Radiative and energy fluxes on the ice slope of ... amazonaws com
Radiative and energy fluxes on the ice slope of East Antarctica G. WENDLER AND J.P. GOSINK
Geophysical Institute University of Alaska Fairbanks, Alaska 99701
The coastal area of Adélie Land has probably the strongest surface winds found anywhere at sea level on Earth (Mawson 1915). These winds are gravitational in origin: cold, heavy air is pulled down the ice slopes. They can become very strong, especially when topographic forcing enhances them as in Adélie Land (Parish 1984). To obtain a better understanding of the energetics, detailed radiative and heat-budget measurements were carried out in summer at a station some 100 kilometers inland from the coast. Figure 1 shows the instrument mast with the standard micrometeorological equipment. The global radiation was high during the period. A clearness index, K, was computed; it is the ratio of the global radiation to the extraterrestrial radiation on the horizontal surface and reduced to mean Earth/Sun distance. It was 0.89 for 0/10 cloudiness, a very high value indeed, even for totally clear skies. This shows that the atmosphere over Antarctica has a very low turbidity and contains very little water vapor. A decrease of Kt with increasing cloudiness was observed, the lowest value being 0.57 for 10/10 cloudiness. More scattering was observed for overcast than for totally clear skies, indicating that variation in the opacity of clouds is larger than the variation in turbidity.
ill r
We found the dependency of K t on cloudiness to be less pronounced than in areas where the surface albedo is lower, e.g., Alaska. The reasons for this are: • The site in Antarctica has lower temperatures compared to Alaska. Hence, the atmosphere can hold less water vapor. Clouds, when formed, are normally "thinner," with a lower opacity. • Multiple reflection between the surface and the base of the cloud is important in Antarctica. We measured a very high surface albedo of 83 percent, which enhances the global radiation, especially during overcast periods. We measured not only the short-wave radiation, but also the long-wave—or thermal infrared—radiative fluxes. We found the long-wave radiation budget—the long-wave incoming minus the long-wave outgoing—to be of greater importance than the short-wave budget (global minus reflected). Figure 2 illustrates that with increasingly positive short-wave radiation budget, the long-wave radiation budget decreases even more strongly. This results in the so-called "radiation paradox" (Wendler 1986), indicating that with increased global radiation (or under clear skies), the all-wave radiation budget is more negative than for cloudy conditions. It is, of course, an effect of the high-surface albedo, and modeling with our data set showed that a surface reflectivity above 60 percent is needed to observe this phenomenon. The components of the heat balance are averaged for a 43day period in figure 3. The short-wave radiation budget supplies most of the energy toward the surface. The second positive flux is that of sensible heat, which means the air above the surface is cooled on the average. Most of the energy received is lost as long-wave radiation. To a lesser degree, energy is also needed for sublimation of snow and to heat the snow cover (Wendler, Ishikawa, and Kodama 1988). Naturally, all these fluxes have strong diurnal variations. The radiation budget is positive during the day and negative during the night, while sensible, latent, and the heat flux in the snow are negative during the day, but positive or close to zero (latent heat flux) during night. In summary, the mean warming of the -100 C -' ID
-70 -60
I-
hi I flh*
Ui > C, 2 0 -I
-50 -40 -30 -20 -10 0 0 10 20 30 40 50 60 70 80 2P 100 SHORT WAVE RADIATION BUDGET in Wm
Figure 1. Photo of the meteorological tower at D-47. Measuring heights are 0.5, 1, 2, and 4 meters.
1988 REVIEW
Figure 2. Relation between the short-wave radiation budget (SWRB) and the long-wave radiation budget (LWRB). (Note: y-axis is inverted.) (Wm 2 denotes watts per square meter.)
179
+72
+9
+3
-9 -5
SWRB - short wave radiation balance LWRB - lon g wave radiation balance s- sensible heat flux 6'.
L- latent heat flux B- heat flux in the snow I- imbalance
Figure 3. Mean fluxes of the components of the heat balance for the period 20 November to 22 December 1985. The values are expressed In watts per square meter and In percent. For the calculation of the imbalance, the sum of all the outgoing fluxes was assumed to be 100 percent.
snow, the sublimation, and the negative radiation budget for most of the day are compensated by a high positive sensible heat flux, which means that the air above the surface is cooled for most of the summer, which explains the frequent occurrence of gravity flow. This study was supported by National Science Foundation grant DPP 84-13367. Our thanks go to many individuals of the U.S. Antarctic Research Program, as well as to the Expeditions Polaires Francaises. We are also grateful to Charles Stearns' group (University of Wisconsin, Madison), who did a great job in maintaining our automatic weather stations.
180
References Mawson, D. 1915. The home of the blizzard; being the story of the Antarctic Australian Expedition, 1911-1914. London: Heinemann. Parish, T.R. 1984. A numerical study of strong katabatic winds over Antarctica. Monthly Weather Review, 112(3), 545-554. Wendler, G. 1986. The "radiation paradox" on the slopes of the antarctic continent. Polarforschung, 56(1/2), 33-41. Wendler, G., N. Ishikawa, and Y. Kodama. 1988. The heat balance of the icy slope of Adélie Land, Eastern Antarctica. Journal of Applied Meteorology, 52-65.
ANTARCTIC JOURNAL
Geophysical Institute University of Alaska Fairbanks, Alaska 99701
The coastal area of Adélie Land has probably the strongest surface winds found anywhere at sea level on Earth (Mawson 1915). These winds are gravitational in origin: cold, heavy air is pulled down the ice slopes. They can become very strong, especially when topographic forcing enhances them as in Adélie Land (Parish 1984). To obtain a better understanding of the energetics, detailed radiative and heat-budget measurements were carried out in summer at a station some 100 kilometers inland from the coast. Figure 1 shows the instrument mast with the standard micrometeorological equipment. The global radiation was high during the period. A clearness index, K, was computed; it is the ratio of the global radiation to the extraterrestrial radiation on the horizontal surface and reduced to mean Earth/Sun distance. It was 0.89 for 0/10 cloudiness, a very high value indeed, even for totally clear skies. This shows that the atmosphere over Antarctica has a very low turbidity and contains very little water vapor. A decrease of Kt with increasing cloudiness was observed, the lowest value being 0.57 for 10/10 cloudiness. More scattering was observed for overcast than for totally clear skies, indicating that variation in the opacity of clouds is larger than the variation in turbidity.
ill r
We found the dependency of K t on cloudiness to be less pronounced than in areas where the surface albedo is lower, e.g., Alaska. The reasons for this are: • The site in Antarctica has lower temperatures compared to Alaska. Hence, the atmosphere can hold less water vapor. Clouds, when formed, are normally "thinner," with a lower opacity. • Multiple reflection between the surface and the base of the cloud is important in Antarctica. We measured a very high surface albedo of 83 percent, which enhances the global radiation, especially during overcast periods. We measured not only the short-wave radiation, but also the long-wave—or thermal infrared—radiative fluxes. We found the long-wave radiation budget—the long-wave incoming minus the long-wave outgoing—to be of greater importance than the short-wave budget (global minus reflected). Figure 2 illustrates that with increasingly positive short-wave radiation budget, the long-wave radiation budget decreases even more strongly. This results in the so-called "radiation paradox" (Wendler 1986), indicating that with increased global radiation (or under clear skies), the all-wave radiation budget is more negative than for cloudy conditions. It is, of course, an effect of the high-surface albedo, and modeling with our data set showed that a surface reflectivity above 60 percent is needed to observe this phenomenon. The components of the heat balance are averaged for a 43day period in figure 3. The short-wave radiation budget supplies most of the energy toward the surface. The second positive flux is that of sensible heat, which means the air above the surface is cooled on the average. Most of the energy received is lost as long-wave radiation. To a lesser degree, energy is also needed for sublimation of snow and to heat the snow cover (Wendler, Ishikawa, and Kodama 1988). Naturally, all these fluxes have strong diurnal variations. The radiation budget is positive during the day and negative during the night, while sensible, latent, and the heat flux in the snow are negative during the day, but positive or close to zero (latent heat flux) during night. In summary, the mean warming of the -100 C -' ID
-70 -60
I-
hi I flh*
Ui > C, 2 0 -I
-50 -40 -30 -20 -10 0 0 10 20 30 40 50 60 70 80 2P 100 SHORT WAVE RADIATION BUDGET in Wm
Figure 1. Photo of the meteorological tower at D-47. Measuring heights are 0.5, 1, 2, and 4 meters.
1988 REVIEW
Figure 2. Relation between the short-wave radiation budget (SWRB) and the long-wave radiation budget (LWRB). (Note: y-axis is inverted.) (Wm 2 denotes watts per square meter.)
179
+72
+9
+3
-9 -5
SWRB - short wave radiation balance LWRB - lon g wave radiation balance s- sensible heat flux 6'.
L- latent heat flux B- heat flux in the snow I- imbalance
Figure 3. Mean fluxes of the components of the heat balance for the period 20 November to 22 December 1985. The values are expressed In watts per square meter and In percent. For the calculation of the imbalance, the sum of all the outgoing fluxes was assumed to be 100 percent.
snow, the sublimation, and the negative radiation budget for most of the day are compensated by a high positive sensible heat flux, which means that the air above the surface is cooled for most of the summer, which explains the frequent occurrence of gravity flow. This study was supported by National Science Foundation grant DPP 84-13367. Our thanks go to many individuals of the U.S. Antarctic Research Program, as well as to the Expeditions Polaires Francaises. We are also grateful to Charles Stearns' group (University of Wisconsin, Madison), who did a great job in maintaining our automatic weather stations.
180
References Mawson, D. 1915. The home of the blizzard; being the story of the Antarctic Australian Expedition, 1911-1914. London: Heinemann. Parish, T.R. 1984. A numerical study of strong katabatic winds over Antarctica. Monthly Weather Review, 112(3), 545-554. Wendler, G. 1986. The "radiation paradox" on the slopes of the antarctic continent. Polarforschung, 56(1/2), 33-41. Wendler, G., N. Ishikawa, and Y. Kodama. 1988. The heat balance of the icy slope of Adélie Land, Eastern Antarctica. Journal of Applied Meteorology, 52-65.
ANTARCTIC JOURNAL
