Geoscience Reference
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is positive for warming and negative for cooling. Calculations are typically based on
monthly means of radiation fluxes. The two components of cloud radiative forcing
compete with each other. Although cloud cover will reduce the downward solar flux
to the surface because of cloud albedo (and to a much lesser extent cloud absorb-
tion), clouds tend to increase the longwave flux to surface. The issue at hand is
which effect wins.
From a study of ten years of surface data from the NOAA Barrow Alaska
Observatory and the nearby Atmospheric Radiation Measurement North Slope of
Alaska site Barrow, X. Dong et al. (
2010
) found that the net cloud radiative forcing
is positive (clouds warm the surface) for all months except June, July, and August,
when the net forcing is negative. The longwave component of the forcing is posi-
tive in all months with a winter minimum and an August peak of about 60 Wm
−2
.
The shortwave forcing is zero in winter (when there is no solar flux) and peaks in
August at about −90 W m
−2
. The net cloud radiation forcing is at is most positive
in October at about 45 W m
−2
and most negative in July at about −40 W m
−2
. The
annual average net cloud radiative forcing was found to be +3.5 W m
−2
. By compar-
ison, surface observations from the one year SHEBA experiment in the ice-covered
Beaufort Sea (Intrieri, Fairall, and Shupe,
2002
) show that the net cloud radiative
forcing is positive except for a brief period in summer, with a much larger annual
average of +27 W m
−2
.
The warming effect of clouds at the surface of the Arctic for most of the year (and
for the annual average) contrasts with lower latitudes, where clouds have an overall net
cooling effect. The key differences are that in the Arctic: (1) solar radiation is small or
zero for much of the year, meaning that the (always) positive longwave forcing effect
dominates; and (2) the surface albedo tends to be high. If the surface albedo is high, the
net shortwave flux at the surface is already fairly small, such that a further reduction
because of cloud albedo (compared to a low albedo surface) will be of lesser importance
than the increase in the longwave flux. The longer period in summer that the net cloud
radiative forcing is negative over Barrow compared to the ice-covered Arctic Ocean
is consistent with the much lower summer albedo over land; with the lower albedo,
the cloud shortwave effect is bigger. Not surprisingly, Dong et al. (
2010
) find that the
longwave component of the forcing at Barrow is positively correlated with cloud frac-
tion, cloud liquid water path and cloud radiative temperature. The shortwave forcing is
negatively correlated with cloud fraction, cloud liquid water content and cloud radiative
temperature (as they increase, the shortwave forcing becomes more negative), and is
positively correlated with surface albedo.
Cloud radiative forcing can also be assessed at the top of the atmosphere (TOA).
J. Curry and E. Ebert (
1992
) examined the seasonal cycle in cloud radiative forcing
at both the surface and TOA for latitude 80°N using a single-column coupled model
(
Figure 5.9
). Their results indicate that the total TOA cloud forcing is close to zero
during winter and strongly negative in midsummer. This is primarily attributed to
the high albedo of clouds. At the surface, values are positive except for two weeks
in mid-summer. The competing effects of the shortwave and longwave forcing are
most pronounced at the surface.
