Monitoring Change in the Arctic - Satellite-based Applications on Climate Change - page 135

annual mean for the Arctic Ocean and surrounding landmasses north of 60 N.
Cloud particle phase is indicated by two numbers: 0 for liquid phase, 1 for solid
phase (ice), and a number between 0 and 1 for averages over time and/or space.
A number less than 0.5 indicates that liquid-phase clouds dominate; a value greater
than 0.5 indicates that ice clouds dominate. The cloud particle effective radius
for liquid droplets is the ratio of the third to second moments of the drop size
distribution. In spring and summer clouds are increasingly in liquid phase. The
effective radius has decreased over the Arctic seas except North Pole and the GIN
Seas in spring. No significant trend in cloud optical depth was found in spring or
summer. On an annual time scale, cloud particle effective radius has been decreas-
ing mainly over the western part of the Arctic and Chukchi Sea. This agrees with
tropospheric warming trends.
The influence of changes in cloud cover on sea-ice extent and vice versa is an
important part of the Arctic climate feedback process, but has not been studied
extensively until recently. On the time scale of a single season, changes in cloud
amount may have a minimal influence on summer sea-ice melt, although there are
clearly interdependencies between trends in cloud cover, surface temperature, and
sea ice extent. Over the past few decades, more than 80% of the observed surface
warming in the western Arctic Ocean during autumn is attributable to decreasing
sea ice, and over 80% of the winter surface cooling in the central Arctic is a result of
changes in cloud cover. In spring, only about half of the surface warming is a result
of changes in cloud cover (Liu et al. 2009 ). Using satellite data, Liu et al. ( 2012 )
found that a 1% decrease in sea ice concentration leads to a 0.36-0.47% increase in
cloud cover, and that 22-34% of the variance in cloud cover can be explained by
changes in sea ice.

9.4 Surface Temperature and Albedo

Surface air temperature has been recorded at land-based and drifting ice meteoro-
logical stations for decades, and a long time series over the Arctic Ocean is
available from Russian “North Pole” (NP) drifting stations, drifting ice buoys,
and coastal station observations (Martin et al. 1997 ; Martin and Munoz 1997 ).
While a valuable source of information, the in situ data do not provide spatial
details that can be obtained from satellite data.
With satellite data, surface temperature is calculated with a split-window infra-
red algorithm using 11 and 12 m brightness temperatures, similar in form to the

traditional method used for sea surface temperatures (Key et al. 1997b ). Over the
annual cycle, the Arctic surface temperature varies most for the landmasses and
least for the Arctic Ocean. Figure 9.8 shows the spatial distribution of the annual
mean surface temperature. Central Greenland has the lowest surface temperature,
as low as

μ

30 C, and the Arctic Ocean and coastal areas are colder than the

landmasses.