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primarily in the 8-12 µm region. The remainder is absorbed in the lower atmosphere
and is re-emitted both upward and downward. The processes controlling the down-
ward flux to the surface are rather involved, and merit further discussion.
Variations in the downward transfer of infrared radiation under clear skies
depend primarily on the profiles of atmospheric temperature, water vapor, and (in
winter) the presence of suspended ice crystals in the atmosphere, known as “dia-
mond dust” (Curry,
1983
; Curry et al.,
1990
) (although, arguably, diamond dust
can be considered to be cloud cover). Water vapor and diamond dust increase the
emissivity. Gasses such as carbon dioxide and methane tend to be well mixed in
the atmosphere, but of course also influence emissivity. Most of the radiation to the
surface emanates from the lowest layers of the troposphere, typically the lowest 100
m or so. In the winter, the moisture content of the Arctic atmosphere is low. In sum-
mer, however, the near-surface specific humidity is about a factor of five higher and
hence has a stronger impact.
Cloud cover tends to increase the downward longwave flux. Clouds are more
emissive than the clear-sky atmosphere. Clouds act to “close” the atmospheric
windows that allow emission from the surface to space. They absorb upwelling
radiation, and then radiate both upward and downward. The effective emissivity of
clouds depends on the cloud optical thickness and height of the cloud base. Low
stratus is an especially effective absorber and radiator. In turn, especially during
winter when the atmosphere is particularly stable and strong surface-based inver-
sions are present, the cloud base temperature often exceeds the surface temperature.
Measurements during the Coordinated Eastern Arctic Experiment for autumn and
winter indicate an increase in downwelling longwave radiation under cloudy skies
of about 90 W m
−2
. The cloud effect may be difficult to isolate, however, as cloudy
conditions are often attended by horizontal heat advection raising atmospheric and
surface temperatures.
5.5.2
Distribution of Downward and Net Longwave Fluxes
Again there are insufficient direct measurements of the downward longwave flux to
compile maps for the Arctic. Although a number of empirical formulae have been
derived that employ the surface air temperature, we rely on the ISCCP-D fields.
Figure 5.6
and
Figure 5.7
provide mean fields for the four mid-season months of the
downward longwave and net longwave fluxes, respectively.
The spatial pattern of the mean downward longwave flux largely follows the dis-
tribution of cloud cover and SAT (see
Chapter 2
). During January, the highest values
are in the Atlantic sector where cloud cover is extensive and optically thick and
where air temperatures and specific humidity are highest. Note that, for all months,
the relatively low values over central Greenland, which is above much of the cloud
cover and where air temperatures and water vapor content are low. Comparisons
between the January, April, and July maps reveal the effects of the seasonal increase
in air temperature and humidity. For all months, however, the largest fluxes are over
the Atlantic sector.
