Arctic Cloud Properties and Radiative Forcing From Observations and Their Role in Sea Ice Decline Predicted by the NCAR CCSM3 Model During the 21st Century - Arctic Sea Ice Decline: Observations, Projections, Mechanisms, and Implications - page 48

Figure 1. Spatial distribution of monthly mean top-of-atmosphere (TOA) albedo (percent, gray scale) and sea ice extent
(thick black line) over the ocean north of 40°N during (a) March and (b) July. The TOA albedo values for each month
are based on the ERBE data during 1985-1989 on 2.5° ´ 2.5° grid. The sea ice extent is defined as the area of the ocean
with sea ice concentrations (SICs) of at least 10%. SICs are from the HadISST1 data, subset to the ERBE time period and
regridded to the ERBE resolution. White area around the North Pole contains no data.

and within the surface albedo range at high SICs. While the
solar zenith angle causes a significant change in the open
ocean surface albedo, the effects of clouds overwhelm these
changes and increase the mean TOA albedo over the open
ocean well above the maximum observed ice-free surface
albedo. In contrast, over the 100% SICs the combination of
sea ice and cloud effects results in the mean TOA albedo
lying within the extremes of observed surface albedo values
(Figure 2).
Figure 3 shows seasonal changes in the downwelling and
reflected SW flux at the TOA, and the sea ice RE, calculated
as the product of the downwelling SW flux averaged over
the Arctic Ocean and sea ice RE with respect to the TOA
albedo for each corresponding month [ Gorodetskaya et al. ,
2006]. The largest impact of sea ice on the SW flux occurs
during the summer: for an incident solar flux of about 450
W m -2 reaching the TOA in the polar latitudes in June (aver-
aged over the ocean north of 70°N), local reduction of SIC
from 100% to 0% results in about 100 W m -2 decrease in
reflected SW radiation at the top of the atmosphere. Sea ice
effect on SW flux is relatively small during the fall and early
spring reducing to zero during the winter months proportion-
ate to the incoming SW flux. Thus, variations in monthly

changes in sea ice cause about 10-20% change in the all-sky
TOA albedo.
Sea ice effect on the TOA albedo is summarized in Fig-
ure 2, which shows the TOA albedo and the surface albedo
as a function of sea ice concentration. The radiative effec-
tiveness of sea ice is defined as RE = TOA albedo (SIC =
100%) - TOA albedo (SIC = 0%), following Yamanouchi
and Charlock [1997]. The average RE of the Arctic sea ice
with respect to the TOA albedo is 0.22 for all-sky condi-
tions [ Gorodetskaya et al. , 2006]. Arctic sea ice experiences
extensive melt during the summer, which decreases the sea
ice surface albedo down to about 38%, while during winter,
freshly fallen snow increases surface albedo up to 84% ac-
cording to ground-based observations [ Curry et al. , 2001].
Surface albedo over the open ocean are calculated as a func-
tion of the solar zenith angle increasing from 3% during
summer to almost 30% during winter. These surface albedo
values define an envelope of the surface albedo ranges shown
by thin lines in Figure 2: the upper line representing winter/
fall/spring surface albedo and the lower line representing the
melt season.
Monthly mean TOA albedo averaged for all-sky condi-
tions is above the range of the surface albedo at low SICs