Geoscience Reference
In-Depth Information
Coming full circle to early observations by Nordenskiold and Nansen, aero-
sol deposition on snow and ice surfaces reduces the surface albedo. Soot on snow
linked to fossil burning appears to be a significant climate forcing in the Arctic
(Hansen and Nazarenko,
2003
) and may contribute to the observed strong decline
in summer sea ice extent.
2.3.6
Clouds
Cloud cover has first order impacts on the Arctic surface radiation balance (see
Chapter 5
). Cloud microphysical and radiative properties are hence a vibrant area
of research. This includes impacts of aerosol loading on cloud albedo and lifetime
as discussed in the preceding section. Although much has been has been learned
from modeling, remote sensing, and special observation programs (e.g., SHEBA,
the Department of Energy Atmospheric Radiation Monitoring program, and IPY
efforts), continuing problems are the paucity of accurate observational data on even
cloud amounts over the Arctic and challenges in obtaining cloud information from
satellite sensors.
Over the Arctic, one can observe many of the same generic low level (e.g., stratus,
stratocumulus), middle level (e.g., altocumulus) and high level (e.g., cirrus) cloud
types as seen in middle latitudes. At the very cold summer mesopause (roughly 85
km) noctilucent clouds form when temperatures fall to 190 K. They are composed
of minute ice crystals through upwelling of trace amounts of water vapor. Despite
the high latitude of the Arctic, convective cloud cover is common over Eurasian and
Alaskan land areas during summer. Convective cloud cover is also frequent during
winter over the Norwegian Sea - cold outbreaks from the north, when reaching the
fairly warm, ice free waters off the Norwegian coast, result in destabilization of
the air.
However, cloud classification can be problematic in the Arctic. J. Curry and
her colleagues (
1996
) identify four “unusual” boundary layer cloud types over the
Arctic Ocean, the first two falling into the general category of low-level, optically
thin Arctic stratus: (1) extensive summertime boundary layer clouds with multiple
layers; (2) mixed-phase (ice and water) boundary layer clouds during the transi-
tional season; (3) low-level ice crystal clouds and “clear-sky” ice crystal precipita-
tion (the latter often termed “diamond dust”) in stable wintertime boundary layers;
and (4) wintertime ice crystal “plumes” emanating from open leads (Schnell et al.,
1989
). Measurements in the western Arctic during SHEBA (Intrieri and Shupe,
2004
) show that true clear sky diamond dust occurs about 13 percent of the time
during November through mid-May. However, the occurrence of diamond dust is
often greatly over-reported (145 percent of the time) by observers, mainly during
polar darkness. Lidar profiles showed that for nearly all these misreports, ice crys-
tals were actually precipitating from clouds.
Overall, cloud cover is least extensive during winter and most extensive during
summer. This seasonality is well expressed over the central Arctic Ocean and is pri-
marily driven by the seasonality in low-level stratus. By comparison, the Atlantic
