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Energy balance estimates during the summer season of glaciers of the Antarctic Peninsula
Global and Planetary Change 22 Ž1999. 117–130 www.elsevier.comrlocatergloplacha
Energy balance estimates during the summer season of glaciers of the Antarctic Peninsula Christoph Schneider
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Department of Physical Geography, UniÕersity of Freiburg, Werderring 4, D-79085 Freiburg, Germany Received 15 September 1997; accepted 19 February 1999
Abstract Two small glaciers in Marguerite Bay on the west coast of the Antarctic Peninsula were chosen to monitor the accumulation and ablation pattern of the snow cover and the reaction of the glacial system in respect with global change. Both glaciers are located at 68808X S and 67806X W in the vicinity of the Argentine research base ‘San Martın’. ´ McClary Glacier is a short valley glacier with an accumulation area not extending higher than 600 m asl. Its short snout joins the snout of Northeast Glacier to form an ice-cliff to the sea. Northeast Glacier is separated from McClary Glacier by a mountain ridge and is fed by a large ice-fall coming down from the plateau of the Antarctic Peninsula at 1500 m asl. As the topography of the glaciers in terms of accumulation area, length and surface elevation differs widely it is assumed that the glaciers react differently to climate variations. However the response time of these systems is believed to be of the same order of magnitude as the anticipated climatic variability. Therefore measures were taken to monitor the seasonal development of the snow cover on the lower parts of the glaciers in order to detect changes at an early stage. During the field campaign in the Austral summer 1994r1995 micro-meteorological measurements were carried out with automatic weather stations ŽAWS. at three locations on the glaciers. More than 40 snow pits were dug during the summer season to obtain data from the snow cover. Data was sampled on snow temperature variations, water equivalent, liquid water content, stratification of the snow cover, crystal types and crystal sizes. Furthermore the accumulation during the whole year from March 1994 to February 1995 was measured with stakes placed on the glaciers. The data from the AWS was used to compute energy available for ablation. This correlated well Ž r s 0.9. with the changes observed in the snow pits. Turbulent heat fluxes account for 55% of the summer-time energy budget at the surface of the snow cover. Sensible heat is the dominant energy source for ablation during the summer. The equilibrium line was estimated between sea-level and 110 m asl. It is assumed that further warming will cause the development of an ablation zone on the two glaciers. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Antarctic peninsula; glaciers; climate change
1. Introduction The Antarctic Peninsula ŽFig. 1. comprises a glaciated landscape with a large variety of different )
Tel: q49-761-203-3548; fax: q49-761-203-3596. E-mail address: [email protected] ŽC. Schneider.
glacial systems such as ice caps, ice sheets, outlet glaciers, alpine type valley glaciers and fringing glaciers ŽFleming et al., 1938; Nichols, 1960.. The spatial pattern of the landscape is also complicated, because of huge differences in annual air temperature and precipitation, both from north to south and from west to east ŽReynolds, 1981; Peel, 1992..
0921-8181r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 8 1 8 1 Ž 9 9 . 0 0 0 3 0 - 2
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Fig. 1. Map of the Antarctic Peninsula. The area of investigation marked as rectangle is located on the west coast of the Antarctic Peninsula in the vicinity of the Argentine research base ‘San Martın’. ´
Local climate variation caused by complicated topography add to this ŽHoskins, 1963.. This special topographic and climatological situation on the Antarctic Peninsula is attracting more and more interest with respect to global change scenarios. Although global circulation models ŽGCM. do not predict extraordinary warming rates when compared to other marginal zones of the Antarctic conti-
nent ŽHoughton et al., 1996., recently much evidence for climate change during the last few decades has been gathered by different authors, especially in this area. The west coast of the Peninsula shows extreme year-to-year variability with respect to air temperature, which is mainly attributed to fluctuations in the sea-ice extent ŽKing, 1994.. A significant warming trend of q5.58C in mid-winter and q1.58C during
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the summer was determined by Smith and Stammerjohn Ž1996. for the time period ranging from 1941 to 1991. Wunderle Ž1996. calculated warming trends of between 0.0288C yry1 ŽOrcadas. and 0.2658C yry1 ŽAdelaide Island. for different stations on the west of the Antarctic Peninsula for similar time periods. Reduction, and even decay of ice shelves has been reported by different authors ŽDoake and Vaughan, 1991; Skvarca, 1993; Rott et al., 1996.. This is associated with a shift to the south of the y58C mean annual isotherm by approximately 200 km on the west coast and 50 km on the east coast of the Peninsula ŽVaughan, 1993; Vaughan and Doake, 1996.. Annual precipitation rates have increased by 20% since 1950, according to Peel Ž1992., with the year-to-year variability of precipitation depending mainly on the variations in the frequency of mesoscale cyclones approaching the Peninsula from the west ŽTurner et al., 1997.. Furthermore, enlargement of snow-free areas during the summer ŽFox and Cooper, 1997. indicate climate change. The glaciers on the Antarctic Peninsula will react to these changing climate conditions. Ice barrier variations on King George Island, Stonington Island and the Debenham Islands have been monitored by means of former maps, aerial photography and remote sensing imagery ŽMuser, 1995; Fox and Thomson, 1995; Wunderle, 1996.. However, since the variation in the location of ice barriers is a complex function of sea level change and the response time of glaciers to climate change, it is not easy to draw conclusions in respect to regional impact of global change. Drewry and Morris Ž1992. estimated the contribution to sea-level rise by the Antarctic Peninsula ice sheet for a 28C rise in mean annual surface temperature over 40 years to be only 1.0 mm. Despite its small contribution to sea level rise significant changes in terms of glaciology can be expected. The Antarctic Peninsula extends over a wide range of subpolar and polar climates and is covered by glaciers, which at lower altitudes experience considerable melting in summer. Whereas glaciated areas on the plateau, where annual mean temperatures are below y108C, will see an increase in solid precipitation, low lying areas in the more northwestern parts of the Peninsula will be exposed to increased summer ablation. Thus, within a very small region we assume different changes of the glacial systems as a consequence of
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their topography. On the plateau increased precipitation will increase the ice mass which will lead to an acceleration of the ice flow and to thickening of the outlet glaciers. In the lower reaches of these glaciers ablation zones may widen or will start to develop. Local glaciers near the sea without ice mass contribution from the plateau may suffer from a negative mass balance due to increased summer ablation. Only very few glaciological studies are related to the energy or mass balance of small glaciers on the Antarctic Peninsula. Published work concentrates on valley glaciers that are tributary to the King George VI ice shelf at 718S ŽJamieson and Wager, 1983; Morris, 1999. and on ice caps and glaciers on the South Shetland Islands at 638S ŽNobel, 1965; Wen et al., 1994; Bintanja, 1995; Ren et al., 1995.. In this study, data is presented on two valley glaciers from the inner part of Marguerite Bay on the west coast of the Antarctic Peninsula. To the author’s knowledge, no detailed glaciological study has yet been published on valley glaciers of the inner part of Marguerite Bay other than the details given by Wunderle Ž1996.. Although first observations were made as early as the thirties and forties of this century by the British Antarctic Survey ŽSkinner, 1970; Rymill, 1983. and the Ronne Antarctic Expedition ŽKnowles, 1945; Ronne, 1945., up to now the knowledge and the time period of the observations was not sufficient to draw conclusions with respect to glaciological changes resulting from the regional impact of climate change. This study presents results from surface energy balance estimates for the summer season on two small glaciers at 688S. Since the response time of the glaciers to climate forcing is probably on the order of decades, this study concentrates on the description of the observed behaviour of the snow cover, relating it to observed meteorological conditions. Ongoing monitoring on the two glaciers may reveal glaciological changes as a consequence of regional climate change.
2. Glacier topography and flow characteristics Northeast and McClary Glaciers are located at 68807X S and 67806X W in the vicinity of the Argentine base San Martın ´ ŽFig. 2., which is located on the
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Fig. 2. A topographic sketch of the investigation area. The projection is polar stereographic with 678W as central meridian and 688S as X Y X Y latitude of true scale. The lower left corner is located at 67817 33 W and 68814 06 S. The numbers on the axis denote distance in kilometres. Zero on the x-axis indicates 678W. The surface areas of Northeast Glacier and McClary Glacier are indicated by different shading. Lines and numbers on the glaciers ŽA1, A6, A15, A17 and A17. refer to the field measurements shown in Figs. 3, 5, 6 and 7. The triangles show the locations of AWS in summer 1994r1995.
Debenham Islands. McClary is a valley glacier about 19 km in total length. The highest parts of its accumulation area, at approximately 600 m asl, are located only 13 km from the ice barrier in the west. The slopes of the flanking mountain ridges also contribute to the accumulation area. The ice mass flows out in opposite directions, to the ice barrier in the west and to the fjord of Swithinbank glacier in the north–northeast. To the south, McClary Glacier
is separated from Northeast Glacier by Butson Ridge, with summits up to 1000 m asl. Both glaciers form a joint ice barrier that runs northwest–southeast from Cape Calmette to Roman Four Promontory ŽFig. 2.. Northeast Glacier is fed by an ice-fall coming down from the plateau. Although this part of the plateau is the narrowest part of the transition from Graham Land to Palmer Land ŽFig. 1. the altitude is still above 1500 m asl. The surface of Northeast Glacier
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Fig. 3. Displacement of ablation stakes on McClary and Northeast Glacier relative to A1. Since A1 moved approximately 20 m during the period from March 1994 to February 1995 this number has to be added to the numbers on the y-axis in order to obtain absolute displacement. The lines of stakes are shown in Fig. 2.
in the valley, which is approximately 20 km long, rises from sea level to about 550 asl. Between the Debenham Islands and Roman Four Promontory the glacier widens out to a piedmont-type glacier ŽNichols, 1960.. The central zone of the glacier is heavily crevassed and in front of this part of the ice barrier a delta-like snout of broken icebergs and smashed sea-ice forms occasionally during the cold season ŽNichols, 1960.. Nichols estimates the flow velocity of this central zone to be more than 150 m per year. This contrasts to the velocity of only 30 m per year measured just south of the crevassed area by Knowles Ž1945.. Close to the Debenham Islands the flow velocity calculated from the displacement of corner reflectors in multitemporal synthetic aperture radar ŽSAR. images of the European Remote Sensing ŽERS-1. satellite was estimated to be some tens
of metres per year. Towards the central part of the glacier the relative displacement of ablation stakes also indicates a distinct rise in velocity towards the central crevassed area ŽFig. 3.. This zone of enhanced flow velocity corresponds with a trough in the bathymetric profile between San Martın ´ and Stonington Island ŽFig. 4.. From the bathymetry the ice thickness close to the ice-cliff can be estimated to be between 70 and 200 m, assuming that the ice-cliff above sea-level varies between 20 and 50 m in height.
3. Mass balance In February 1994, 45 ablation stakes were arranged along two lines and in a rectangle on the
Fig. 4. Bathymetric profile between Stonington Island San Martın ´ ŽDebenham Island. ŽFig. 2.. The profile was derived from an unpublished sea floor map at a scale of 1:10,000 of the ‘Servicio de Hidrografia Naval, Armada Argentina’.
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glacier close to the Debenham Islands Žlines A1 to Y7 and A1 to A17, ŽFig. 2.. Fig. 5 gives the pattern of accumulation and ablation during the following year for some of these stakes. It can be concluded that accumulation processes prevail over ablation processes until late October. The depletion of the snow cover during the summer months ŽFig. 5. shows the combined effect of snow melt, wind drift, densification of the snow pack and evaporation. From snow pits ŽFig. 6. the snow density of the snow pack is estimated to be 0.4 kg my3 at the end of the winter season and 0.5 kg my3 in summer, resulting in a mass balance for the winter of 1994 of q320 mm water equivalent. At the end of the summer, in late February, the mean of the net balance of all stakes yields a water equivalent of q200 mm. However, Fig. 7 shows that, whereas the correlation between accumulation and altitude in winter is not
significant, for the year as a whole it is. This is attributed to the fact that air temperature drives surface ablation. From Fig. 7 the equilibrium line altitude is calculated to be 110 m asl for the glacial year 1994r1995. Wunderle Ž1996. gives a value of 560 mm mean annual accumulation for 1993r1994 and reports that no ablation zone had developed at all. However, in contrast to 1994r1995, the mean air temperature from November to February was 2.28C lower in 1993r1994. In addition, SAR images from the first ERS-1 satellite reveal that the altitude of the transition between the percolation zone and the completely frozen snow pack was approximately 330 m lower in the 1993r1994 summer season than in 1994r1995 ŽSchneider, 1998.. It can be concluded that mass balance in the lower zones of the glacier is determined by summer energy balance and therefore highly dependent on summer air temperatures.
Fig. 5. Ablation and accumulation according to stake measurements. The y-axis gives the snow depth above the ablation surface of the preceding summer. The stakes were placed in a row from A1 to A15 as indicated in Fig. 2. The stakes B4 and B6 belong to a row parallel to the line from A1 to A6 running between A6 and D6 in Fig. 2. To derive accumulation in mm water equivalent these values must be multiplied by the snow density.
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4. Energy balance estimates 4.1. Methodology of computation The purpose of the 1994r1995 summer campaign on Northeast and McClary Glacier was to relate micro-meteorological measurements made with automatic weather stations ŽAWS. to surface ablation and the extent of the percolation zone. In this paper
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only the relation to surface ablation is discussed. The relation to the extent of the percolation zone is subject to another paper ŽSchneider, 1998.. In the following the term ‘ablation’ means melting and removal by downward percolation or evaporation from the uppermost layer of the snow cover. ‘Ablation’ is compared to the short time energy balance at the surface of the snow cover and it is different from the mass balance of the entire glacier column at a
Fig. 6. Time series of a selection of snow pits at A1 ŽFig. 2. from January 1995.
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Fig. 7. Correlation of accumulation and ablation with altitude asl for the individual stakes for the winter season Žleft. and for the whole glacial year Žright.. Stakes along the line from A1 to A17 as shown in Fig. 2 were used.
distinct location. Hence, ‘ablation’ is considered synonymous to the term ‘surface ablation’ because refreezing of water in deeper layers of the firn is not considered here. Three AWS, supplied by Campbell Scientific ŽUK. were operated from 20 December 1994 to 20 February 1995. The locations of the AWS on the glaciers are marked with triangles in Fig. 2. All AWS were equipped with sensors for net radiation, short-wave irradiance, reflected short-wave radiation, wind direction, wind velocity, and air temperature and air humidity at 2.0 m and 0.5 m above the surface. Snow temperatures were measured at three depths. All sensors were read every 10 s and stored as 10 min means in a storage module attached to the data logger. The systems were powered by 10-W solar panels. Energy balance was calculated according to: B s R q H q L.
Ž 1.
B denotes the sum of energy fluxes from or into the atmosphere. Thus, B gives the net energy available for melting snow or changing its thermal characteristics. R represents the net radiation, H the sensible heat flux and L the latent heat flux. H and L were computed according to the bulk transfer equations given by Oke Ž1970. and Moore Ž1983.. This formu-
lation is discussed in detail by Braithwaite Ž1995. and Blackadar Ž1997.: Hsy
r cp k 2 u Ž z2 . z2 z2 ln ln z 0,u z 0,T
ŽQ Ž z2 . y Q 0 .
ž / ž /
= Ž 1 y 5Rb .
2
Ž 2. 2
Lsy
r L v 0.622 k u Ž z 2 . p ln
z2
z2
Ž e Ž z 2 . y e0 .
ž / ž / z 0,u
2
= Ž 1 y 5Rb . .
ln
z 0,q
Ž 3.
The variables denote: r , density of air; c p , specific heat at constant pressure of air Ž1005 J kgy1 Ky1 .; k , van Karman constant Ž0.4.; L v , latent heat of evaporation or sublimation wŽ2.514 or 2.849.10 6 J kgy1 .x; p, air pressure; uŽ z 2 ., wind velocity at height z 2 ; z 2 , height above snow surface Ž2.0 m.; z 0,u , z 0,T , z 0,q , roughness lengths for momentum, heat and water vapour; Q Ž z 2 ., potential air temperature at height z 2 ; Q 0 , potential air temperature at the snow surface Ž08C.; eŽ z 2 ., water vapour pressure at height z 2 ; e 0 , water vapour pressure at the snow surface Ž6.1 hPa.; Rb, bulk Richardson number.
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For time periods with air temperatures above 08C the surface temperature was set to 08C and the water vapour pressure Ž e 0 . was fixed to 6.1 hPa assuming 100% relative humidity at the snow surface. During periods with air temperatures below zero, the gradient between measurements of humidity and temperature at z 1 s 0.5 m and z 2 s 2.0 m was used to calculate turbulent energy exchange. In this case surface roughness length does not occur explicitly in the equations: Hsy
r c p k 2 Ž u Ž z 2 . y u Ž z1 . . ln
z2
ž / z1
= Ž Q Ž z 2 . y Q Ž z 1 . . Ž 1 y 5RbX . Lsy
2
Ž 4.
r L v 0.622 k 2 Ž u Ž z 2 . y u Ž z 1 . . p ln
z2
2
ž / z1
= Ž e Ž z 2 . y e Ž z 1 . . Ž 1 y 5RbX .
2
Ž 5.
However, since uŽ z 2 . was computed from uŽ z1. assuming a logarithmic wind profile, z 0 also is inherent in the formulations. The expression containing the bulk Richardson number Ž Rb . accounts for stable stratification in the boundary layer. The number itself is calculated using Rb s
g ŽQ Ž z2 . y Q 0 . Ž z2 y z0 .
Q uŽ z2 .
2
Ž 6.
with
Qs
Q Ž z2 . q Q 0
Ž 7.
2
and RbX s
g Ž Q Ž z 2 . y Q Ž z1 . . Ž z 2 y z1 .
Q X Ž u Ž z 2 . y u Ž z1 . .
2
Ž 8.
with
QXs
Q Ž z 2 . q Q Ž z1 . 2
ments. Most of these measurements were close to neutral conditions and Rb was set to zero for these cases. Radiation shields of temperature probes could not be ventilated because of insufficient power supply. Therefore, air temperature was corrected for the effect of radiative heating using a parametrisation based on short-wave irradiance Ž I . and wind velocity Ž uŽ z ..: DT s y
2
Ž 9.
g denotes acceleration due to gravity. No correction for unstable stratification was needed since unstable situations occurred in less than 5% of the measure-
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I 1080
Ž 12.79ey4 .02 uŽ z .q0.33 .
Ž 10 .
This formulation was derived as fit to data given in a technical note by Young Company ŽMI, USA. Ž1987.. At wind speeds above 3.5 m sy1 the correction was omitted because it returns values smaller than 0.18 K at I s 600 W my2 . The mean wind speed during the time period of the measurements was 4.6 mrs. Absolute values of the turbulent heat fluxes can also be overestimated because of inversion layers between the snow surface and heights below 0.5 m. Calculation of the bulk Richardson number cannot incorporate the effect of inversions, which are confined to the lowermost boundary layer ŽMorris et al., 1997.. However, the inversion is weakened by pre-dominantly positive radiation balance during the summer. Furthermore, mean wind speed was large enough to remove the surface inversion by turbulent mixing. However, it cannot be ruled out that on some occasions with small wind speeds a bias is introduced to the computations of turbulent heat fluxes. After computing H and L the energy available at the snow surface Ž B . was calculated according to Ž1. ŽFig. 8.. The only parameters in the equations that can be adjusted to obtain optimal agreement between measured and computed ablation are the surface roughness lengths. Using 10y3 m for momentum and heat and 10y5 m for water vapour yielded the best overall result, with a correlation coefficient of r s 0.9 ŽFig. 9.. The x-axis offset of the regression is 1.7 mm and the gradient of the fit is 0.82. The negative deviation of the gradient from 1 indicates, that high ablation rates are systematically underestimated by the computations. Differences between the surface roughness length for momentum, temperature and vapour have been
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Fig. 8. Energy balance estimates from 20 December 1994 to 21 February 1995 at the location A1 ŽFig. 2.. The bars show weekly averages. The black bar, net energy, gives the mean weekly energy flux from or to the snow surface and is the sum of the three other fluxes in the graph. Hence net energy indicates the flux of energy available for ablation, snow metamorphosis, warming or cooling of the snow pack.
widely discussed Žsee, e.g., Hogg et al., 1982; Greuell and Konzelmann, 1994; Braithwaite, 1995. and values of roughness lengths over snow ranging from 10y5 mm to 5 = 10y3 mm are listed, e.g., in the work of Kuhn Ž1979., Moore Ž1983. and Morris Ž1989.. However, the necessity of taking z e smaller than z o and z h suggests, that there is a systematic error in the measurement of the humidity gradient. Forcing all of the three roughness lengths to the same value would result in z 0 s z e s z h f 10y4 m. This is very close to values of surface roughness quoted by Morris Ž1989. and King and Connolley Ž1997. for smooth snow surfaces. Since this seems reasonable, a considerable negative bias in the computation of the latent heat flux is suspected. In other
words: in this study surface roughness lengths are used to account for all the systematic errors in the measurements, which can not be explained individually. 4.2. Measured ablation Energy balance estimates were related to the ablation as derived from the changes in the snow pits, assuming that ablation would occur only if a surplus of energy at the surface temporarily coincided with positive air temperatures. Heat flux into the glacier was neglected because snow temperatures in the uppermost 2 m of the snow pack were between y0.38C and 08C throughout the field campaign and
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4.3. Error analysis
Fig. 9. Correlation of observed ablation in time series of snow pits ŽFig. 6. on the x-axis and modelled ablation from the data sampled at the three AWS. The locations of the AWS are marked in Fig. 2 with open triangles. The correlation coefficient is r s 0.9, the x-axis offset is 1.7 and the slope of the regression fit is 0.82.
showed no distinct gradient with depth. Time series of snow pits from one location show a distinct pattern of ice lenses and stratification ŽFig. 6.. The snow pits were dug adjacent to the AWS ŽFig. 2.. Pits were dug every 2 to 5 days. They were dug approximately 0.5 m apart from the previous snow pit. The removal of snow from the uppermost layers by melting makes characteristic ice lenses move up towards the surface during the summer season. A sequence of four snow pits shown in Fig. 6 demonstrates this: three ice lenses at 0.5 m depth on 29 December 1994 can be identified at 0.3 m on 16 January 1995, at 0.15 m depth on 24 January 1995 and at the surface on 29 January 1995. Using the density of the snow measured in the pits and the depth changes of identifiable layers in subsequent pits the water equivalent of the melted snow was derived. Since it was not always possible to conclusively identify ice lenses, only 15 different time intervals could be chosen during the field campaign from the locations of the AWS. These periods vary in length between 2 and 7 days, because changes in the snow cover larger than the error associated with this technique were only found when snow pits from time intervals of several days were compared.
Assuming a statistical error of "0.28C for temperature and "5% for relative humidity a relative error of 22% for sensible heat flux and 15% for latent heat flux was computed for mean atmospheric conditions according to the Gaussian error propagation laws. The validity of the logarithmic wind profile and the temporal variations in surface roughness lengths were not introduced into the computation as additional error sources. According to the error propagation, the total relative error of the energy balance calculation was 25% or 4.7 W my2 for mean conditions, assuming 15% relative error for the measurement of the net radiation. The error in measured ablation was also derived using error propagation formulations. An error of 0.01 kg my3 for the snow density, a mean snow density of 0.5 kg my3 and an error of 10 mm in height for the depth of characteristic layers yields an error of 7 mm of water equivalent per measurement. The error for the difference between two measurements then is calculated to 10 mm or approximately 7.6 W my2 for a time interval of 5 days between measurements. Accumulated ablation in the time intervals ranged from 14 to 89 mm water equivalent. 4.4. Discussion Mean values of atmospheric conditions and computed heat fluxes are given in Table 1. Table 1 and Fig. 8 show extraordinarily large values for both of the turbulent heat fluxes in comparison to the radia-
Table 1. Mean atmospheric conditions and mean heat fluxes for the period of observations from 20 December 1994 to 21 February 1995 at location A1 in Fig. 2 Air temperature Ž2 m. Wind speed Ž2 m. Relative humidity Radiation balance Sensible heat flux Latent heat flux Turbulent heat fluxes Total atmospheric heat flux
q0.8 C 4.6 m sy1 70.1% 8.6 W my2 Ž46%. 35.5 W my2 Ž190%. y25.5 W my2 Žy137%. 10.0 W my2 Ž54%. 18.6 W my2
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tion heat flux. When the sum of latent and sensible heat flux is calculated, it appears that turbulent heat fluxes account for 55%, whereas radiation balance accounts only for 45%, of the total energy balance. Small contribution from net radiation to the energy balance can be explained by low shortwave radiation because of high albedo, varying between 0.75 and 0.9 throughout the summer. Marguerite Bay experiences less cloudiness than areas further north on the Peninsula because of lee side effects occurring when air masses overflow Alexander Island from the south-west, Adelaide Island from the north or the plateau of Graham Land from the east. In consequence, the downward long-wave radiation flux is fairly low. This also reduces net radiation. Comparatively high air temperature, due to the advection of warm air from the sea in the west increases the input of sensible heat. The absolute values of the turbulent fluxes are large for both latent and sensible heat, but the fluxes have opposite signs. This situation is unusual when compared to other studies on summer-time energy balance on glaciers. On many glaciers summer-time energy balance during ablation is characterised by dominant energy input from radiation, whereas sensible heat contributes only a minor portion of the energy balance. On many occasions latent heat flux contributes to the energy budget by condensation on the glacier surface but it is often negligibly small ŽDe la Casiniere, ` 1974; Male and Granger 1981; Bintanja 1995; Konzelmann and Braithwaite 1995; Paterson, 1994, p. 68 ff... However, opposite signs of the turbulent heat fluxes must be expected when gradients of temperature and absolute humidity point into opposite directions. This is the case when air temperature is only a little above 08C, snow surface is at 08C with 6.1 hPa water vapour pressure at the surface, and absolute humidity in the boundary layer is low. During January 1995 mean air temperature on the glacier was q2.38C, and with 65% of relative humidity the average water vapour pressure was q4.7 hPa. Hence, although mean sensible heat flux must be directed downward, the mean latent heat flux was directed upward triggering evaporation from the snow surface. Foehnic winds also promote this effect. Such winds from the northeast associated with air temperatures up to q58C and low relative humidity were observed on several days.
Steffen Ž1995. obtains a similar distribution between net radiation, sensible heat flux and latent heat flux for a location on the Greenland ice sheet during a period just before the onset of summer ablation. Konzelmann and Braithwaite Ž1995. also report similar behaviour of the two turbulent heat fluxes during summer on the margin of the Greenland ice sheet, but their data show that latent heat flux accounts only for a small percentage of the total energy loss. Jamieson and Wager Ž1983. obtained similar results for energy budget terms for the summer season at Spartan Glacier on Alexander Island Ž71803X S. both in respect to absolute values of the single fluxes and in respect to the relation between sensible heat flux and net radiation. As in this study they suggest that the measurements of turbulent fluxes are affected by considerable errors. However, both studies show sensible heat flux dominates over radiation as an energy source for ablation. Jamieson and Wager Ž1983. also find opposite signs for sensible and latent heat flux. But in contrast to Fig. 8, on Spartan Glacier this is confined to short periods only. In conclusion, the results given in Fig. 8 can only be regarded as good estimates of energy fluxes. Since the correlation with observed ablation is fair and since net radiation was measured directly, it can be concluded that the sum of the turbulent heat fluxes is reasonable. It accounts for 55% of the total net energy input from the atmosphere to the snow pack during the summer. Stake measurements ŽFig. 5 and Fig. 7. show that during the summer at 120 m asl 0.6 m of snow were removed from the top layer. With a mean snow density Žsee Fig. 6. of 500 kg my3 this makes 300 mm of water equivalent. The computed ablation for the summer season was calculated to 375 mm water equivalent. The accumulated ablation as derived from the snow pits was 315 mm water equivalent. Hence, values of total ablation derived by the different methods are in good agreement.
5. Outlook Ongoing measurements on the glaciers will permit the detection of future changes. Analysis of the
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summer energy budget and the mass balance close to the ice cliffs at approximately 150 asl show that this area, is very sensitive to air temperature variations, because sensible heat transfer is the major energy source for ablation in summer. Equilibrium line is approximately at sea-level. Further warming would shift the equilibrium line to higher altitudes and with the development of an ablation zone, albedo would decrease. In consequence, major changes must be expected to the energy and mass balance in the lowest parts of the glaciers. This can be monitored permanently by radar remote sensing and field work based at the nearby Argentine base San Martın. ´ The energy balance computation presented in this paper may serve as a basis to evaluate the potential of ERS-1 SAR images for monitoring climate fluctuations in this region. A statistical approach can be used to produce spatial estimates of energy balance and to derive the areal extent of the percolation zone on the glaciers ŽSchneider et al., 1997.. In a further paper ŽSchneider, 1998. this is related to SAR images, where actual wet snow areas and frozen snow areas also can be delineated.
Acknowledgements The author wishes to thank Georg Kaser, Karen Lewis and Stephan Hofinger for their helpful comments on the paper. This research was supported by the German Federal Secretary of Education and Research ŽBMBF. within the programme ‘Dynamic Processes in Antarctic Geosystems’ ŽDYPAG. ŽContract Number: 03PL016A. and by the ESA pilot study ‘Monitoring Of Dynamic Processes in Antarctic Geosystems’ ŽMODPAG., ŽContract Number: AO2.D149.. The author would like to thank the Instituto Antarctico Argentino ŽIAA., the British Antarctic Survey ŽBAS. and the German AlfredWegener-Institut fur ¨ Polar- und Meeresforschung ŽAWI. for their support with respect to logistics and field equipment. The author is grateful for the invaluable assistance and discussions in the field provided by the Argentinean and German collaborators. The author is thankful to Stefan Wunderle who made available field data and remote sensing data from the summer campaign 1993r1994.
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References Bintanja, R., 1995. The local energy balance of the Ecology Glacier, King George Island, Antarctica: measurements and modelling. Antarct. Sci. 7, 315–325. Blackadar, A.K., 1997. Turbulence and Diffusion in the Atmosphere. Springer-Verlag, Berlin. Braithwaite, R.J., 1995. Aerodynamic stability and turbulent sensible-heat flux over a melting ice surface, the Greenland ice sheet. J. Glaciol. 41, 562–571. De La Casiniere, ` A.C., 1974. Heat exchange over a melting snow surface. J. Glaciol. 13, 55–72. Doake, C.S.M., Vaughan, D.G., 1991. Rapid disintegration of the Wordie Ice Shelf in response to atmospheric warming. Nature 350, 328–330. Drewry, D.J., Morris, E.M., 1992. The response of large ice sheets to climatic change. Philos. Trans. R. Soc. London 338, 235– 242. Fleming, W.L.S., Stephenson, A., Roberts, B.B., Bertram, G.C.L., 1938. Notes on the scientific work of the British Graham Land expedition. 1934–37. Geogr. J. R. Geogr. Soc. 91, 508–532. Fox, A.J., Cooper, A.P.R., 1997. Climate change indicators from archival aerial photography of the Antarctic Peninsula region. Ann. Glaciol. 27, 636–642. Fox, A.J., Thomson, J.W., 1995. Melting ice and changing coastlines, Antarctica, Proceedings of the 1995 annual symposium of the British Cartographic Society, 14–17 September 1995. University of Exeter, British Cartographic Society, Exeter, 8 pp. Žno page numbers.. Greuell, W., Konzelmann, T., 1994. Numerical modelling of the energy balance and the englacial temperature of the Greenland Ice sheet. Calculations for the ETH-Camp location ŽWest Greenland, 1155 m a.s.l... Planet. Global Change 9, 91–114. Hogg, I.G.G., Paren, J.G., Timmis, R.J., 1982. Summer heat and ice balances on Hodges Glacier, South Georgia, Falkland Islands Dependencies. J. Glaciol. 28, 221–238. Hoskins, A.K., 1963. Block terraces in the Neny Fjord area, Marguerite Bay, Graham Land. Br. Antarct. Surv. Bull. 1, 45–49. Houghton, J.T., Meira Filho, L.G., Callander, B.A., Harris, N., Kattenberg, A., Maskell, K., 1996. Climate change 1995, the science of climate change. IPCC report. Cambridge Univ. Press, Cambridge. Jamieson, A.W., Wager, A.C., 1983. Ice, water and energy balances of Spartan Glacier, Alexander Island. Br. Antarct. Surv. Bull. 52, 155–186. King, J.C., 1994. Recent climate variability in the vicinity of the Antarctic Peninsula. Int. J. Climatol. 14, 357–369. King, J.C., Connolley, W.M., 1997. Validation of the surface energy balance over the Antarctic ice sheets in the U.K. Meteorological Office Unified Climate Model. J. Climate 10, 1273–1287. Knowles, P.H., 1945. Glaciology of southern Palmer Peninsula, Antarctica. Proc. Am. Philos. Soc. 89, 174–176. Konzelmann, T., Braithwaite, R.J., 1995. Variations of ablation, albedo and energy balance at the margin of the Greenland ice
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sheet, Kronprins Christian Land, eastern north Greenland. J. Glaciol. 41, 174–182. Kuhn, M., 1979. On the computation of heat transfer coefficients from energy-balance gradients on a glacier. J. Glaciol. 22, 263–272. Male, D.H., Granger, R.J., 1981. Snow surface energy exchange. Water Resour. Res. 17, 609–627. Moore, R.D., 1983. On the use of bulk aerodynamic formulae over melting snow. Nord. Hydrol., 193–206. Morris, E.M., 1989. Turbulent transfer over snow and ice. J. Hydrol. 105, 205–223. Morris, E.M., 1999. Surface ablation rates on Moraine Corrie Glacier, Antarctica. Planet. Global Change 22, 221–231. Morris, E.M., Bader, H.-P., Weilenmann, P., 1997. Modelling temperature variations in polar snow using DAISY. J. Glaciol. 43, 180–191. Muser, D., 1995. Der Gletscherruckzug auf King George Island, ¨ Sud-Shetland-Inseln zwischen 1956 und 1992. Unpublished ¨ masteral thesis, Universitat ¨ Freiburg, Germany, Freiburg. Nichols, R.L., 1960. Geomorphology of Marguerite Bay area, Palmer Peninsula, Antarctica. Bull. Geol. Soc. Am. 71, 1421– 1450. Noble, H.M., 1965. Glaciological observations at Admiralty bay, King George Island, in 1957–58. Br. Antarct. Surv. Bull. 5, 1–11. Oke, T.R., 1970. Turbulent transport near the ground in stable conditions. J. Appl. Meteorol. 9, 778–786. Peel, D.A., 1992. Spatial temperature and accumulation rate variations in the Antarctic Peninsula. In: Morris, E.M. ŽEd.., The contribution of Antarctic Peninsula ice to sea level rise, British Antarctic Survey, Ice and Climate Special Report No. 1. British Antarctic Survey, Cambridge, UK, pp. 11–15. Ren, J., Qin, D., Petit, J.R., Jouzel, J., Wang, W., Liu, C., Wang, X., Qian, S., Wang, X., 1995. Glaciological studies on Nelson Island, South Shetland Islands, Antarctica. J. Glaciol. 41, 408–412. Reynolds, J.M., 1981. The distribution of mean annual temperatures in the Antarctic Peninsula. Br. Antarct. Surv. Bull. 54, 123–131. Ronne, F., 1945. The main southern sledge journey from East Base, Palmer Land, Antarctica. Proc. Am. Philos. Soc. 89, 13–22. Rott, H., Skvarca, P., Nagler, T., 1996. Rapid Collapse of Northern Larsen Ice Shelf. Science 271, 788–792.
Rymill, J.R., 1983. British Graham Land Expedition, 1934–37 — part II. Geogr. J. R. Geogr. Soc. 91, 425–438. Schneider, C., 1998. Monitoring climate variability on the Antarctic Peninsula by means of observations of the snow cover. Ann. Glaciol. 27, in print. Schneider, C., Wunderle, S., Friedrich, M., 1997. Snow cover investigations by means of ERS-SAR imagery on the Antarctic Peninsula. EARSeL Adv. Remote Sensing 5, 71–81. Skinner, A.C., 1970. Field report on the geology of the Mount Wilcox area, between Calmette and Square Bay Coasts, McClary and Swithinbank Glaciers; also completion of Butson Ridge and Centre Island geology field work. Unpublished report of the British Antarctic Survey, B.A.S. Reference Number: G3r1970rE, Cambridge. Skvarca, P., 1993. Fast recession of the northern Larsen Ice Shelf monitored by space images. Ann. Glaciol. 17, 317–321. Smith, R.C., Stammerjohn, S.E., 1996. Surface air temperature variations in the western Antarctic Peninsula region. In: Ross, R.M., Hofmann, E.E., Quetin, L.B. ŽEds.., Foundations for Ecological Research West of the Antarctic Peninsula. Antarctic Research Series 70 American Geophysical Union, Washington, DC, pp. 105–121. Steffen, K., 1995. Surface energy exchange at the equilibrium line on the Greenland ice sheet during onset of melt. Ann. Glaciol. 21, 13–18. Turner, J., Colwell, S.R., Harangozo, S.A., 1997. Variability of precipitation over the western Antarctic peninsula from synoptic observations. J. Geophys. Res. 102, 13999–14007. Vaughan, D.G., 1993. Implications of the break-up of Wordie Ice Shelf, Antarctica for sea level. Antarct. Sci. 5, 403–408. Vaughan, D.G., Doake, C.S.M., 1996. Recent atmospheric warming and retreat of ice shelves on the Antarctic Peninsula. Nature 379, 328–330. Wen, J., Kang, J., Xie, Z., Han, J., Lluberas, A., 1994. Climate, mass balance and glacial changes on small dome of Collins Ice Cap, King George Island, Antarctica. Antarct. Res. 5, 52–61. Wunderle, S., 1996. Die Schneedeckendynamik der Antarktischen Halbinsel und ihre Erfassung mit aktiven und passiven Fernerkundungsverfahren. Freiburger Geographische Hefte 48 Institut fur ¨ Physische Geographie, Universitat ¨ Freiburg, Freiburg, Germany.
Energy balance estimates during the summer season of glaciers of the Antarctic Peninsula Christoph Schneider
)
Department of Physical Geography, UniÕersity of Freiburg, Werderring 4, D-79085 Freiburg, Germany Received 15 September 1997; accepted 19 February 1999
Abstract Two small glaciers in Marguerite Bay on the west coast of the Antarctic Peninsula were chosen to monitor the accumulation and ablation pattern of the snow cover and the reaction of the glacial system in respect with global change. Both glaciers are located at 68808X S and 67806X W in the vicinity of the Argentine research base ‘San Martın’. ´ McClary Glacier is a short valley glacier with an accumulation area not extending higher than 600 m asl. Its short snout joins the snout of Northeast Glacier to form an ice-cliff to the sea. Northeast Glacier is separated from McClary Glacier by a mountain ridge and is fed by a large ice-fall coming down from the plateau of the Antarctic Peninsula at 1500 m asl. As the topography of the glaciers in terms of accumulation area, length and surface elevation differs widely it is assumed that the glaciers react differently to climate variations. However the response time of these systems is believed to be of the same order of magnitude as the anticipated climatic variability. Therefore measures were taken to monitor the seasonal development of the snow cover on the lower parts of the glaciers in order to detect changes at an early stage. During the field campaign in the Austral summer 1994r1995 micro-meteorological measurements were carried out with automatic weather stations ŽAWS. at three locations on the glaciers. More than 40 snow pits were dug during the summer season to obtain data from the snow cover. Data was sampled on snow temperature variations, water equivalent, liquid water content, stratification of the snow cover, crystal types and crystal sizes. Furthermore the accumulation during the whole year from March 1994 to February 1995 was measured with stakes placed on the glaciers. The data from the AWS was used to compute energy available for ablation. This correlated well Ž r s 0.9. with the changes observed in the snow pits. Turbulent heat fluxes account for 55% of the summer-time energy budget at the surface of the snow cover. Sensible heat is the dominant energy source for ablation during the summer. The equilibrium line was estimated between sea-level and 110 m asl. It is assumed that further warming will cause the development of an ablation zone on the two glaciers. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Antarctic peninsula; glaciers; climate change
1. Introduction The Antarctic Peninsula ŽFig. 1. comprises a glaciated landscape with a large variety of different )
Tel: q49-761-203-3548; fax: q49-761-203-3596. E-mail address: [email protected] ŽC. Schneider.
glacial systems such as ice caps, ice sheets, outlet glaciers, alpine type valley glaciers and fringing glaciers ŽFleming et al., 1938; Nichols, 1960.. The spatial pattern of the landscape is also complicated, because of huge differences in annual air temperature and precipitation, both from north to south and from west to east ŽReynolds, 1981; Peel, 1992..
0921-8181r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 8 1 8 1 Ž 9 9 . 0 0 0 3 0 - 2
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Fig. 1. Map of the Antarctic Peninsula. The area of investigation marked as rectangle is located on the west coast of the Antarctic Peninsula in the vicinity of the Argentine research base ‘San Martın’. ´
Local climate variation caused by complicated topography add to this ŽHoskins, 1963.. This special topographic and climatological situation on the Antarctic Peninsula is attracting more and more interest with respect to global change scenarios. Although global circulation models ŽGCM. do not predict extraordinary warming rates when compared to other marginal zones of the Antarctic conti-
nent ŽHoughton et al., 1996., recently much evidence for climate change during the last few decades has been gathered by different authors, especially in this area. The west coast of the Peninsula shows extreme year-to-year variability with respect to air temperature, which is mainly attributed to fluctuations in the sea-ice extent ŽKing, 1994.. A significant warming trend of q5.58C in mid-winter and q1.58C during
C. Schneiderr Global and Planetary Change 22 (1999) 117–130
the summer was determined by Smith and Stammerjohn Ž1996. for the time period ranging from 1941 to 1991. Wunderle Ž1996. calculated warming trends of between 0.0288C yry1 ŽOrcadas. and 0.2658C yry1 ŽAdelaide Island. for different stations on the west of the Antarctic Peninsula for similar time periods. Reduction, and even decay of ice shelves has been reported by different authors ŽDoake and Vaughan, 1991; Skvarca, 1993; Rott et al., 1996.. This is associated with a shift to the south of the y58C mean annual isotherm by approximately 200 km on the west coast and 50 km on the east coast of the Peninsula ŽVaughan, 1993; Vaughan and Doake, 1996.. Annual precipitation rates have increased by 20% since 1950, according to Peel Ž1992., with the year-to-year variability of precipitation depending mainly on the variations in the frequency of mesoscale cyclones approaching the Peninsula from the west ŽTurner et al., 1997.. Furthermore, enlargement of snow-free areas during the summer ŽFox and Cooper, 1997. indicate climate change. The glaciers on the Antarctic Peninsula will react to these changing climate conditions. Ice barrier variations on King George Island, Stonington Island and the Debenham Islands have been monitored by means of former maps, aerial photography and remote sensing imagery ŽMuser, 1995; Fox and Thomson, 1995; Wunderle, 1996.. However, since the variation in the location of ice barriers is a complex function of sea level change and the response time of glaciers to climate change, it is not easy to draw conclusions in respect to regional impact of global change. Drewry and Morris Ž1992. estimated the contribution to sea-level rise by the Antarctic Peninsula ice sheet for a 28C rise in mean annual surface temperature over 40 years to be only 1.0 mm. Despite its small contribution to sea level rise significant changes in terms of glaciology can be expected. The Antarctic Peninsula extends over a wide range of subpolar and polar climates and is covered by glaciers, which at lower altitudes experience considerable melting in summer. Whereas glaciated areas on the plateau, where annual mean temperatures are below y108C, will see an increase in solid precipitation, low lying areas in the more northwestern parts of the Peninsula will be exposed to increased summer ablation. Thus, within a very small region we assume different changes of the glacial systems as a consequence of
119
their topography. On the plateau increased precipitation will increase the ice mass which will lead to an acceleration of the ice flow and to thickening of the outlet glaciers. In the lower reaches of these glaciers ablation zones may widen or will start to develop. Local glaciers near the sea without ice mass contribution from the plateau may suffer from a negative mass balance due to increased summer ablation. Only very few glaciological studies are related to the energy or mass balance of small glaciers on the Antarctic Peninsula. Published work concentrates on valley glaciers that are tributary to the King George VI ice shelf at 718S ŽJamieson and Wager, 1983; Morris, 1999. and on ice caps and glaciers on the South Shetland Islands at 638S ŽNobel, 1965; Wen et al., 1994; Bintanja, 1995; Ren et al., 1995.. In this study, data is presented on two valley glaciers from the inner part of Marguerite Bay on the west coast of the Antarctic Peninsula. To the author’s knowledge, no detailed glaciological study has yet been published on valley glaciers of the inner part of Marguerite Bay other than the details given by Wunderle Ž1996.. Although first observations were made as early as the thirties and forties of this century by the British Antarctic Survey ŽSkinner, 1970; Rymill, 1983. and the Ronne Antarctic Expedition ŽKnowles, 1945; Ronne, 1945., up to now the knowledge and the time period of the observations was not sufficient to draw conclusions with respect to glaciological changes resulting from the regional impact of climate change. This study presents results from surface energy balance estimates for the summer season on two small glaciers at 688S. Since the response time of the glaciers to climate forcing is probably on the order of decades, this study concentrates on the description of the observed behaviour of the snow cover, relating it to observed meteorological conditions. Ongoing monitoring on the two glaciers may reveal glaciological changes as a consequence of regional climate change.
2. Glacier topography and flow characteristics Northeast and McClary Glaciers are located at 68807X S and 67806X W in the vicinity of the Argentine base San Martın ´ ŽFig. 2., which is located on the
120
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Fig. 2. A topographic sketch of the investigation area. The projection is polar stereographic with 678W as central meridian and 688S as X Y X Y latitude of true scale. The lower left corner is located at 67817 33 W and 68814 06 S. The numbers on the axis denote distance in kilometres. Zero on the x-axis indicates 678W. The surface areas of Northeast Glacier and McClary Glacier are indicated by different shading. Lines and numbers on the glaciers ŽA1, A6, A15, A17 and A17. refer to the field measurements shown in Figs. 3, 5, 6 and 7. The triangles show the locations of AWS in summer 1994r1995.
Debenham Islands. McClary is a valley glacier about 19 km in total length. The highest parts of its accumulation area, at approximately 600 m asl, are located only 13 km from the ice barrier in the west. The slopes of the flanking mountain ridges also contribute to the accumulation area. The ice mass flows out in opposite directions, to the ice barrier in the west and to the fjord of Swithinbank glacier in the north–northeast. To the south, McClary Glacier
is separated from Northeast Glacier by Butson Ridge, with summits up to 1000 m asl. Both glaciers form a joint ice barrier that runs northwest–southeast from Cape Calmette to Roman Four Promontory ŽFig. 2.. Northeast Glacier is fed by an ice-fall coming down from the plateau. Although this part of the plateau is the narrowest part of the transition from Graham Land to Palmer Land ŽFig. 1. the altitude is still above 1500 m asl. The surface of Northeast Glacier
C. Schneiderr Global and Planetary Change 22 (1999) 117–130
121
Fig. 3. Displacement of ablation stakes on McClary and Northeast Glacier relative to A1. Since A1 moved approximately 20 m during the period from March 1994 to February 1995 this number has to be added to the numbers on the y-axis in order to obtain absolute displacement. The lines of stakes are shown in Fig. 2.
in the valley, which is approximately 20 km long, rises from sea level to about 550 asl. Between the Debenham Islands and Roman Four Promontory the glacier widens out to a piedmont-type glacier ŽNichols, 1960.. The central zone of the glacier is heavily crevassed and in front of this part of the ice barrier a delta-like snout of broken icebergs and smashed sea-ice forms occasionally during the cold season ŽNichols, 1960.. Nichols estimates the flow velocity of this central zone to be more than 150 m per year. This contrasts to the velocity of only 30 m per year measured just south of the crevassed area by Knowles Ž1945.. Close to the Debenham Islands the flow velocity calculated from the displacement of corner reflectors in multitemporal synthetic aperture radar ŽSAR. images of the European Remote Sensing ŽERS-1. satellite was estimated to be some tens
of metres per year. Towards the central part of the glacier the relative displacement of ablation stakes also indicates a distinct rise in velocity towards the central crevassed area ŽFig. 3.. This zone of enhanced flow velocity corresponds with a trough in the bathymetric profile between San Martın ´ and Stonington Island ŽFig. 4.. From the bathymetry the ice thickness close to the ice-cliff can be estimated to be between 70 and 200 m, assuming that the ice-cliff above sea-level varies between 20 and 50 m in height.
3. Mass balance In February 1994, 45 ablation stakes were arranged along two lines and in a rectangle on the
Fig. 4. Bathymetric profile between Stonington Island San Martın ´ ŽDebenham Island. ŽFig. 2.. The profile was derived from an unpublished sea floor map at a scale of 1:10,000 of the ‘Servicio de Hidrografia Naval, Armada Argentina’.
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glacier close to the Debenham Islands Žlines A1 to Y7 and A1 to A17, ŽFig. 2.. Fig. 5 gives the pattern of accumulation and ablation during the following year for some of these stakes. It can be concluded that accumulation processes prevail over ablation processes until late October. The depletion of the snow cover during the summer months ŽFig. 5. shows the combined effect of snow melt, wind drift, densification of the snow pack and evaporation. From snow pits ŽFig. 6. the snow density of the snow pack is estimated to be 0.4 kg my3 at the end of the winter season and 0.5 kg my3 in summer, resulting in a mass balance for the winter of 1994 of q320 mm water equivalent. At the end of the summer, in late February, the mean of the net balance of all stakes yields a water equivalent of q200 mm. However, Fig. 7 shows that, whereas the correlation between accumulation and altitude in winter is not
significant, for the year as a whole it is. This is attributed to the fact that air temperature drives surface ablation. From Fig. 7 the equilibrium line altitude is calculated to be 110 m asl for the glacial year 1994r1995. Wunderle Ž1996. gives a value of 560 mm mean annual accumulation for 1993r1994 and reports that no ablation zone had developed at all. However, in contrast to 1994r1995, the mean air temperature from November to February was 2.28C lower in 1993r1994. In addition, SAR images from the first ERS-1 satellite reveal that the altitude of the transition between the percolation zone and the completely frozen snow pack was approximately 330 m lower in the 1993r1994 summer season than in 1994r1995 ŽSchneider, 1998.. It can be concluded that mass balance in the lower zones of the glacier is determined by summer energy balance and therefore highly dependent on summer air temperatures.
Fig. 5. Ablation and accumulation according to stake measurements. The y-axis gives the snow depth above the ablation surface of the preceding summer. The stakes were placed in a row from A1 to A15 as indicated in Fig. 2. The stakes B4 and B6 belong to a row parallel to the line from A1 to A6 running between A6 and D6 in Fig. 2. To derive accumulation in mm water equivalent these values must be multiplied by the snow density.
C. Schneiderr Global and Planetary Change 22 (1999) 117–130
4. Energy balance estimates 4.1. Methodology of computation The purpose of the 1994r1995 summer campaign on Northeast and McClary Glacier was to relate micro-meteorological measurements made with automatic weather stations ŽAWS. to surface ablation and the extent of the percolation zone. In this paper
123
only the relation to surface ablation is discussed. The relation to the extent of the percolation zone is subject to another paper ŽSchneider, 1998.. In the following the term ‘ablation’ means melting and removal by downward percolation or evaporation from the uppermost layer of the snow cover. ‘Ablation’ is compared to the short time energy balance at the surface of the snow cover and it is different from the mass balance of the entire glacier column at a
Fig. 6. Time series of a selection of snow pits at A1 ŽFig. 2. from January 1995.
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Fig. 7. Correlation of accumulation and ablation with altitude asl for the individual stakes for the winter season Žleft. and for the whole glacial year Žright.. Stakes along the line from A1 to A17 as shown in Fig. 2 were used.
distinct location. Hence, ‘ablation’ is considered synonymous to the term ‘surface ablation’ because refreezing of water in deeper layers of the firn is not considered here. Three AWS, supplied by Campbell Scientific ŽUK. were operated from 20 December 1994 to 20 February 1995. The locations of the AWS on the glaciers are marked with triangles in Fig. 2. All AWS were equipped with sensors for net radiation, short-wave irradiance, reflected short-wave radiation, wind direction, wind velocity, and air temperature and air humidity at 2.0 m and 0.5 m above the surface. Snow temperatures were measured at three depths. All sensors were read every 10 s and stored as 10 min means in a storage module attached to the data logger. The systems were powered by 10-W solar panels. Energy balance was calculated according to: B s R q H q L.
Ž 1.
B denotes the sum of energy fluxes from or into the atmosphere. Thus, B gives the net energy available for melting snow or changing its thermal characteristics. R represents the net radiation, H the sensible heat flux and L the latent heat flux. H and L were computed according to the bulk transfer equations given by Oke Ž1970. and Moore Ž1983.. This formu-
lation is discussed in detail by Braithwaite Ž1995. and Blackadar Ž1997.: Hsy
r cp k 2 u Ž z2 . z2 z2 ln ln z 0,u z 0,T
ŽQ Ž z2 . y Q 0 .
ž / ž /
= Ž 1 y 5Rb .
2
Ž 2. 2
Lsy
r L v 0.622 k u Ž z 2 . p ln
z2
z2
Ž e Ž z 2 . y e0 .
ž / ž / z 0,u
2
= Ž 1 y 5Rb . .
ln
z 0,q
Ž 3.
The variables denote: r , density of air; c p , specific heat at constant pressure of air Ž1005 J kgy1 Ky1 .; k , van Karman constant Ž0.4.; L v , latent heat of evaporation or sublimation wŽ2.514 or 2.849.10 6 J kgy1 .x; p, air pressure; uŽ z 2 ., wind velocity at height z 2 ; z 2 , height above snow surface Ž2.0 m.; z 0,u , z 0,T , z 0,q , roughness lengths for momentum, heat and water vapour; Q Ž z 2 ., potential air temperature at height z 2 ; Q 0 , potential air temperature at the snow surface Ž08C.; eŽ z 2 ., water vapour pressure at height z 2 ; e 0 , water vapour pressure at the snow surface Ž6.1 hPa.; Rb, bulk Richardson number.
C. Schneiderr Global and Planetary Change 22 (1999) 117–130
For time periods with air temperatures above 08C the surface temperature was set to 08C and the water vapour pressure Ž e 0 . was fixed to 6.1 hPa assuming 100% relative humidity at the snow surface. During periods with air temperatures below zero, the gradient between measurements of humidity and temperature at z 1 s 0.5 m and z 2 s 2.0 m was used to calculate turbulent energy exchange. In this case surface roughness length does not occur explicitly in the equations: Hsy
r c p k 2 Ž u Ž z 2 . y u Ž z1 . . ln
z2
ž / z1
= Ž Q Ž z 2 . y Q Ž z 1 . . Ž 1 y 5RbX . Lsy
2
Ž 4.
r L v 0.622 k 2 Ž u Ž z 2 . y u Ž z 1 . . p ln
z2
2
ž / z1
= Ž e Ž z 2 . y e Ž z 1 . . Ž 1 y 5RbX .
2
Ž 5.
However, since uŽ z 2 . was computed from uŽ z1. assuming a logarithmic wind profile, z 0 also is inherent in the formulations. The expression containing the bulk Richardson number Ž Rb . accounts for stable stratification in the boundary layer. The number itself is calculated using Rb s
g ŽQ Ž z2 . y Q 0 . Ž z2 y z0 .
Q uŽ z2 .
2
Ž 6.
with
Qs
Q Ž z2 . q Q 0
Ž 7.
2
and RbX s
g Ž Q Ž z 2 . y Q Ž z1 . . Ž z 2 y z1 .
Q X Ž u Ž z 2 . y u Ž z1 . .
2
Ž 8.
with
QXs
Q Ž z 2 . q Q Ž z1 . 2
ments. Most of these measurements were close to neutral conditions and Rb was set to zero for these cases. Radiation shields of temperature probes could not be ventilated because of insufficient power supply. Therefore, air temperature was corrected for the effect of radiative heating using a parametrisation based on short-wave irradiance Ž I . and wind velocity Ž uŽ z ..: DT s y
2
Ž 9.
g denotes acceleration due to gravity. No correction for unstable stratification was needed since unstable situations occurred in less than 5% of the measure-
125
I 1080
Ž 12.79ey4 .02 uŽ z .q0.33 .
Ž 10 .
This formulation was derived as fit to data given in a technical note by Young Company ŽMI, USA. Ž1987.. At wind speeds above 3.5 m sy1 the correction was omitted because it returns values smaller than 0.18 K at I s 600 W my2 . The mean wind speed during the time period of the measurements was 4.6 mrs. Absolute values of the turbulent heat fluxes can also be overestimated because of inversion layers between the snow surface and heights below 0.5 m. Calculation of the bulk Richardson number cannot incorporate the effect of inversions, which are confined to the lowermost boundary layer ŽMorris et al., 1997.. However, the inversion is weakened by pre-dominantly positive radiation balance during the summer. Furthermore, mean wind speed was large enough to remove the surface inversion by turbulent mixing. However, it cannot be ruled out that on some occasions with small wind speeds a bias is introduced to the computations of turbulent heat fluxes. After computing H and L the energy available at the snow surface Ž B . was calculated according to Ž1. ŽFig. 8.. The only parameters in the equations that can be adjusted to obtain optimal agreement between measured and computed ablation are the surface roughness lengths. Using 10y3 m for momentum and heat and 10y5 m for water vapour yielded the best overall result, with a correlation coefficient of r s 0.9 ŽFig. 9.. The x-axis offset of the regression is 1.7 mm and the gradient of the fit is 0.82. The negative deviation of the gradient from 1 indicates, that high ablation rates are systematically underestimated by the computations. Differences between the surface roughness length for momentum, temperature and vapour have been
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Fig. 8. Energy balance estimates from 20 December 1994 to 21 February 1995 at the location A1 ŽFig. 2.. The bars show weekly averages. The black bar, net energy, gives the mean weekly energy flux from or to the snow surface and is the sum of the three other fluxes in the graph. Hence net energy indicates the flux of energy available for ablation, snow metamorphosis, warming or cooling of the snow pack.
widely discussed Žsee, e.g., Hogg et al., 1982; Greuell and Konzelmann, 1994; Braithwaite, 1995. and values of roughness lengths over snow ranging from 10y5 mm to 5 = 10y3 mm are listed, e.g., in the work of Kuhn Ž1979., Moore Ž1983. and Morris Ž1989.. However, the necessity of taking z e smaller than z o and z h suggests, that there is a systematic error in the measurement of the humidity gradient. Forcing all of the three roughness lengths to the same value would result in z 0 s z e s z h f 10y4 m. This is very close to values of surface roughness quoted by Morris Ž1989. and King and Connolley Ž1997. for smooth snow surfaces. Since this seems reasonable, a considerable negative bias in the computation of the latent heat flux is suspected. In other
words: in this study surface roughness lengths are used to account for all the systematic errors in the measurements, which can not be explained individually. 4.2. Measured ablation Energy balance estimates were related to the ablation as derived from the changes in the snow pits, assuming that ablation would occur only if a surplus of energy at the surface temporarily coincided with positive air temperatures. Heat flux into the glacier was neglected because snow temperatures in the uppermost 2 m of the snow pack were between y0.38C and 08C throughout the field campaign and
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4.3. Error analysis
Fig. 9. Correlation of observed ablation in time series of snow pits ŽFig. 6. on the x-axis and modelled ablation from the data sampled at the three AWS. The locations of the AWS are marked in Fig. 2 with open triangles. The correlation coefficient is r s 0.9, the x-axis offset is 1.7 and the slope of the regression fit is 0.82.
showed no distinct gradient with depth. Time series of snow pits from one location show a distinct pattern of ice lenses and stratification ŽFig. 6.. The snow pits were dug adjacent to the AWS ŽFig. 2.. Pits were dug every 2 to 5 days. They were dug approximately 0.5 m apart from the previous snow pit. The removal of snow from the uppermost layers by melting makes characteristic ice lenses move up towards the surface during the summer season. A sequence of four snow pits shown in Fig. 6 demonstrates this: three ice lenses at 0.5 m depth on 29 December 1994 can be identified at 0.3 m on 16 January 1995, at 0.15 m depth on 24 January 1995 and at the surface on 29 January 1995. Using the density of the snow measured in the pits and the depth changes of identifiable layers in subsequent pits the water equivalent of the melted snow was derived. Since it was not always possible to conclusively identify ice lenses, only 15 different time intervals could be chosen during the field campaign from the locations of the AWS. These periods vary in length between 2 and 7 days, because changes in the snow cover larger than the error associated with this technique were only found when snow pits from time intervals of several days were compared.
Assuming a statistical error of "0.28C for temperature and "5% for relative humidity a relative error of 22% for sensible heat flux and 15% for latent heat flux was computed for mean atmospheric conditions according to the Gaussian error propagation laws. The validity of the logarithmic wind profile and the temporal variations in surface roughness lengths were not introduced into the computation as additional error sources. According to the error propagation, the total relative error of the energy balance calculation was 25% or 4.7 W my2 for mean conditions, assuming 15% relative error for the measurement of the net radiation. The error in measured ablation was also derived using error propagation formulations. An error of 0.01 kg my3 for the snow density, a mean snow density of 0.5 kg my3 and an error of 10 mm in height for the depth of characteristic layers yields an error of 7 mm of water equivalent per measurement. The error for the difference between two measurements then is calculated to 10 mm or approximately 7.6 W my2 for a time interval of 5 days between measurements. Accumulated ablation in the time intervals ranged from 14 to 89 mm water equivalent. 4.4. Discussion Mean values of atmospheric conditions and computed heat fluxes are given in Table 1. Table 1 and Fig. 8 show extraordinarily large values for both of the turbulent heat fluxes in comparison to the radia-
Table 1. Mean atmospheric conditions and mean heat fluxes for the period of observations from 20 December 1994 to 21 February 1995 at location A1 in Fig. 2 Air temperature Ž2 m. Wind speed Ž2 m. Relative humidity Radiation balance Sensible heat flux Latent heat flux Turbulent heat fluxes Total atmospheric heat flux
q0.8 C 4.6 m sy1 70.1% 8.6 W my2 Ž46%. 35.5 W my2 Ž190%. y25.5 W my2 Žy137%. 10.0 W my2 Ž54%. 18.6 W my2
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tion heat flux. When the sum of latent and sensible heat flux is calculated, it appears that turbulent heat fluxes account for 55%, whereas radiation balance accounts only for 45%, of the total energy balance. Small contribution from net radiation to the energy balance can be explained by low shortwave radiation because of high albedo, varying between 0.75 and 0.9 throughout the summer. Marguerite Bay experiences less cloudiness than areas further north on the Peninsula because of lee side effects occurring when air masses overflow Alexander Island from the south-west, Adelaide Island from the north or the plateau of Graham Land from the east. In consequence, the downward long-wave radiation flux is fairly low. This also reduces net radiation. Comparatively high air temperature, due to the advection of warm air from the sea in the west increases the input of sensible heat. The absolute values of the turbulent fluxes are large for both latent and sensible heat, but the fluxes have opposite signs. This situation is unusual when compared to other studies on summer-time energy balance on glaciers. On many glaciers summer-time energy balance during ablation is characterised by dominant energy input from radiation, whereas sensible heat contributes only a minor portion of the energy balance. On many occasions latent heat flux contributes to the energy budget by condensation on the glacier surface but it is often negligibly small ŽDe la Casiniere, ` 1974; Male and Granger 1981; Bintanja 1995; Konzelmann and Braithwaite 1995; Paterson, 1994, p. 68 ff... However, opposite signs of the turbulent heat fluxes must be expected when gradients of temperature and absolute humidity point into opposite directions. This is the case when air temperature is only a little above 08C, snow surface is at 08C with 6.1 hPa water vapour pressure at the surface, and absolute humidity in the boundary layer is low. During January 1995 mean air temperature on the glacier was q2.38C, and with 65% of relative humidity the average water vapour pressure was q4.7 hPa. Hence, although mean sensible heat flux must be directed downward, the mean latent heat flux was directed upward triggering evaporation from the snow surface. Foehnic winds also promote this effect. Such winds from the northeast associated with air temperatures up to q58C and low relative humidity were observed on several days.
Steffen Ž1995. obtains a similar distribution between net radiation, sensible heat flux and latent heat flux for a location on the Greenland ice sheet during a period just before the onset of summer ablation. Konzelmann and Braithwaite Ž1995. also report similar behaviour of the two turbulent heat fluxes during summer on the margin of the Greenland ice sheet, but their data show that latent heat flux accounts only for a small percentage of the total energy loss. Jamieson and Wager Ž1983. obtained similar results for energy budget terms for the summer season at Spartan Glacier on Alexander Island Ž71803X S. both in respect to absolute values of the single fluxes and in respect to the relation between sensible heat flux and net radiation. As in this study they suggest that the measurements of turbulent fluxes are affected by considerable errors. However, both studies show sensible heat flux dominates over radiation as an energy source for ablation. Jamieson and Wager Ž1983. also find opposite signs for sensible and latent heat flux. But in contrast to Fig. 8, on Spartan Glacier this is confined to short periods only. In conclusion, the results given in Fig. 8 can only be regarded as good estimates of energy fluxes. Since the correlation with observed ablation is fair and since net radiation was measured directly, it can be concluded that the sum of the turbulent heat fluxes is reasonable. It accounts for 55% of the total net energy input from the atmosphere to the snow pack during the summer. Stake measurements ŽFig. 5 and Fig. 7. show that during the summer at 120 m asl 0.6 m of snow were removed from the top layer. With a mean snow density Žsee Fig. 6. of 500 kg my3 this makes 300 mm of water equivalent. The computed ablation for the summer season was calculated to 375 mm water equivalent. The accumulated ablation as derived from the snow pits was 315 mm water equivalent. Hence, values of total ablation derived by the different methods are in good agreement.
5. Outlook Ongoing measurements on the glaciers will permit the detection of future changes. Analysis of the
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summer energy budget and the mass balance close to the ice cliffs at approximately 150 asl show that this area, is very sensitive to air temperature variations, because sensible heat transfer is the major energy source for ablation in summer. Equilibrium line is approximately at sea-level. Further warming would shift the equilibrium line to higher altitudes and with the development of an ablation zone, albedo would decrease. In consequence, major changes must be expected to the energy and mass balance in the lowest parts of the glaciers. This can be monitored permanently by radar remote sensing and field work based at the nearby Argentine base San Martın. ´ The energy balance computation presented in this paper may serve as a basis to evaluate the potential of ERS-1 SAR images for monitoring climate fluctuations in this region. A statistical approach can be used to produce spatial estimates of energy balance and to derive the areal extent of the percolation zone on the glaciers ŽSchneider et al., 1997.. In a further paper ŽSchneider, 1998. this is related to SAR images, where actual wet snow areas and frozen snow areas also can be delineated.
Acknowledgements The author wishes to thank Georg Kaser, Karen Lewis and Stephan Hofinger for their helpful comments on the paper. This research was supported by the German Federal Secretary of Education and Research ŽBMBF. within the programme ‘Dynamic Processes in Antarctic Geosystems’ ŽDYPAG. ŽContract Number: 03PL016A. and by the ESA pilot study ‘Monitoring Of Dynamic Processes in Antarctic Geosystems’ ŽMODPAG., ŽContract Number: AO2.D149.. The author would like to thank the Instituto Antarctico Argentino ŽIAA., the British Antarctic Survey ŽBAS. and the German AlfredWegener-Institut fur ¨ Polar- und Meeresforschung ŽAWI. for their support with respect to logistics and field equipment. The author is grateful for the invaluable assistance and discussions in the field provided by the Argentinean and German collaborators. The author is thankful to Stefan Wunderle who made available field data and remote sensing data from the summer campaign 1993r1994.
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