycame from the snow and 80 percent the snow, while the remainder (96 ly. day") was absorbed. fromthe air. During clear, coldperiods atthe South, During the same period, the snow surface radiated440 ly. Pole the snow and air supply about equal amount's of en- day", while the atmospheric (back) radiation returned 70 ergy to make up the surface radiation loss (Hanson [SI). percent (809 ly. day-l) of that energy. The net radiation Liljequist [8] found that with clearskies at hlaudheirn, averaged "35 ly. day". along the coast of Antarct'ica, roughly 40 percent of the required energy comes from the snow and 60 percent from 4. ANNUALENERGY EXCHANGE, ANTARCTICA the air. Some idea of the heat budget of the atmosphere during During the 6 months of sunlight at the South Pole, the this period can be obtained from the airborne radiation solar radiation which was incident on the snow totaled measurementswhich were takenwith Suorniairborne 9.71 x lo* ly. The snow reflected 7.96X lo4 ly., indicat- radiometers (Suomi et al. [14]). Data from the clear-sky ing an average albedo of 82 percent for the sunlit period. flight on April 27, 1959 (fig. 2) indicat'e a radiative loss of Unlike solar radiation, the emission of long-wave radia- 240 ly. day-l at 50 mb. Assuming this loss is representa- t,ion by the snow is continuous throughout the year. Dur- tive of April 1958, and adding the small amount of heat ing 1958, the snow surfaceradiation averaged301 ly. day", which was conducted from the snow (11 ly. day"), the of which about 72 percent (218 ly. day") was returned by net cooling rate from the surfaceto 50 mb. becomes atmospheric(back) radiation. This percentage is rela- 1.49' C. day". This is about 6 times greater than t'heob- tively unchanged from summer to winter even though t,he served cooling rate (0.26'C. day"). Presumably, subsi- sky is much clearer duringwinter (fig. 3). Withother dence and advection provide the necessary energy t80ac- things being equal,clear skies would certainly tend tolower count for the discrepancy. thispercentage. Apparently, a compensatingfactor is that t,hesurface temperature inversion is moreintense 3. SUMMER MONTHAT THE SOUTH POLE during winter; this would allow a greater return of bhe Eventhough the South Polar plateau receives more surface radiation. solar radiation at midsummerthan any other area on The net mdiation at theSouth Pole (2800 m.) averaged earth, the temperature of the snow surface remains well about "35 ly. day" during 1958. Liljequist [7] found an below freezing. In January, the warmest month of the annual loss of 25 ly. day" at Maudheim, and Loewe [9] summer, thetemperature averages near -27O C., and found a loss of 20 ly. day-1 at Port Martin. Both stations rarely exceeds about -17' C. are located along the coast of Ant,arctica. Rusin [13] has During January 1958, with continuous sunlight at the reported an annual net radiation of -6 to "8 ly. day" at South Pole, the incomingsolar (sun and sky)radiation Mirny, anot,her coastal station, and -19 to -22 ly. day" Unauthenticated | Downloaded 09/26/21 02:19 AM UTC &1AY 1961 MONTHLY WEATHER REVIEW 175 LANGLEY/DAY TEMPERATURE ("C) 1: 1: JAN FE0 MAR APR MAY JUN JUL AUG SEP OCT NOV DEC FIGERE3.-Average cloud coverage during the period January1957 through L)ecemhc:r 1960 at Amundsen-Scott(South Pole) Station, Antarctica. FIGURE2.--Net radiation profile, temperature profile, and tempera- ture-change profile computedfrom radiometersonde (Suomi et al. [14]) ascent at Amundsen-Scott(South Pole) Station, Antarctica, OB00 GMT April 27, 1059. The €act that a smaller percentage of the extraterrestrial radiation reaches the surface in the Arct'ic may be due to a number of factors: possibly there are thicker cloud sys- tems in the Arct,ic, t'hc surface albedo is less in the Arctic, at Pionerskaya (2700 m.) in the interior of eastern Antr anti theoptical thickness of the Arct,ic atmosphere is arctica. There is 1it't)ledoubt that, as a whole, the surface greater because of the comparatively lower surface eleva- of Antarctica has a negative net radiation, although cer- tion of theArctic.
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