Geoscience Reference
In-Depth Information
albedo over heterogeneous surfaces such as snow-covered vegetation or sea ice for
use in climate models is an area of active research (see Barry,
1996
). For exam-
ple, the SHEBA program combined local measurements of albedo (e.g.,
Figure 5.3
)
with aircraft surveys to determine the areal coverage of different surface types.
5.4.6
Distribution of Surface Albedo
Although albedo is a frequently measured variable, surface or aircraft observations
are insufficient to allow the construction of maps to assess the basic spatio-temporal
patterns across the Arctic. However, satellite-derived surface albedo is a standard
variable of ISCCP-D and APP-x. Monthly fields from APP-x are displayed for April
through September (
Figure 5.5
).
Over land areas, regional albedos for April typically range from 0.40 to 0.80.
Those over the Arctic Ocean are from 0.70-0.80. The high albedos over most of the
Arctic are of course attributed to snow cover. Over the ice-covered ocean,
Figure 5.4
integrates high-albedo snow surfaces along with lower albedo features such as open
or recently refrozen leads in the ice cover, or areas where winds may have blown
the sea ice clear of snow. Similarly, over land, the high albedo of the snow cover
will be integrated with effects such as vegetation canopies, or areas where the snow
cover is thin enough that darker underlying surfaces (e.g., tundra) “show through.”
Furthermore, even in cold conditions, snow will age, acting to reduce the albedo.
The sharp decline in albedo toward the North Atlantic relates to the decline in ice
concentration in the marginal ice zone.
The difference between the April pattern and the May pattern illustrates the effects
of surface melt. Albedos over land areas begin to drop from their April values, most
apparent over the southern Arctic lands. Albedos remain high over the Arctic Ocean
where it is colder and the snow has not begun to melt. There is a further reduction
in albedo between May and June. By July, the snow cover is completely removed
from the land surface and some of it has melted from the sea ice cover, forming melt
ponds (
Plate 5
). However, albedos near the pole are still around 0.50. Albedos still
remain high over central Greenland. Albedos are at a minimum during August and
start to increase again in September because of the deposition of fresh snow cover
and initial freeze-up of the sea ice surface.
5.5
Longwave Radiation Fluxes
5.5.1
Radiative Processes
During the polar night, the radiation budget is almost entirely determined by long-
wave fluxes (starlight and moonlight provide a negligible shortwave flux). Referring
back to Equation
5.4
, infrared radiation is emitted by the surface as a function of
its temperature and emissivity, scaled by the Stefan-Boltzman constant. Part of the
emitted energy from the surface escapes to space through the atmospheric windows,
