Radiative effects of clouds, ice sheet and sea ice in the Antarctic

Transcription

1 Snow and fee Covers: Interactions with the Atmosphere and Ecosystems (Proceedings of Yokohama Symposia J2 and J5, July 1993). IAHS Publ. no. 223, Radiative effects of clouds, ice sheet and sea ice in the Antarctic TAKASHI YAMANOUCHI National Institute of Polar Research, Kaga, Itabashi-ku, Tokyo 173, Japan THOMAS P. CHARLOCK NASA Langley Research Center, Hampton, Virginia , USA Abstract The effects of clouds, ice sheet and sea ice on the radiation budget in the Antarctic were examined using the ERBE data and surface observations at Antarctic stations in 1987/1988. Long-wave radiation emitted by clouds was found to heat the surface throughout the year and strongly cool the atmosphere over Antarctica. The elevation of the ice sheet surface reduced the outgoing long-wave radiation, making the radiation budget in the two polar regions asymmetric. Sea ice had a significant impact on radiation; however, cloud distribution reduced the effect. INTRODUCTION The Antarctic ice sheet and surrounding sea ice are the main factors characterizing the role of the Antarctic in the global climate. The effects of ice sheet and sea ice together with clouds on the radiation budget in the Antarctic were examined using the Earth Radiation Budget Experiment (ERBE) (Barkstrom, 1984; Harrison et al, 1990) data, surface observations at the Antarctic stations and SSM/I sea ice concentrations (National Snow and Ice Data Center, 1992) in 1987/1988. ERBE daily or monthly average data at 2.5 x 2.5 grid (ERBE S-4 data set) and instantaneous pixel data at the satellite pass (S-8 data set) are used as the main data source. Surface radiation budget observations had been made at the Japanese Antarctic Station Syowa (69 00'S, 39 35'E; Yamanouchi, 1989), and at the US Amundsen Scott South Pole Station (90 S; Dutton et al., 1989). RADIATIVE EFFECT OF CLOUDS - COMPARISON OF SURFACE AND SATELLITE OBSERVATION ERBE top of the atmosphere radiative fluxes and albedo are compared with surface observations at Syowa Station as one of the example of observations on sea ice, and at the South Pole as one of the typical observations at the interior of the continent to see the radiative effect of clouds. In order to get a better compatibility with the surface, one point measurements, S-8 pixel data of ERBE, which are of about 50 km scale, are used. Cloud amount dependence of the outgoing long-wave radiation (OLR) at the top of the atmosphere (TOA) and the net long-wave radiation at the surface are shown for July

2 30 Takashi Yamanouchi & Thomas P. Charlock and January at Syowa and South Pole Station in Fig. 1. Except for the winter month at the South Pole, long-wave radiative effects of clouds are heating at the TO A, large heating at the surface and cooling for the atmosphere. In the winter month at the South Pole, radiative effects of clouds by the long-wave are very different: a small cooling at the TOA and heating at the surface and cooling for the atmosphere. Short-wave radiations are effective only during summer months. At both Syowa and the South Pole Stations, short-wave radiative effects of clouds are negative at the TOA by increasing albedo and negative at the surface mainly by reducing the incoming solar radiation. For net radiation, the cloud effect is negative in January; the short-wave surpasses the effects of the long-wave. Radiative effects of clouds at Asuka Station derived from the AVHRR data were heating by the short-wave in summer months (Aoki & Yamanouchi, 1992), showing a difference from the present results. Syowa, July 1987 Syowa, January j i TOA 4 i > CD = 100 ""O cr 50 --! -..; SFC o o Cloud amount {n/10) Cloud amount (n/10) South Pole, July 1987 South Pole, January Cloud amount (n/10) Cloud amount (n/10) Fig. 1 Cloud amount dependence of outgoing long-wave radiation at the top of the atmosphere (solid circle) and net long-wave flux at the surface (white square), Syowa Station and the South Pole, July 1987 and January Solid and dashed lines are regression lines.

3 Radiative effects of clouds, ice sheet and sea ice 31 Radiative effects of clouds by the long-wave estimated from the brightness temperature of AVHRR for a larger area in East Antarctica were similar to the present results; cooling in winter at the interior of the continent, and heating in summer (Murata & Yamanouchi, 1994). These results characterize the long-wave radiative effect of clouds in winter at the interior of the continent, where the strong surface inversion develops at the surface. RADIATIVE EFFECT OF AN ICE SHEET The radiative effect of an ice sheet comes through the albedo effect of the high reflective snow or ice surface and the long-wave effect due to its high surface elevation with low surface temperature. In the first order approximation, the albedo at the TOA (planetary albedo) is simply higher by around 65% over the continent (ice sheet). The relation of the albedo to the surface elevation (Drewry, 1993) is directly examined and seems to have a slight dependence on the surface elevation. Since this is the albedo at the TOA, even if the surface albedo does not change, the albedo at the TOA can increase by the decrease of the atmospheric thickness. The relation is compared with the radiative transfer calculation for the atmosphere with a constant surface albedo of 80%, using the code by Chou (1992). Calculated curves, with the solar zenith angle 6 0 as cos 6 0 = 0.2 and 0.4, clearly explain the elevation dependence of data points. In West Antarctica, a clear dependence is not seen and albedo for the whole area is higher than that of East Antarctica. The higher albedo is expected from the higher surface albedo, and the higher accumulation rate at the surface, or higher amounts of clouds (also leading to higher accumulation by precipitation). The OLR has a clear relation to the surface elevation. Examined in latitude band S (Fig. 2(b)), a strong negative dependence of about -20 W irf 2 km" 1 on elevation is clearly seen, at elevations higher than about 1.6 km. Up to 1.6 km in height, the OLR still has a slight dependence of about -5 to - 10Wm" 2 km'' though the dependence diverges largely ^ j^rr^t^?$&, -180 E E 160 X 13 u 140 > (b) OLR 40 (a) Albedo 3n Surface elevation (km) ^120 en Surface elevation (km) Fig. 2 Surface elevation dependence of ERBE top of the atmosphere (a) albedo for full sky between 60 and 90 E, and calculated albedo for clear sky with fixed surface albedo of 80 % for cosine of solar zenith angle 0.2 (dashed) and 0.4 (solid), (b) outgoing long-wave radiation along latitude band S, October

4 32 Takashi Yamanouchi & Thomas P. Charlock These dependences of OLR on the surface elevation must be related to the surface temperature. The dependence of the surface temperature on the surface elevation was already reported by Satow (1978) and Kikuchi et al. (1992). Two different elevation height dependences of surface temperature are also seen from ISCCP clear sky composite surface temperature, about -loto -15Kkm _1 at 1.6 km to the top and about 2 to 4 K km" 1 below 1.6 km. The average height dependence of about 12 K km" 1 at 220 K for the high plateau, also similar to the amount reported by Satow (1978), corresponds to the height dependence of the black body radiation, 30 W m" 2 km" 1, which is much steeper than the present elevation dependence of OLR, about -20 W m" 2 km" 1 found in Fig. 2(b). The OLR is uniquely expressed by the surface temperature (T s ) in each case, in a single close to linear curve, and a small variation of OLR at the particular T s, indicating a closer relationship between them. The black body radiation <xt s 4 of these surface temperatures T s is calculated and compared with the OLR. The ratio of the OLR to <rt*, defined as "effective emissivity", is close to 1 at the high plateau area, above about 2.5 km; and around 0.8 at the coastal area. The effective emissivity is not strictly determined by the atmospheric conditions such as temperature; however, it is rather determined by the surface elevation, irrespective of the seasons. It means that there is no apparent effect of the atmosphere (no greenhouse effect!) at the interior (a) Albedo S Ice concentration (%) Ice concentration (%) Ice concentration Ice concentration {%) Fig. 3 Sea ice concentration dependence of ERBE top of the atmosphere (a) albedo, (b) short-wave absorption, (c) outgoing long-wave radiation and (d) net radiation for full sky along latitude band S, October

5 RADIATIVE EFFECT OF SEA ICE Radiative effects of clouds, ice sheet and sea ice 33 Sea ice has a strong impact on radiation, mainly through the albedo effect in the sunlit season. The sea ice also has a major effect on radiation by virtue of capping the upper layer of the ocean. With negative sea ice anomalies, the upwelling long-wave radiation is much greater. The sea ice increases the TOA albedo by about 20% and reduces the OLR by some amount. The short-wave effect is stronger than the long-wave effect except in some winter months and the net effect is strong cooling: about -50 W m" 2 in October (Fig. 3). These radiation effects of sea ice are similar but less than that of clouds over open water; however, these results were obtained under the distribution of clouds and do not show the independent effect of sea ice under clear sky. The TOA albedo under clear sky over fast ice is 43 to 56%, while that over open water is 13 to 15 % in January. Sea ice effectiveness under clear sky is about 30 to 40%, which is more than twice the 15 to 20% derived under average clouds. It can be said that clouds are masking the radiative effect of sea ice by more than a half. On the other hand, compared to the cloud effectiveness of 30% over the open water, cloud effectiveness over the 100% sea ice is only around 10% (see above). This means the sea ice is reducing the radiative effect of clouds. Sea ice and clouds are interacting together for the radiative effect. If there is some distinct dependence of cloud distribution on sea ice, then the radiative effect of sea ice is compensated by clouds furthermore. It depends on how cloud distribution relates to sea ice distribution; the relation of clouds and sea ice is a subject which needs clarification. 80 Continental slope CD < Continental interior Sea ice x Jr %kh ** Cloud (over Vf?f \ P enwater ) V* \ \ ^ Clear sky (over open water) K: -^** Outgoing longwave radiation (W/m2) Fig. 4 Scatter diagram of top of the atmosphere albedo vs. outgoing long-wave radiation for clear sky over open water ( S; small solid square), full sky over open water ( S; cross), full sky over sea ice ( S; solid circle), full sky over continental slope ( S; open square) and full sky over continental interior ( S; asterisk). Solid line in the left bottom of the figure shows the slope of the relation between albedo and OLR when both the effects are identical at 60 S.

6 34 Takashi Yamanouchi & Thomas P. Charlock TOTAL RADIATIVE EFFECTS The relation of both the long-wave and short-wave (albedo) effect of each process in the Antarctic are compared, as an example, in October 1987 (Fig. 4). Starting from the points for clear sky over open water, clouds have a large effect; however, the slope of the effect is close to the relation indicating that the long-wave and short-wave effect are almost cancelled, but a little larger in the short-wave effect. The effect of sea ice is much stronger in the short-wave, showing a steeper slope, which means the sea ice is a strong cooling factor. On the other hand, the effect of surface elevation of the ice sheet is much stronger in the long-wave, namely the heating effect, through lower emission at lower temperatures. However, the primary effect of the ice sheet compared from the open ocean is also stronger in the short-wave and both effects are nearly identical. The radiative effect of the high plateau is almost balancing the short-wave and long-wave. REFERENCES Aoki,T.& Yamanouchi, T. ( 1992) Cloud radiative forcingover the snow-covered surface around Asuka Station, Antarctica. Proc. NIPRSymp. PolarMeteorol. Glacial., 5, Barkstrom.B. R. (1984) The Earth Radiation Budget Experiment (ERBE). Bull. Am. Met. Soc. 65, Chou, M.-D. (1992) A solar radiation model for use in climate studies. J. Atmos. Sci. 49, Drewry, D. J. (1983) The Surface of the Antarctic Ice Sheet, Antarctica: Glaciological and Geophysical Folio, ed. by D. J. Drewry, Sheet No 2, Scott Polar Research Inst., Cambridge, England. Dutton, E. G., Stone R. S. & DeLuisi, J. J. (1989) South Pole surface radiation balance measurements, April 1986 to February NOAA Data Report ERL ARL-17, Air Resources Laboratory, Silver Spring, Maryland, USA. Harrison, E. F., Minnis, P., Barkstrom, B. R., Ramanathan, V., Cess, R. D. & Gibson, G. G. (1990) Seasonal variation of cloud radiative forcing derived from the Earth Radiation Budget Experiment./. Geophys. Res. 95, Kikuchi, T., Satow, K., Ohata, T., Yamanouchi, T. & Nishio, F. (1992) Wind and temperature regime in Mizuho Plateau, East Antarctica. Int. J. Remote Sensing 13, Murata, A. & Yamanouchi, T. (1994) Distribution characteristics of clouds over East Antarctica in 1987 obtained from AVHRR. Submitted to J. Met. Soc. Japan. National Snow and Ice Data Center ( 1992) DMSP SSM/I brightness temperature and sea ice concentrationgrids for the polar regions on CD-ROM; Users Guide. National Snow and Ice Data Center Special Report-1, CIRES, University of Colorado, Boulder, Colorado, USA. Satow, K. (1978) Distribution of 10 m snow temperatures in Mizuho Plateau. Mem. Nat. Inst. Polar Res., Spec, issue, 7, Yamanouchi, T. (1989) Antarctic climate research data. Part 1, Radiation data at Syowa Station, Antarctica from February 1987 to January JARE Data Report 144 (Meteorol. 22).