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Arctic Multiyear Ice Concentration
Retrieval from SSM/I Data Using
the NASA Team Algorithm
with Dynamic Tie Points
Yufang Ye and Georg Heygster
Abstract We retrieve multiyear ice concentration with the NASA Team algorithm
and a modi
ed NASA Team algorithm with dynamic tie points. The latter is used to
compensate the temporal impact of brightness temperature variations. To assess the
performance of both methods, analyses were made for all winters from 1989 to
2012. The method with dynamic tie points yields higher estimates in most years.
Both methods show a clear declining trend of the multiyear ice area from 1989 to
2012, which is consistent with the decrease of the annual minimum ice extent.
Furthermore, the ice extent of multiyear ice in most years decreases during each
winter, re
ecting the expected loss of multiyear ice by export to lower latitudes. A
sensitivity study to evaluate the in
fl
uence of each tie point on multiyear ice con-
centration retrieval shows that the retrieval is most sensitive to the tie points of
multiyear ice and
fl
first year ice at 19 GHz vertical polarization. Therefore, these tie
points need to be determined more precisely if dynamic tie points are used for
multiyear ice retrieval with the NASA Team algorithm.
Keywords Multiyear ice concentration
SSM/I
Dynamic tie points
NASA
team algorithm
1 Introduction
Arctic sea ice is a sensitive climate indicator (Serreze and Francis
2006
). Its area has
declined dramatically over the past 30 years. The decrease is most pronounced in
September when the annual sea ice minimum occurs, so that the area of multiyear
ice (ice which has survived at least one summer) is decreasing (Nghiem et al.
2007
;
Perovich and Richter-Menge
2009
; Stroeve et al.
2005
). Multiyear ice is important
because it differs from
first year ice in physical and biological aspects. Multiyear ice
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