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GLOBAL AND PLANETARY CHANGE
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Comparison of heat balance characteristics at five glaciers in the Southern Hemisphere
Yukari Takeuchi a*, Renji Naruse b, Kazuhide Satow c, Nobuyoshi Ishikawa b
a Nagaoka Institute of Snow and Ice Studies, NIED, STA, Suyoshi, Nagaoka 940, Japan b Institute of Low Temperature Science, Hokkaido University, Sapporo 060, Japan c Nagaoka College of Technology, Nagaoka 940, Japan
Received 29 September 1997; accepted 19 February 1999
Abstract
Ablation characteristics of five glaciers in Patagonia and New Zealand were compared. Investigated glaciers were Tyndall and Moreno in southern Patagonia, Soler and San Rafael in northern Patagonia, and Franz Josef in New Zealand. Micro-meteorological observations were carried out at the glaciers and the heat balance components were estimated. At Franz Josef and Soler glaciers, the sensible heat flux is the largest and the latent heat flux is the second, and they are larger than the net radiation. At San Rafael Glacier, the net radiation is the largest and the latent heat flux is the smallest component, which is similar to Moreno and Tyndall glaciers. Though the latent heat flux is the smallest component at San Rafael Glacier, it is more than twice as large as that at Tyndall Glacier and contributes substantially to ice melting. The ratios of heat balance components were very different among glaciers, but the total heat flux ranged from about 240 to 300 W m_2 showing little difference among glaciers. © 1999 Elsevier Science B.V. All rights reserved.
Keywords: heat balance; glacier; Patagonia; New Zealand
1. Introduction
Among glaciers in mid-latitude of the southern hemisphere, some glaciers in the Patagonian area and in New Zealand have been well studied. Consequently, ablation characteristics and meteorological features of these glaciers have been known (Suggate, 1950; Owens et al., 1984; Kobayashi and Saito, 1985a, 1985b; Ohata et al., 1985a, 1985b; Fukami and Naruse, 1987; Fukami et al., 1987; Kondo and
* Corresponding author. Tel.: + 81-25-262-7052; fax: + 81-25262-7050.
E-mail address: yukari@ngs.niigata-u.ac.jp (Y. Takeuchi)
Inoue, 1988; Ishikawa et al., 1992; Takeuchi et al., 1995a, 1995b).
Because altitudes of ablation areas of most of these glaciers are below 1000 m a.s.l. in mid-latitude, large ablation rates have been reported. Normally, because the upper westerly jet stream induces strong westerly humid winds over these areas, the western sides of mountains have a large amount of precipitation, while the eastern sides have less precipitation. The distinct east-west contrast in meteorological conditions of the Northern Patagonia Icefield was pointed out by Ohata et al. (1985b).
In this paper, observations and details of ablation characteristics at two glaciers in southern Patagonia are shown. Then the heat balance characteristics will
0921-8181/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0921-8181 (99)00037-5
120° 140° 160'
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160° 140° 120° 100° 807&£/60°
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Fig. 1. Locations of glaciers in Patagonia and New Zealand.
202 Y. Takeuchi et al. / Global and Planetary Change 22 (1999) 201-208
be compared with those from glaciers in northern Patagonia and in New Zealand.
2. Sites and methods of observations
Observations were carried out at Moreno and Tyndall glaciers in southern Patagonia in the summer of 1993. The map of southern Patagonia is shown in Fig. 1. Moreno Glacier (officially called Glaciar Perito Moreno) flows northeastward from the Southern Patagonia Icefield and terminates in a channel of Lake Argentino. The glacier area is about 257 km2, a length of 30 km, and a mean width of 4 km in the ablation zone (Naruse and Aniya, 1992). Tyndall Glacier flows southward and terminates in a proglacial lake. The glacier area is about 355 km2 and with a length of 40 km. The width of the ablation zone is from 3.5 km to 10 km and the length is about 16-22 km (Naruse and Aniya, 1992).
The main observation site was located on a nearly-flat bare-ice in the ablation zone of each glacier. Air temperature, relative humidity, wind speed and radiation at 1 m above the glacier surface were continuously measured and recorded with a portable data logger. The sensor of the hygrother-
mometer was inserted in a double pipe to shield it from the global radiation and was ventilated by a micro-fan with a solar battery during daytime. To withstand strong winds and large ablation rates, the instruments were fixed using many stones or by drilling deep holes in the glacier ice. Maintenance of all instruments was made every day. The ablation was measured once or twice a day using seven stakes. Albedo at each stake was measured by an albedo meter from 12 h to 14 h on clear days. A view of the observation site at Moreno Glacier is shown in Fig. 2.
Franz Josef Glacier in the Southern Alps, New Zealand is 10 km long and varies in width from 0.5 km to 7 km, and flows northwestward from an accumulation zone of about 31 km2 (Ishikawa et al., 1992). Soler Glacier flows eastward while San Rafael Glacier northwestward, discharging from the Northern Patagonia Icefield. Soler Glacier is separated from the icefield by an icefall about 700 m high. It is about 7 km long and 1.5 km wide with an area of 12 km2. San Rafael Glacier is 45 km long and 4 km wide; its elevation varies from sea level to well over 1000 m (Nakajima, 1985). All these glaciers terminate below 1000 m a.s.l. and are among the lowest lying mid-latitude glaciers in the world. In this study,
Fig. 2. Meteorological station at Moreno Glacier. (a) A three-cup anemometer and a hygro-thermometer covered with a ventilation pipe. (b) An albedo meter (left) and a net radiometer (right).
observations which were made in December are compared, only the study at Moreno Glacier was made in November. The sites and periods of investigation are summarized in Table 1. All observation sites were set up on a flat portion in the ablation area of each glacier. Surface ablation and micrometeoro-logical measurement were made in the same way as Moreno and Tyndall glaciers, and height of the instruments above the surface was between 1 m and 1.5 m.
heat for ice melting. Heat conduction to the subsurface ice layer is zero, because ice near the surface was saturated with water at 0°C during the observation period. Fluxes toward the surface are regarded as positive, and those away from the surface as negative. Values of NR were measured directly by an all wave net radiometer. The turbulent heat fluxes (S and L) were calculated using a bulk aerodynamic approach following Stull (1988). The formulae are:
S = Pa Cp D H (Tz - T0 ), (2)
3. Heat balance computation and
The heat balance equation at the melting glacier surface can be written as follows:
nr + S + l -m = 0, (1)
where NR is the all wave net radiation, S the sensible heat flux, L the latent heat flux and M the
L s PaLeDE(0.622/Pa)(e, - e0), (3)
where pa is the air density, Cp the specific heat of air at constant pressure, Tz the air temperature at height z, T0 the surface temperature, Le the latent heat of vaporization of water, Pa the atmospheric
Table 1
(a) The sites and periods of investigation
Glacier Location Elevation (m.a.s.l.) Observation period Reference
Franz Josef 43°24' S, 500 Dec. 15-17, Owens et al.
170-12' W 1981 (1984)
Soler 46°54' S, 400 Dec. 15-29, Kobayashi and
73°10'W 1983 Saito (1985a)
San Rafael 46°41' S, 103 Dec. 29-Jan. 1, Ohata et al.
73°51' W 1983/84 (1985a)
Moreno 50°28' S, 330 Nov. 12-27, Takeuchi et al.
73°02'W 1993 (1995a)
Tyndall 51°15' S, 700 Dec. 9-17, Takeuchi et al.
73° 15' W 1993 (1995a)
(b) Meteorological variable at each site for observation period
Glacier Air temperature (°C) Relative humidity (%) Wind speed (ms y 1 ) Radiation (MJ m y 2 day y 1 ) Roughness length (mm) Ablation (cm-ice day y 1 ) Reference
Franz Josef - - - - .7 2. - .4 2. 7.2 Owens et al. (1984)
Soler 8.6 - 4.1 22.6 0.46 9.8 Kobayashi and Saito (1985a)
San Rafael 9.8 65.4 2.9 17.2 2.0 8.0 Ohata et al. (1985a)
Moreno 7.9 56.5 4.9 20.5 0.9 4.9 Takeuchi et al. (1995a)
Tyndall 5.1 75.3 6.6 20.2 0.6 6.0 Takeuchi et al. (1995a)
pressure, ez and e0 are the vapor pressure at heights z and the surface, DH the bulk exchange coefficient for heat, and DE the bulk exchange coefficient for water vapor. The temperature and vapor pressure at the melting ice surface are taken as 0oC and 6.11 h Pa, respectively.
It is assumed that, under neutral conditions, DH and DE are equal to the momentum exchange coefficient given by:
Do = k2U\ ln (z/zo)] "2, (4)
where k is the von Karman constant (= 0.4), Uz the wind speed at height z, and z0 the roughness length for momentum. When the atmospheric conditions are not neutral, the exchange coefficients should be corrected by using the following stability functions presented by Thom (1975):
D = D0(1 - 5Rb)2 0 < Rb < 0.25,
0.75
D = D 0 (1 - 16 R b ) 0 > R
b, (5)
where Rb is the bulk Richardson Number defined by:
Rb = g(T - T0)z/TUz2, (6)
where g is the gravitational acceleration and T the mean absolute temperature of air. The roughness length for momentum z0 was calculated from wind speeds at two heights under neutral conditions (I Rb I - 0.01) using the equation:
Zo = exp [(U2 ln z 1 - Ui ln z2)/(U2 - Ui) . (7)
The values of z0 were 0.9 mm on Moreno Glacier, 0.6 mm on Tyndall Glacier (Takeuchi et al., 1995b). In the case of other glaciers, the values of z0 were 2.4-2.7 mm on Franz Josef Glacier (Ishikawa et al., 1992), 0.46 mm on Soler Glacier (Kobayashi and Saito, 1985b) and 2.0 mm on San Rafael Glacier (Ohata et al., 1985a).
Heat for ice melting calculated by the heat balance method was compared with that measured by the stake at each glacier and it found that both values agree well and the methods of heat balance computation have good accuracy to predict melting.
4. Characteristics of heat balance and ablation at the glaciers in southern Patagonia
Daily variations of heat balance components at Moreno and Tyndall glaciers are shown in Fig. 3.
These are mean values for fine days at each glacier. The heat source for ice melting consisted mainly of net radiation and sensible heat flux at both glaciers. In the daytime, the amount of net radiation was more than a half of the total heat flux, indicating that net radiation is the most important factor for ice melting. While in the nighttime, in spite of negative net radiation, ice was melting continuously by the sensible heat from the atmosphere.
The mean daily ablation rate measured with seven stakes during the observation period ranged from 4.9 to 7.3 cm day-1 at Moreno Glacier and from 6.0 to 6.9 cm day-1 at Tyndall Glacier. When the mean daily ablation rate is plotted against the albedo at each site, a linear relation results (Fig. 4). The slope
Net radiation
Sensible heat flu;
...A
2 4 6 8 10 12 14 16 18 20 22 24
x
13
CO
CD
X
Local Time
Fig. 3. Mean daily variations of heat balance components on fine days at (a) Moreno and (b) Tyndall glaciers.
Fig. 4. Relationships between the albedo and the mean daily ablation rate in ice thickness at (a) Moreno and (b) Tyndall glaciers. Letters S1 to S7 are the stake numbers.
of the lines are considered to indicate the global radiation, and the values at which the lines intersect the vertical axis depend mainly on turbulent heat flux. At Moreno Glacier, which has a very rough surface with many seracs and crevasses, three stakes (S3, S4 and S5) were placed at the ridges of seracs, where the turbulent heat flux may have been larger than that of another stakes (S1, S2, S6 and S7) on the flat area. On the other hand, at Tyndall Glacier, since all stakes were set up on the flat and smooth surface without distinct seracs and crevasses, there was no difference in the turbulent heat flux among stakes.
5. Comparison of heat balance features
The observations at southern Patagonia glaciers in summer were compared with heat balance studies at other glaciers in the southern hemisphere (Fig. 5). At Franz Josef and Soler glaciers, the sensible heat flux is the largest component and the latent heat flux for condensation is the second, and they are both larger than the net radiation. At San Rafael Glacier, the net radiation is the largest and the latent heat flux for condensation is the smallest component. Although the latent heat flux is the smallest component at San Rafael Glacier, it is more than twice as large as that
at Tyndall Glacier and contributes substantially to ice melting.
In spite of the large differences in the ratio of heat balance components among glaciers, it became clear that the total heat fluxes (heat used for ice melting) are similar.
(0
0
I
400
300
200
100
0
-100
-200
-300
NR □ S
II M
Franz
Josef
Soler
San
Rafael
Moreno Tyndall
Fig. 5. Comparison of heat balance on glaciers in the mid-latitude of the southern hemisphere in December, except at Moreno Glacier in November. NR is the net radiation, S the sensible heat flux, L the latent heat flux (positive sign for condensation) and M the heat for melting.
6. Discussion
Generally, the climate of glaciers in Patagonia and New Zealand is characterized by the strong influence of the mountain ranges running north-south perpendicular the Southern hemisphere westerlies. As a result, there is the distinct contrast in the meteorological conditions in the western and eastern side of the mountains. One difference is the cloud distribution and related meteorological factors. Another difference is the wind system, such as the westerly circulation, and the occurrence of strong winds (Fohn) on the eastern side.
The latent heat flux were very different among the five glaciers as seen in Fig. 5. The amount of latent heat flux depends mainly on vapor pressure gradient and wind speed as shown in Eq. 3. The vapor pressure gradient depends on the air temperature and the atmospheric water content because the vapor pressure at the melting glacier surface is 6.11 hPa.
The latent heat fluxes at Franz Josef, Soler and San Rafael glaciers are larger than those at Moreno and Tyndall glaciers. Because of lower latitudes, the air temperature in December at the first three glaciers is higher than that at the latter glaciers. This is one cause of the larger vapor pressure gradient at the former glaciers. The largest sensible heat flux at Soler Glacier in the eastern side are probably due to the strong wind considered to be Fohn. On the other hand, the easterly local glacier wind component at Franz Josef and San Rafael glaciers in the western side may oppose the large-scale westerly wind. Only at Moreno Glacier, the latent heat flux is negative. At Soler Glacier situated on the eastern side of the Patagonian icefield like Moreno Glacier, evaporation predominated in November (Fukami and Naruse, 1987), while condensation predominated in December. Therefore, at Moreno Glacier, if air temperature should increase in December, condensation may predominate and ice melting may increase like at Soler Glacier. At Tyndall Glacier, lower air temperature makes the amount of the latent heat flux for condensation very smaller in spite of the humidity and the wind speed are not so small.
Causes for the agreement of the total heat flux among the five glaciers cannot be discussed in this study. If total heat flux depends on global radiation and large-scale climatic conditions, this agreement
may not be a mere accident, because these glaciers are all located at mid-latitude in southern hemisphere and data were mean values of December, then the differences of global radiation could not be so large. Moreover, large-scale climatic conditions around glaciers in Patagonia and New Zealand are very similar. The ratio of heat balance components may indicate the difference in the local climatic and geographic conditions at each glacier. In other words, mean value of meteorological elements which control the total heat flux, such as air temperature, humidity, wind speed and radiation, may depend on global radiation and large-scale climatic conditions and they may be related to each others.
7. Conclusions
Meteorological observations at Moreno and Tyndall glaciers in southern Patagonia showed that the heat source for ice melting consisted mainly of net radiation and sensible heat flux at both glaciers. The amount of net radiation was more than half of the total heat flux during the daytime, while ice was melting continuously by the sensible heat in spite of negative net radiation in the nighttime.
These results were compared with previous works at other glaciers in the southern hemisphere: in northern Patagonia and New Zealand. The ratios of heat balance components and especially the amount of latent heat flux were very different among glaciers, but the total heat flux ranged from about 240 to 300 W m2, showing little differences among glaciers.
Acknowledgements
The authors are grateful to the following members of the Glacier Research Project in Patagonia 1993 for the supports to the field observations at Moreno and Tyndall glaciers: Mr. Pedro Skvarca, instituto Antártico Argentino, Dr. Gino Casassa, Centro Austral Antartico, Universidad de Magallanes, Mr. Kenro Nishida, Kyoto University and Mr. Kenichi Mat-suoka, Institute of Low Temperature Science, Hokkaido University. They are also grateful to Dr. Andrew G. Fountain, Portland State University and Dr. Georg Kaser, Universitat Innsbruck for valuable
comments on the manuscript. This study was supported by a grant for International Scientific Research Program (No. 05041049) of the Ministry of Education, Science, Sports and Culture of Japan.
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