Geoscience Reference
In-Depth Information
promoting multiple scattering between the surface and atmosphere. The clear sky
flux can be estimated with the aid of radiative transfer models (Schweiger and Key,
1992
). As noted in
Chapter 2
, the term “Arctic haze” has come into widespread use
to describe the frequent occurrence of aerosol layers that are especially noticeable
in late winter and spring, representing soot, dust, and sulfate aerosols emitted by
industrial complexes which are then transported into the Arctic. The effect of aero-
sols on incoming solar radiation involves both scattering and absorption, depending
on their properties.
The clear-sky transmittance over the Arctic, taken as the ratio between the
incoming clear-sky solar flux at the surface and the TOA flux, typically ranges from
0.70 to 0.90. The clear-sky transmittance decreases as latitude increases because
the atmospheric path length increases (the sun strikes the earth at a more grazing
angle - the longer resulting atmospheric path length means more scatterers and
absorbers). As path length decreases with elevation (the higher the elevation, the
lower the density of atmospheric scatterers and absorbers), the clear-sky flux will
be greater over surfaces such as the Greenland ice sheet. Measurements atop the
Greenland ice sheet by T. Konzelmann and A. Ohmura (
1995
) point to a clear-sky
transmittance of about 0.80. Although to a first order, the clear-sky atmosphere is
hence relatively transparent with respect to solar radiation, non-cloud scattering and
absorption is by no means insignificant in the Arctic.
5.3.2
Cloud Cover
A very important factor influencing global radiation is cloud cover. The Arctic is
cloudy, especially during summer when low-level stratus is prevalent. Mean fields
of cloud cover were presented back in
Figure 2.24
. By far, the dominant effect of
clouds is to reduce the shortwave flux received at the surface, largely owing to their
high albedo (60-75 percent for Arctic stratus). Cloud absorption is generally only a
few percent (Herman and Curry,
1984
). However, attenuation of the downward flux
by clouds is partially offset for high albedo surfaces because of multiple reflections
from the surface to the clouds and back to the surface (Wendler, Easton, and Ohtake,
1981
; Shine
1984
). The proportion of the downwelling shortwave flux resulting from
multiple reflections can be significant (Loewe,
1963
). In summer over sea ice and
glaciers, one frequently encounters “flat light” or “whiteout” conditions, associated
with a high surface albedo and low cloud or fog. These conditions represent a haz-
ard to aviation, and, as the first author can attest, complicate navigation when trav-
eling via snowmachine. In turn, the effect of radiative transmission through clouds
is to change the spectral distribution of transmitted solar radiation. Although cloud
absorption is generally small in terms of the total solar energy involved, clouds
preferentially absorb in the near infrared. Because the surface albedo is wavelength-
dependent, this leads to an increase in surface albedo under overcast compared with
clear-sky conditions. We will return to this issue later.
As a young graduate student fascinated by clouds, the author made several hun-
dred measurements of associations between the downward solar flux and cloud
