Shortwave Net Radiation

From a climatological point of view, knowledge about short-wave net radiation (or the absorbed solar radiation) is more important than about the potential global solar radiation reaching the earth's surface. Short-wave net radiation depends mainly on the declination of the sun and surface albedo. Along the same latitude only the albedo determines the amount of absorbed energy hy the earth's surface. On a local scale (in mountainous areas), however, differences in the elevation of the land, its aspect and inclination also significantly control the amount of solar radiation which is received.

3.3.1 Albedo

As mentioned earlier, albedo is a very important factor in the shortwave balance of the surface. The computation of mean monthly values of albedo for the entire Arctic is a very difficult task. Problems arise for different reasons, e.g. the lack of insufficient in situ measurements or measurements taken from aircraft, dynamical changes of the area and physical characteristics of vegetation, snow, and ice covers, which mainly influence the albedo. In recent years, however, the chance of receiving the real distribution of albedo changes (and other components of the radiation balance) in the Arctic has markedly grown thanks to the possibilities provided by satellite techniques.

At present there are only a few publications which give the mean monthly distribution of albedo in the Arctic. Larsson and Orvig (1961, 1962) and Larsson (1963) published their results first in the form of maps and then in the form of stereograms. These maps were compiled from different kinds of information about natural vegetation, large scale physiographic features, snow cover, sea-ice cover, and glacicrised areas etc. Marshunova and Chernigovskii (1971), using the same method as Larsson and Orvig, also constructed maps showing mean albedo values for March, May, July, and September. It seems to me that the best record of the albedo in the Arctic can be obtained from satellites such as those recently presented by Robinson et al. (1992), Schweiger et al. (1993), and for Greenland by Haefliger (1998).

Albedo in March in the Arctic Ocean and seas covered by sea ice and snow cover is, according to Marshunova and Chernigovskii (1971), about 82% (Figure 3.4). The northern parts of Russia and North America, including the Canadian Arctic Archipelago and probably Greenland, have similar albedo values. A significant drop in albedo (from 82% to only 20%) is observed between the regions covered by sea ice and open waters. For May it is

Figure 5.4, Mean monthly albedo (March, May, July, and September) (¡11 percentages) in the Arctic (after Marshunova and C'hcrnigovskti 1971). 1 - boundary between ice and water, 2 boundary between ice of 1-5 and 5-8 points, 3 - boundary between ice of 5-8 and 8-10 points, 4 - forest limit.

possible to compare albedo values received by traditional methods and satellite methods. Marshunova and Chernigovskii's (1971) maps show the greatest correspondence with maps published by Robinson et al. (1992), This correspondence is surprisingly high because nowhere does the difference exceed 3%. Albedo, according to Marshunova and Chcrnigovskii (1971), in the areas of the Arctic Ocean and the seas surrounding the Arctic covered by sea ice, varies between 78% and 82% (Figure 3.4), while according to Robinson et al. (1992) it varies from 75% to 80%. In July, the correspondence is only a little worse than in May, but the differences rarely exceed 5%. In the central part of the Arctic the albedo, according to Marshunova and

Chernigovskii, oscillates between 60-65%, while on Robinson et al.'s map these values vary from 55-60%. The albedo near the sea-ice edge is equal to 50-55% (Marshunova and Chernigovskii 1971) and about 45-50% (Robinson et al. 1992). Albedo of the drifting ice (Barents, Norwegian, Greenland seas and in Baffin Bay) oscillates between 25—40%. In July the albedo is at its lowest in the whole year and on the tundra it reaches a minimum value of 16% to 18%.

In September, the surface reflectivity of the central Arctic is 70-83% (Figure 3.4). The albedo of the tundra increases to 25-35% (Marshunova and Chernigovskii 1971). The highest mean monthly values of the surface albedo of the Arctic seas occur in February and March (82%), and the lowest in July (55%).

3.3.2 Absorbed Global Solar Radiation

The magnitude of absorbed global solar radiation on every point of the Earth is determined by the incoming global radiation and by the reflective characteristics (albedo) of the underlying surface. Both these quantities change significantly in the annual cycle. Moreover, as may be seen from previous sections, the existing network of actinoinetric stations in the Arctic is very scarce. Therefore, the drawing of maps presenting the distribution of absorbed radiation in the whole Arctic is rather difficult. Reviewing the literature we only find a few teams of authors who have attempted to present such a distribution. Gavrilova (1959, 1963) was the first to publish maps presenting the monthly amounts of absorbed radiation in the Arctic. A little later, Vowinckel and Orvig (1962) also presented their results. Some of the maps from this paper were also later included in their better known articles (Vowinckel and Orvig 1964b, 1970). The third known attempt was made by Marshunova and Chernigovskii, first only for the Soviet Arctic (Chernigovskii and Marshunova 1965) and then also for the whole Arctic (Marshunova and Chernigovskii 1971). In the last work, all material (fortunately aside from maps) is limited to the non-Soviet Arctic, in accordance with the title. It is worth noting, however, that all these published geographical distributions of mean monthly and annual amounts of absorbed radiation are more rough approximations of the reality than in the case of incoming radiation.

In January, only areas to the south of 68"N receive solar radiation. However, within the Arctic these fluxes of solar radiation are small. Moreover, they are almost entirely (80-85%) reflected back by the snow cover. As a result, the zero isoline of the absorbed radiation more or less passes near the Arctic Circle (see Gavrilova 1963).

In April (Figure 3.5), the whole area covered by sea ice and snow (the Arctic Ocean, the Laptev Sea, and the central and northern parts of other Arctic seas, as well as the northern part of the Taymyr Peninsula and Greenland, and the Canadian Arctic Archipelago) absorbs radiation at a rate of 1.52.0 kcal/cm2 (6.3-8.4 kJ/cm2). On the other hand, the highest values are absorbed by the open water areas in the Norwegian and Barents seas, as well as in Baffin Bay. In the southern parts of the continental Arctic, the absorbed radiation oscillates between 2 kcal/cm2 and 3 kcal/cm2 (8.4-12,5 kJ/cm2).

Figure 3.5. Mean totals of absorbed radiation in the Arctic for April, July, and October (in kcal/cnv/month) and for the year (in kcal/cmVyear) (after Marshunova and Chernigovskii 1971).

In July, most Arctic regions absorb their highest values of solar radiation (see Marshunova and Chernigovskii 1971), The Arctic Occan receives from 5 kcal/cm2 (20.9 kJ/cin2) in the vicinity of the North Pole to about 6 kcal/ cm2 (25.1 kJ/cm2) along the latitude 80 N. Further to the south, the absorbed radiation systematically rises to about 10 kcal/cm2 (41.8 kJ/cm2) in the northern continental part of the Russian Arctic and Alaska, and in the southern part of the Canadian Arctic Archipelago. Similar values and even greater, up to 12 kcal/cm2 (50.2 kJ/cm2), occur in the Norwegian Sea, in the pure water of the Barents and Greenland seas, and in Baffin Bay. In the central part of

Baffin Bay, values exceeding even 13 kcal/cm2 (54.3 kJ/cm2) are observed (Figure 3.5).

In Octobcr (Figure 3.5), the central parts of the Arctic, up to about the latitude 80°N, do not absorb any solar radiation (polar night). The 0.5 kcal/ cm2 (2.1 kJ/cm2) isoiine runs between mainly 70°N and 75°N. The highest values of absorbed radiation (> 2 kcal/cm2 [8.4 kJ/cm2]) are in the southern parts of the Canadian Arctic and probably in the coastal parts of southern Greenland (the southernmost parts of the Arctic).

On an annual basis (Figure 3.5), the maximum values of absorbed radiation (50-55 kcal/cm2 [209-230 kJ/cm2]) occur in the southernmost parts of the Arctic (the southern Canadian Arctic) and in the Norwegian and Barents seas, where, for the greater part of the year, an open water or thin drifting ice is observed. The sums of the absorbed radiation systematically decrease in a northerly direction and oscillate between 17 kcal/cm2 and 20 kcal/cm2 (71.1 83.6 kJ/cm2) in the vicinity of the North Pole. Vowinckcl and Orvig (1962, 1964b) give significantly higher values for this area of the Arctic: c. 28-30 kcal/cm2 (117,0-125.4 kJ/cm2). However, Badgley (1961) received similar results to those of the Russian authors. The mean July and August absorption of solar radiation in years with slight ice formation, in comparison with years with heavy ice conditions in the Arctic, is 1.4-1.5 times greater (see Marshunova and Chernigovskii 1971, their Tabic 21).

0 0

Post a comment