1.  TITLE 

1.1  Data Set Identification.

     NMC/ECMWF Reanalysis Sea ice.

     (Monthly ; NMC/ECMWF)

1.2  Data Base and Data Base Table Name.

     Not applicable.

1.3  CD-ROM File Name.


     Note:  capital letters indicate fixed values that appear on the CD-ROM 
     exactly as shown here, lower case indicates characters (values) that 
     change for each path and file.

     The format used for the filenames is: YyyMmm.sfx, where yy is the last 
     two digits of the year (e.g., Y87=1987), and mm is the month of the year 
     (e.g., M12=December).  The filename extension (.sfx), identifies the data 
     set content for the file (see Section 8.2) and is equal to .SIC for this 
     data set.

1.4  Revision Date Of This Document.

     April 5, 1995

                     2.  INVESTIGATOR(S)

2.1  Investigator(s) Name And Title.

     Atsushi Nomura
     ECMWF Reanalysis Project
     Shinfield Park, Reading
     Berkshire  RG2 9AX, UK

     Robert Grumbine
     #206 5200 Auth Rd.
     World Weather Building
     Camp Springs, MD 20746

2.2  Title Of Investigation.

     ECMWF Reanalysis Project and the NMC/NCAR Reanalysis Project.

2.3  Contacts (For Data Production Information).

              |       Contact 1                |
2.3.1 Name    |Mr. Robert Grumbine             |
2.3.2 Address |#206 5200 Auth Rd.              |
              |World Weather Building          |
      City/St.|Camp Springs, MD                |
      Zip Code|20746                           |
2.3.3 Tel.    |(301) 763-8133                  |
2.3.4 Email   |        |
              | |

2.4  Requested Form of Acknowledgment.

     Please use the following citation whenever these data are used:

     A. Nomura and R. Grumbine, personal communication, 1995.

                        3.  INTRODUCTION

3.1  Objective/Purpose.

     Sea ice concentrations derived from satellite-based passive microwave 
     observations have been collected continuously since 1978 for a number of 
     scientific programs.  The basic data are archived at the National Snow 
     and Ice Data Center (NSIDC), World Data Center for Glaciology - A.  These 
     data have been used for climate change detection, operational sea ice 
     forecasting, climate modeling, and climate reanalysis.

3.2  Summary of Parameters.

     Sea ice concentration (fraction of grid cell area which is covered
     by sea ice) expressed as a percent.

3.3  Discussion.

     The underlying data set which has been collected are passive microwave
     brightness temperatures.  These data are useful for several scientific
     purposes, especially for estimating sea ice concentration.  The basic
     data are collected on a large swath scanning microwave radiometer, at
     a resolution of approximately 25 km from a polar orbiting satellite.
     The satellite covers the mid-latitudes with about a three day
     repeat time.  High latitude points are passed nearly every day.  

     The brightness temperatures are converted to ice concentrations by
     use of the NASA Team algorithm [Cavalieri, 1992].  The annual average
     accuracy, estimated by comparison with LANDSAT imagery (itself estimated
     to be accurate to about 4%) collocated and at the same time, is 7%, with 
     a bias of -4%.  That is, the passive microwave estimate is biased about 
     4% low.  The errors are greater in summer, but the verification itself is 
     suspect owing to errors which also affect the LANDSAT comparison set 
     [Cavalieri, 1992].

     An ice concentration of 15% typically corresponds to the ice edge.
     Values less than that are normally weather contamination.  A 38%
     concentration corresponds to the main body of the ice pack [Cavalieri, 
                   4.  THEORY OF MEASUREMENTS

This data set consists of global gridded data of monthly sea ice 
concentrations derived from passive microwave radiometry.  In the polar 
regions, the contributions from the atmosphere are low, in the absence of 
storms, for the wavelength span of passive microwave radiometers (0.8 cm to 
4.5 cm).  The contributions from the surface can be expressed as a product of 
the emissivity and the physical temperature of the radiating layer.  This 
linear relationship holds because the Rayleigh-Jeans approximation to the 
Planck blackbody law is valid for passive microwave radiometers (SMMR, SSM/I) 
wavelength intervals over the range of physical temperatures encountered on 
the Earth.  Radiances are frequently described in terms of brightness 
temperature.  Depending on the polarized component and the wavelength of the 
radiation, the emissivities of ice-free open water lie in the range 0.28 - 
0.75, and those of sea ice lie in the range 0.52 - 96.  Except at wavelengths 
shorter than 1 cm, there is no overlap in the water versus ice emissivity 
ranges, and sea ice can readily be discerned against the background of open 
ocean. Moreover, sea ice emissivities vary with ice type, allowing some ice 
types to be distinguished, within certain limitations, by multichannel 
microwave observations.  For additional information on Microwave properties of 
sea ice and open ocean see Gloersen et al. (1992).

                        5.  EQUIPMENT

5.1  Instrument Description. 

     5.1.1  Platform (Satellite).

            Nimbus 7 SMMR for 25 October 1978 to 20 August 1987
            DMSP F-8 SSMI for 9 July 1987 to 31 December 1991 

     5.1.3  Key Variables.

              Brightness temperature for vertical and horizontal polarization 
              at 18.0, 21.0, and 37.0 GHz.

              Brightness temperature for vertical and horizontal polarization
              at 19.3 and 37.0 GHz, vertical polarization at 22.2 GHz.

     5.1.4  Principles of Operation.

            See Gloersen et al. (1992).

     5.1.5  Instrument Measurement Geometry.

            See Gloersen et al. (1992).

     5.1.6  Manufacturer of Instrument.

            See Gloersen et al. (1992).

5.2  Calibration.

     5.2.1  Specifications.

            See Gloersen et al. (1992).


            See Gloersen et al. (1992).

     5.2.2  Frequency of Calibration.

            See Gloersen et al. (1992).

     5.2.3  Other Calibration Information.

            See Gloersen et al. (1992).

                          6.  PROCEDURE

6.1  Data Acquisition Methods.

     The brightness temperature data are available from the NSIDC on CD-ROM, 
     as are the NASA Team algorithms for processing the brightness 
     temperatures to obtain ice concentrations.  Already mapped ice 
     concentrations are also available from NSIDC, again on CD-ROM.

6.2  Spatial Characteristics.

     The NSIDC data are mapped on a polar stereographic grid with a true 
     latitude of 70 N or S, and resolution of 25 km at that latitude.  The 
     data here have been averaged to a 1 degree regular latitude-longitude 

     6.2.1  Spatial Coverage.

            The coverage is global.  Data in each file are ordered from North 
            to South and from West to East beginning at 180 degrees West and 
            90 degrees North.  Point (1,1) represents the grid cell centered 
            at 89.5 N and 179.5 W (see section 8.4).

     6.2.2  Spatial Resolution.

            The data are given in an equal-angle lat/long grid that has a 
            spatial resolution of 1 X 1 degree lat/long.

6.3  Temporal Characteristics.

     6.3.1  Temporal Coverage

            January 1987 through December 1988.

     6.3.2  Temporal Resolution


                          7.  OBSERVATIONS

7.1  Field Notes.

     Not applicable.

                        8.  DATA DESCRIPTION

8.1  Table Definition With Comments.

     Not applicable.

8.2  Type of Data.

|                 8.2.1                  |           |        |             |
|Parameter/Variable Name                 |           |        |             |
|    |               8.2.2               |   8.2.3   | 8.2.4  |    8.2.5    |
|    |Parameter/Variable Description     |Range      |Units   |Source       |
|SEA_ICE                                 |           |        |SMMR, SSMI,  |
|    |Sea ice concentration as percent   |min = 0,   |[%]     |NASA Team    |
|    |of pixel area covered by ice.      |max = 100  |        |algorithm    |
|    |                                   |           |        |as quality   |
|    |                                   |           |        |controlled   |
|    |                                   |           |        |by NMC/NCAR  | 
|    |                                   |           |        |and ECMWF    |
|    |                                   |           |        |reanalysis   |
|    |                                   |           |        |             |

8.3  Sample Data Record.

     Not applicable.

8.4  Data Format.

     The CD-ROM file format is ASCII, and consists of numerical fields of 
     varying length, which are space delimited and arranged in columns and 
     rows.  Each column contains 180 numerical values and each row contain 360 
     numerical values.  

          Grid arrangement

             I  = 1 IS CENTERED AT 179.5W
             J  = 1 IS CENTERED AT 89.5N

             90N - | - - - | - - - | - - - | - -
                   | (1,1) | (2,1) | (3,1) |
             89N - | - - - | - - - | - - - | - -
                   | (1,2) | (2,2) | (3,2) |
             88N - | - - - | - - - | - - - | - -
                   | (1,3) | (2,3) | (3,3) |
             87N - | - - - | - - - | - - - |
                  180W   179W    178W   177W


8.5  Related Data Sets.

     The basic ice concentration and brightness temperature data sets are
     available from the NSIDC at the full time and space resolution described
     above.  These data do contain some extraneous apparent ice, so processing
     such as that described in section 9 is necessary prior to use.

                      9.  DATA MANIPULATIONS

9.1  Formulas.

     9.1.1  Derivation Techniques/Algorithms.

            Not applicable.

9.2  Data Processing Sequence.

     9.2.1  Processing Steps and Data Sets.

            1. Obtain brightness temperature grids and NASA Team algorithm 
               from NSIDC.
            2. Apply NASA Team algorithm to brightness temperatures to obtain 
               estimated ice concentrations.
            3. If SST at time of collection is greater than +1 C, set ice 
               concentration to zero.
            4. Regrid onto a 1 degree latitude longitude grid.

            SST is obtained from the Reynolds 'improved' SST available on this 
            CD-ROM (Reynolds and Smith, 1993).

     9.2.2  Processing Changes.

            The NASA Team algorithm tie points are re-calibrated for each 

9.3  Calculations.

     9.3.1  Special Corrections/Adjustments.

            The NASA Goddard DAAC applied the inverse of it's land/sea mask to 
            the sea ice data.  Land values are 0.

9.4  Graphs and Plots.

     The SMMR ice atlas [Gloersen, et al., 1992] provides color maps of
     monthly averaged ice concentration and growth.

                             10.  ERRORS

10.1  Sources of Error.

      The concentration algorithm can falsely report melting ice as being sea 
      surface.  This leads to the underestimation of ice concentration, 
      particularly in summer, mentioned above.  In situations with high winds, 
      waves, or rain, it is possible for the algorithm to report ice as being 
      present when there is in fact none.  The Nomura [1993] quality control 
      described above removes most of this contamination.  

10.2  Quality Assessment.

      The quality of the ice concentration algorithm is assessed in
      Cavalieri [1992].  The Nomura quality control is assessed in 
      Nomura [1993].

      10.2.1  Data Validation by Source.

              Quality control applied by the NMC/ECMWF Reanalysis project.   
              At high latitudes, pixels are revisited on approximately a daily 
              basis (poleward of about 70).  At mid latitudes, a given pixel 
              is visited at least once every three days.

      10.2.2  Confidence Level/Accuracy Judgment.

              Most of the concerns regarding the accuracy and precision of the 
              ice concentration data apply to use of a single days' 
              concentration map, at full resolution.  After averaging over 
              time to produce a monthly averaged map, the weather 
              contamination problem is largely removed, as few polar areas 
              have persistent heavy rain or winds.  The spatial averaging 
              similarly removes much of the ice edge and melting effects.  The 
              time of greatest error will be in the summer hemisphere ice 
              pack, when extensive melt ponds can be present in the Arctic.  
              (This is less of a problem in the Antarctic as the ice is 
              thinner and doesn't support melt ponds as large or as long as 
              the Arctic's.)

              If an ice-no ice decision is desired, a 50% concentration is 
              recommended for all seasons other than summer, then to lower 
              that cutoff to 45% for the Arctic in summer.

      10.2.3  Measurement Error for Parameters and Variables.

              The annual average accuracy, estimated by comparison with 
              LANDSAT imagery (itself estimated to be accurate to about 4%) 
              collocated and at the same time is 7%, with a bias of -4% 
              [Cavalieri, 1992].  That is, the passive microwave estimate is 
              biased about 4% low.  

      10.2.4  Additional Quality Assessment Applied.

              Not available at this revision.

                            11.  NOTES

11.1  Known Problems With The Data.

      None reported at this revision.

11.2  Usage Guidance.

      The concentrations are respectable as concentrations.  Deriving
      features, such as the ice edge, are not recommended as the data 
      have been spatially reduced from the original data set.  

11.3  Other Relevant Information.

      None at this revision.

                          12.  REFERENCES

12.1  Satellite/Instrument/Data Processing Documentation. 

      Gloersen, P., W. J. Campbell, D. J. Cavalieri, J. C. Comiso, C. L. 
          Parkinson, and H. J. Zwally, 1992. Arctic and Antarctic Sea Ice, 
          1978-1987: Satellite Passive-Microwave Observations and Analysis, 
          NASA SP-511, 290 pp, Washington, D.C.
      Cavalieri, D. J., ed.,  1992. NASA Sea Ice Validation Program for the 
          Defense Meteorological Satellite Program Special Sensor Microwave 
          Imager: Final Report,  NASA Technical Memorandum 104559, 126 pp, 
          Washington, D.C.
      Cooperative Institute for Research in Environmental Sciences, 1992. DMSP 
          SSM/I Brightness Temperature and Sea Ice Concentration Grids for the 
          Polar Regions on CD-ROM, User's Guide, NSIDC Special Report - 1.

12.2  Journal Articles and Study Reports. 

      Nomura, A.  1993. Sea surface temperature and sea ice data for the ECMWF 
          Reanalysis (ERA) system  ECMWF Re-analysis project, report no. 2, 

12.3  Archive/DBMS Usage Documentation. 

      Contact the EOS Distributed Active Archive Center (DAAC) at NASA Goddard 
      Space Flight Center (GSFC), Greenbelt Maryland (see Section 13 below).
      Documentation about using the archive or information about access to the 
      on-line information system is available through the GSFC DAAC User 
      Services Office.

                             13.  DATA ACCESS

13.1  Contacts for Archive/Data Access Information.

      GSFC DAAC User Services
      NASA/Goddard Space Flight Center
      Code 902.2
      Greenbelt, MD 20771

      Phone:     (301) 286-3209
      Fax:       (301) 286-1775

13.2  Archive Identification.

      Goddard Distributed Active Archive Center
      NASA Goddard Space Flight Center
      Code 902.2
      Greenbelt, MD 20771

      Telephone:  (301) 286-3209
      FAX:        (301) 286-1775

13.3  Procedures for Obtaining Data.

      Users may place requests by accessing the on-line system, by sending 
      letters, electronic mail, FAX, telephone, or personal visit.

      Accessing the GSFC DAAC Online System:

      The GSFC DAAC Information Management System (IMS) allows users to 
      ordering data sets stored on-line.  The system is open to the public.

      Access Instructions:

      Node name:
      Node number:
      Login example: telnet
      Username:  daacims
      password:  gsfcdaac

      You will be asked to register your name and address during your first

      Ordering CD-ROMs:

      To order CD-ROMs (available through the Goddard DAAC) users should 
      contact the Goddard DAAC User Support Office (see section 13.2).

13.4  GSFC DAAC Status/Plans.

      The ISLSCP Initiative I CD-ROM is available from the Goddard DAAC.


14.1  Tape Products. 


14.2  Film Products. 


14.3  Other Products. 


                      15.  GLOSSARY OF ACRONYMS

SSMI           Special Sensor Microwave Imager
SMMR           Scanning Multichannel Microwave Radiometer
NSIDC          National Snow and Ice Data Center
NASA           National Association for the selection of acronyms
ECMWF          European Center for Medium Range Weather Forecasting
NMC            National Meteorological Center (US)