Geoscience Reference
In-Depth Information
implementing a sustainable network to monitor and understand emerging changes
in the Arctic system.
The Arctic community has greatly benefited from an explosive growth in numer-
ical modeling. The models range widely from single-column models, designed to
examine processes such as sea ice growth and melt at a particular site, to regional
models of the ice-ocean system, to fully coupled (ice-ocean-atmosphere) global
models. Models are being constantly updated and improved in terms of resolution
and processes treated. As this topic was being written, there was considerable focus
on evaluating simulations contained in the Coupled Model Intercomparison Project
(CMIP5) archive that were used in the Intergovernmental Panel on Climate Change
(IPCC) fifth assessment report. This follows on the heels of CMIP3, which archived
output from models used in the fourth IPCC report.
An important area in modeling is data assimilation. Data assimilation can be
thought of as an optimal blending of models and observations. It recognizes that
both data and models are imperfect, but when blended with appropriate physical
constraints, they can provide valuable products. Data assimilation is a central ele-
ment of numerical weather prediction. Atmospheric reanalyses are a key need in
reliable modern Arctic research. Atmosphere reanalyses are retrospective forms of
numerical weather prediction in which historical observations (from radiosondes,
satellites, aircraft, and other sources) are ingested into a numerical weather model to
provide gridded analyses (typically every six hours) spanning many years of atmo-
spheric pressure heights, winds, humidity, and other variables, as well as forecasts
of variables such as precipitation. The best known first generation reanalysis is the
National Centers for Environmental Prediction/National Center for Atmospheric
Research (NCEP/NCAR) reanalysis project (Kalnay et al.,
1996
) which is still in
operation. Outputs based on NCEP/NCAR are available back to 1948. ERA-40 repre-
sents a second generation atmospheric reanalysis developed by the European Centre
for Medium Range Weather Forecasts (ECMWF). Third-generation atmospheric
reanalyses include the NASA Modern Era Retrospective-Analysis for Research
and Applications (MERRA) (Rienecker et al.,
2011
), the National Oceanic and
Atmospheric Administration CFSR (Climate Forecast System Reanalysis) (Saha
et al.,
2010
) and ERA-Interim, hereon referred to as ERA-I (Dee et al.,
2011
). The
Twentieth Century Reanalysis uses surface pressure observations only to contract
atmospheric fields as far back as 1871 (Compo et al.,
2011
).
1.4
The Changing Arctic System
Arctic research over the past two decades has increasingly focused on assessing and
understanding major environmental changes unfolding in the region, which encom-
pass the oceans, land, and atmosphere. Whereas Arctic change was largely reserved
for the final chapter in the first edition of this textbook, this structure is no longer
tenable - change is now such a dominant part of the Arctic Climate System that is
now must be addressed throughout the topic. Here, we only briely set the stage.
