**Evaluation of Special Sensor Microwave / Imager Sea-Ice Products**

**Kim PARTINGTON**

**Cheryl BERTOIA**

U.S. National Ice Center

4251 Suitland Road, FOB 4

Washington, D.C., U.S.A. 20395-5120

Voice 301-457-5314 ext. 302 Fax 301-457-5300

E-mail: cbertoia@natice.noaa.gov

## Abstract

Existing SSM/I algorithms are imperfect at mapping total and partial ice
concentrations. This paper reviews recent findings based on comparisons
of sea-ice products against other satellite data and U.S. National Ice
Center (NIC) ice charts.

INTRODUCTION

Sea-ice products from the Defense Meteorological Satellite Program
(DMSP) Special Sensor Microwave / Imager (SSM/I) are continually adding
to the already long time series of passive microwave observations of
conditions in the polar regions. This time series has the potential to
yield information on the high latitude climate signal. Furthermore,
SSM/I data has become a routine set of data for use in the production of
operational ice charts \[1\] and is the primary source of information on
ice conditions that is used to initialize the U.S. operational ice-ocean
model, PIPS \[2\]. Given the range of important scientific and
operational analysis that depends on these data, it is imperative to
have a complete appreciation of the weaknesses and strengths of the
SSM/I-derived sea-ice products. The study reviewed here was used as the
basis for recommending updates to the operational SSM/I sea-ice products
generated by the U.S. Fleet Numerical Modeling and Oceanography Center
(FNMOC).

CURRENT OPERATIONAL SEA-ICE PRODUCT

The current operational sea-ice product generated by FNMOC for NIC is
the CAL/VAL algorithm \[3\]. This was designed in the early 1990s as a
modified version of the AES-York algorithm \[4\]. It was designed in
particular to provide an accurate location for the ice edge and so uses
the high spatial resolution 37 GHz channel in areas of low ice
concentration. Elsewhere, it used a combination of 19V and 37V channels
with various consistency checks to enable filtering for weather and
rough ocean conditions.

In Summer, 1998, following a request from NIC, the NASA Team algorithm
was implemented at FNMOC and the two products provided in parallel. The
NASA Team algorithm was implemented on the basis of its popularity
within the scientific community and the fact that it has been relatively
well evaluated, compared to most other SSM/I ice concentration
algorithms \[5\]. NIC agreed to undertake an evaluation of these two
algorithms as a pre-cursor to recommending how FNMOC and NIC should
proceed in the development and use of its operational sea-ice product.
To broaden the evaluation, NIC implemented internally the Bootstrap
algorithm \[6\] and NASA Team algorithm for thin ice \[7\], the latter
algorithm being designed to work in areas where there is no multi-year
ice.

COMPARATIVE EVALUATION OF

SSM/I ICE CONCENTRATION ALGORITHMS

Table 1 shows how differences between estimates of ice concentration
from different algorithms can be extremely large. The table gives
examples of differences between the CAL/VAL and NASA Team algorithms for
regions in the northern and southern hemispheres, on the 9^th^ December
1998.

+-------------------+----------+---------+----------+----------------+
| Region            | Season   | Mean    | R.m.s.   | Mean 0%        |
|                   |          |         |          |                |
| (9 Dec. 1998)     |          | Conc.   | Conc.    | Conc. edge     |
|                   |          |         |          |                |
|                   |          | Diff.   | Diff.    | position       |
|                   |          | (%)     | (%)      |                |
|                   |          |         |          | diff. (km)     |
+-------------------+----------+---------+----------+----------------+
| Central Arctic    | W        | 2.6     | 3.34     | -71.0          |
+-------------------+----------+---------+----------+----------------+
| E. Greenland      | W        | 8.8     | 5.4      | -23.0          |
+-------------------+----------+---------+----------+----------------+
| Baffin Bay        | W        | 13.5    | 6.2      | -23.0          |
+-------------------+----------+---------+----------+----------------+
| Sea of Okhotsk    | W        | 29.0    | 8.9      | -44.0          |
+-------------------+----------+---------+----------+----------------+
| Bellinghausen     | S        | 14.5    | 5.3      | -24.0          |
+-------------------+----------+---------+----------+----------------+
| Weddell Sea       | S        | 15.3    | 6.2      | -21.0          |
+-------------------+----------+---------+----------+----------------+

Table 1. Differences between the CAL/VAL and NASA Team algorithm ice
concentrations by region and season. A "+" sign indicates that the
CAL/VAL algorithm predicts more ice than the NASA Team algorithm. "W"
indicates winter and "S" indicates summer.

![](media/image1.png){width="2.2in" height="1.6493055555555556in"}

![](media/image2.png){width="2.2in" height="1.65in"}

Figure 1. One-dimensional profiles showing CAL/VAL -- NASA Team
algorithm differences (in % ice concentration) in the Arctic (top
figure) and Antarctic (bottom figure).

Figure 1 shows how the difference between the two algorithms varies from
the interior of the pack towards the edge. It can be seen that the
differences are substantial and vary systematically with location, being
greatest in the marginal ice zone in the Arctic during winter and in the
interior of the pack in the Antarctic during summer. The CAL/VAL
algorithm shows significantly more ice than the NASA Team algorithm over
the vast majority of both regions, often \>10% in the Arctic and \>20%
in the Antarctic. However, there is a narrow but significant region at
the ice margin of a few tens of km width in which the NASA Team
algorithm shows more ice than the CAL/VAL algorithm and this is season
and region independent.

The area of most significant difference in the north is found in the Sea
of Okhotsk on 9 December 1998. An NIC ice chart is available for that
time which was analyzed using relatively cloud-free DMSP Operational
Line-Scan System (OLS) data and shows a high proportion of new and young
ice. Figure 2 compares the ice chart with predictions from different
algorithms.

![](media/image3.png){width="2.375in" height="3.1666666666666665in"}

![](media/image4.png){width="2.3631944444444444in"
height="3.0569444444444445in"}

Figure 2. Comparison of algorithms with the NIC ice chart for the Sea of
Okhotsk, 9 December 1998. White indicates 100% ice concentration and
black 0% ice concentration.

INTERPRETATION OF DIFFERENCES

The differences recorded here are significant for any application. The
differences can be explained by a number of factors.

*Ice edge position difference*

The difference in ice edge position found for the NASA Team and CAL/VAL
algorithms is probably related to the different spatial resolutions of
the 37 GHz and 19 GHz channels. The CAL/VAL algorithm makes use of the
37GHz channels at the ice margin and so has better spatial resolution
than the NASA Team algorithm. The difference in the effective field of
views of these two frequencies is between 14 and 39 km, depending on
direction relative to the ground track \[3\] and this is consistent with
the differences in ice edge position that are observed. This explanation
is consistent with the fact that the ice edge difference appears to be
independent of season and hemisphere.

*Ice type sensitivities*

Comparison of the algorithm predictions in Figure 2 suggests that there
is severe under-estimation of new and/or young ice types by the NASA
Team algorithm. The Bootstrap algorithm also under-predicts ice
concentration as does, to a lesser extent, the CAL/VAL algorithm. The
NASA Team algorithm modified for thin ice appears to work reasonably
well in this area. Concentrations are close to those suggested by the
ice chart and in fact, the Sea of Okhotsk is a prime candidate for use
of the NASA Team algorithm modified for thin ice. The differing
sensitivities of the algorithms to thin ice types are illustrated
further in Figure 3, which shows ice concentrations calculated using
controlled observations of different ice types (the "correct"
concentration is 0% for open water and 100% for ice).It can be seen that
there is a wide range of sensitivities of the algorithms to different
types of thin ice.

![](media/image5.wmf)

Figure 3. Ice concentrations predicted by different algorithms from
controlled observations of different surface types. Observations
recorded by Grenfell \[8\], Wensnehan et al. \[9\] and the 1998 North
Water experiment.

The pattern if differences in ice concentrations shown in Figure 2 can
be explained largely in terms of different algorithm sensitivities to
thin ice types. Mapping of multi-year ice using SSM/I is also subject to
difficulties, possibly related to flooding of the surface and/or thick
snow cover masking the underlying "fresh" ice.

CONCLUSIONS AND RECOMMENDATIONS

This study has shown that major differences in ice concentration result
from the use of different algorithms. The differences are sufficiently
large to justify a different approach to the use of SSM/I in operational
mapping of sea-ice, which should be based on data fusion or data
assimilation techniques. Such an approach is demonstrated in \[1\].

ACKNOWLEDGEMENTS

The authors would like to thank A. Trakic (NIC), A. Navard and N.
Sandidge (NRL) for assistance with software and J. Haferman (FNMOC) for
assistance with the provision of data.

REFERENCES

\[1\] K.C. Partington, *Data fusion for use of passive microwave data in
operational sea-ice monitoring*, this proceedings.

\[2\] A. Cheng and R. Preller, *The development of an ice-ocean coupled
model in the northern hemisphere*, Naval Research Laboratory technical
report NRL/FR/7322-95-9637, 1996.

\[3\] J.R. Hollinger, R. Lo, G. Poe, R. Savage and J. Pierce, *Special
Sensor Microwave / Imager Calibration/Validation*, Final Report,
Washington DC: Naval Research Laboratory, pp. 10-1 to 10-20, 1991.

\[4\] R. Ramseier, I.G. Rubinstein and A.F. Davies, *Operational
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\[5\] D. Cavalieri, P. Gloersen, P. and W.J. Campbell, *Determination of
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\[6\] J. Comiso, *Characteristics of Arctic winter sea-ice from
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\[7\] D. Cavalieri*, A microwave technique for mapping thin sea-ice*,
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\[8\] T.C Grenfell, *Surface microwave passive microwave observations of
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\[9\] M. Wensnehan, G.A. Maykut, T.C. Grenfell and D.P. Winebrenner,
*Passive microwave remote sensing of thin sea ice using Principal
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