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of cloud LW forcing to LWP has been found during the late
spring and summer when LWP is large, while high sensitiv-
ity of cloud LW forcing to changes in cloud LWP is found in
winter and spring when cloud LWP values are small [
Chen
et al.
, 2006]. As will be discussed in section 4, the mag-
nitude of the cloud LWP during the present climate has a
substantial effect on the cloud SW and LW forcing changes
during the 21st century.
Figure 7 reproduced from
Zuidema et al.
[2005] illustrates
the change in the cloud forcing with respect to the net ra-
diative fluxes as a function of cloud optical depth based on
low-level mixed phase clouds observed at constant height
during 4 days in May 1998 over the SHEBA site. It shows
that cloud LW forcing dominates the total cloud forcing for
small cloud optical thickness (less than 3). After passing a
threshold of 6, further increase in the cloud optical thickness
has no influence on the LW radiation, and net cloud forcing
is mostly determined by the shortwave component. Cloud
effect on the net SW radiation depends on surface reflectiv-
ity causing large spread in cloud SW and total forcing, in
particular for large cloud optical thickness.
The relative magnitude of the cloud SW and LW forcing
discussed above has a large effect on the surface temper-
ature and sea ice thickness. Over perennial sea ice, cloud
warming effect overpasses the cooling effect during most
of the year. During the winter polar night and transition sea-
sons, cloudy conditions are associated with increased down-
welling LW flux and warmer surface air temperatures over
sea ice [
Walsh and Chapman
, 1998]. During the summer
melt period, surface temperatures hover around 0°C, while
the intensity of net surface melt is strongly influenced by the
surface albedo and timing of melt onset: earlier melt onset
increases the amount of solar radiation absorbed during the
entire melt season [
Perovich et al.
, 2007].
Perovich et al.
[2002] explained the melt onset during SHEBA by the de-
crease in surface albedo due to a rain event which occurred
at the end of May. On the other hand, in late spring warm air
masses enter the Arctic from lower latitudes. This increases
both the cloud liquid water content and effective emitting
temperature [
Stramler
, 2006]. Increased downwelling LW
flux during spring can provide energy for initiating the sur-
face melt [
Zhang et al.
, 1996;
Weston et al.
, 2007]. Several
studies using sea ice thermodynamic models found a strong
relationship between changes in cloud radiative forcing and
sea ice thickness.
Curry et al.
[1993] showed that a large
increase in sea ice thickness occurs in response to reduction
in annual mean cloudiness. According to
Shine and Crane
[1984], surface albedo can reverse the effects of cloudiness
increase during the summer. They found that cloud increase
outside of summer months leads to sea ice thinning, while
if clouds increase during July and August, their radiative
cooling will be greater than their longwave forcing, slow-
ing down sea ice melt and thus leading to overall thicker
ice if the forcing persists for years. In the next section, we
will discuss changes in the Arctic cloud forcing predicted for
the 21st century and their role in the sea ice cover changes
as simulated by the NCAR CCSM3 coupled global climate
model.
4. ROLE OF CLOUDS IN SEA ICE CHANGES DURING
THE 21ST CENTURY
Figure 7.
(a) Longwave, (b) shortwave, and (c) total cloud sur-
face forcing with respect to the net radiative fluxes as a function of
cloud optical depth. Dotted lines denote optical depths of 3 and 6,
based on observations during the SHEBA from 1 to 10 May 1998.
From
Zuidema et al.
[2005], with permission of the American Me-
teorological Society.
Coupled models participating in the IPCC AR4 assess-
ment show large differences in future Arctic sea ice thickness
and extent when forced with CO
2
emissions as specified by
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