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Northern Hemisphere EASE-Grid Weekly Snow Cover and Sea Ice Extent Version 3

Summary

The Northern Hemisphere EASE-Grid Weekly Snow Cover and Sea Ice Extent Version 3 product and Version 3.1 update combine snow cover and sea ice extent at weekly intervals from 23 October 1978 through 24 June 2007, and snow cover alone from 3 October 1966 through 23 October 1978. (Sea ice data were not available prior to 23 October 1978.) The data set is the first representation of combined snow and sea ice measurements derived from satellite observations for the period of record. Designed to facilitate study of Northern Hemisphere seasonal fluctuations of snow cover and sea ice extent, the data set also includes monthly climatologies describing average extent, probability of occurrence, and variance. Data are provided in the Northern Hemisphere 25 km equal-area grid (NSIDC Nl EASE-Grid).

Snow cover extent is based on the digital NOAA-NESDIS Weekly Northern Hemisphere Snow Charts, revised by D. Robinson (Rutgers University) and regridded to the EASE-Grid. The original NOAA-NESDIS weekly snow charts are derived from the manual interpretation of AVHRR, GOES, and other visible-band satellite data. Sea ice extent is regridded to EASE-Grid from Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I Passive Microwave Data. Both NOAA and NASA provided funding for the production of this data set.

Citing These Data

To broaden awareness of our services, NSIDC requests that you acknowledge the use of data sets distributed by NSIDC. Please refer to the citation below for the suggested form, or contact NSIDC User Services for further information. We also request that you send us one reprint of any publication that cites the use of data received from our Center. This helps us to determine the level of use of the data we distribute. Thank you.

Armstrong, R. L., and M. J. Brodzik. 2005, updated 2007. Northern Hemisphere EASE-Grid weekly snow cover and sea ice extent version 3. Boulder, Colorado USA: National Snow and Ice Data Center. Digital media.

Overview Table

Table of Contents

1. Contacts and Acknowledgments
2. Detailed Data Description
3. Data Access and Tools
4. Data Acquisition and Processing
5. References and Related Publications
6. Document Information

1. Contacts and Acknowledgments

Investigator(s) Name and Title

Richard Armstrong
449 UCB, University of Colorado, Boulder, CO 80309-0449 USA

Mary Jo Brodzik
449 UCB, University of Colorado, Boulder, CO 80309-0449 USA

Technical Contact

Acknowledgements

The National Oceanic and Atmospheric Administration (NOAA) and the National Aeronautics and Space Administration (NASA) provided funding for the production of this data set.

The EASE-Grid land-ocean-coastline-ice (LOCI) mask was derived from the Boston University version of global 1 km land cover from MODIS 2001, version 4, courtesy of K. Knowles, National Snow and Ice Data Center, Boulder, CO.

2. Detailed Data Description

Format

Data are stored in flat binary files with arrays of 721 columns by 721 rows of 1-byte unsigned integers. Each data file contains one array.

Data Granularity

Data files represent weekly Northern Hemisphere snow cover and sea ice extent.

File and Directory Structure

Data on the FTP site are arranged into four subdirectories. The full directory structure is outlined below.

File Size

The data files are 508 KB each. The browse image files are 22-23 KB each.

File Naming Convention

Weekly Data Files

File naming convention:  NLyyyymmd1-yyyymmd2.v03.SI or .v03.1.SI

where:

NL = prefix indicating that data are in the Nl (Northern Hemisphere 25 km EASE-Grid)
yyyymmd1 = four-digit year, two-digit month, two-digit day of month for beginning of the NOAA week
yyyymmd2 = four-digit year, two-digit month, two-digit day of month for end of the NOAA week
.v03[.1].SI = file extension indicating Version 3 snow and ice data or Version 3.1 update file

The file "NL19950626-19950702.v03.SI" contains Version 3 snow and ice data for 26 June 1995 through 2 July 1995.

Monthly Probability of Occurrence Files

File naming convention:  NLparmFRQmm.v03.DAT

where:

NL = prefix indicating that data are in the Nl (Northern Hemisphere 25 km EASE-Grid)
parm = SNO or ICE
FRQ = frequency data
mm = two-digit month
.v03.DAT = file extension indicating a Version 3 binary data file

The files "NLSNOFRQ.PNG" and "NLICEFRQ.PNG" are images of all twelve months' probability of occurrence data in PNG format. PNG files are suitable for browsing with any Web browse tool.

The files "NLSNOFRQ.PS" and "NLICEFRQ.PS" are color PostScript versions of the same files.

Monthly Average Extent Files

File naming convention:  NLparmAVGmm.v03.DAT

where:

NL = prefix indicating that data are in the NL (Northern Hemisphere 25 km EASE-Grid)
parm = SNO or ICE
AVG = average extent data
mm = two-digit month
.v03.DAT = file extension indicating a Version 3 binary data file

The file "NLSI_AVG.PNG" is a PNG-formatted version of all twelve months' average extent data. The file "NLSI_AVG.PS" is a color PostScript version of the same file.

Monthly Variance Files

File naming convention:  NLparmVARmm.v03.DAT

where:

NL = prefix indicating that data are in the NL (Northern Hemisphere 25 km EASE-Grid)
parm = SNO or ICE
VAR = variance data
mm = two-digit month
.v03.DAT = file extension indicating a Version 3 binary data file

The file "NLSI_VAR.PNG" is a PNG-formatted version of all twelve months' variance data. The file "NLSI_VAR.PS" is a color PostScript version of the same file.

Geolocation Arrays

Data for the NL EASE-Grid are included in the "tools" directory. Each file contains 4-byte signed integer arrays (721 columns x 721 rows) representing latitude and longitude at the center of each cell for the NL grid in hundred thousandths of degrees (i.e. five decimal places). The data are compressed as both little-endian (a.k.a. PC- or VAX-byte order) and big-endian (a.k.a. UNIX-byte order). In these files, .LSB indicates "least-significant byte," (i.e. PC byte-order), and .MSB stands for "most-significant byte" (unix byte-order). The fill value (locations at the corners that are not in the Northern Hemisphere) is designated with 14316.55765.

File naming convention:  NLpppooo.GZ

where:

NL = NL EASE-Grid
ppp = parameter (LAT, LON)
ooo = byte order (MSB, LSB)

Spatial Coverage

The data grid covers the Northern Hemisphere.

Spatial Coverage Map

Spatial Resolution

Spatial resolution is 25 km.

Projection

Data are gridded to a Lambert's equal-area azimuthal projection based on a spherical model of the Earth with a radius of 6371.228 km, which yields a sphere with the same surface area as an ellipsoid using the International Datum. The parameters are:

r = 2 * R/C sin(lambda) * sin (PI/4 - phi/2) + r0
s = 2 * R/C * cos(lambda) * sin(PI/4 - phi/2) + s0
h = sin(PI/4 - phi/2)
k = csc(PI/4 - phi/2)

where:

r = column coordinate
s = row coordinate
h = particular scale along meridians
k = particular scale along parallels
lambda = longitude in radians
phi = latitude in radians
R = radius of the Earth
C = nominal cell size
r0 = map origin column
s0 = map origin row

Grid Description

The grid is 721 columns by 721 rows (0,720). The North Pole is aligned with the center of the pixel at (360,360) (column 360, row 360, the center pixel).

Temporal Coverage

The data set includes snow cover data from 3 October 1966 through 24 June 2007, and sea ice data from 23 October 1978 through 24 June 2007.

SMMR sea ice data are available approximately every other day from 26 October 1978 through July 1987. Weekly sea ice extent was derived from an average of three or four ice source files for the SMMR time period. SSM/I sea ice data are available approximately every day from 13 July 1987 through 24 June 2007. Weekly sea ice extent was derived from an average of seven ice source files for the SSM/I time period.

Temporal Resolution

Each file represents weekly data corresponding with the NOAA/NESDIS snow chart weeks.

No files exist for the following weeks:

• Four weeks spanning 1 July 1968 to 28 July 1968
• 21 weeks spanning 2 June 1969 to 26 October 1969
• 12 weeks spanning 5 July 1971 to 26 September 1971

Files for the following weeks have no ice information:

• All weeks prior to 23 October 1978
• Five weeks spanning 7 December 1987 to 10 January 1988

Parameter or Variable

Parameter Description

Snow extent: binary, indicating snow or no snow

Sea ice extent: binary, indicating presence or absence of sea ice when sea ice concentration is greater than 15 percent in a majority of a given week's sea ice files. The 15 percent ice concentration isopleth is a conventionally accepted cutoff for the ice edge (Johannessen et al. 1996).

Unit of Measurement

Binary presence or absence of snow or sea ice

Parameter Source

• Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I Passive Microwave Data
• Near Real-Time Sea Ice - Goddard (NRTSI)
• Digitized NOAA/NESDIS Northern Hemisphere Snow Extent data, revised by D. Robinson, Rutgers University

SSM/I and SMMR ice classifications were derived from the NASA Team total ice (first year ice plus multiyear ice) grids.

Parameter Range

Weekly Data Files: binary array of unsigned byte (8-bit) data, ranging from 0 to 255. The majority of values are not used.

Metadata for Weekly Data Files: ASCII text file containing respective pixel counts and other relevant information for each weekly data file. For example, the metadata record for the weekly data file "NL19781023-19781029.v03.SI" (data for the week of 23-29 October 1978) is:

File_Name                :NL19781023-19781029.v03.SI

Start_Date               :1978-10-23

Stop_Date                :1978-10-29

Data_Set_Parameter_Name  :Northern Hemisphere Weekly Snow Cover and Sea Ice Extent Version 3

Bytes                    :1

Data_Type                :UNSIGNED INTEGER

Map_Name                 :Nl

Map_Scale                : 25.0675 kilometers

Area_Per_Pixel           :628.3795 square kilometers

Columns                  :721

Rows                     :721

Snow_Pixels              : 33116

QC_Snow_Pixels           :  4894

Land_Pixels              :119710

Ice_Pixels               : 14326

QC_Ice_Pixels            :  2661

Ocean_Pixels             :230165

QC_Ocean_Pixels          :  725

Unclassifiable_Pixels    :   296

Corner_Pixels            :113948

Total_Pixels             :519841

Sea_Ice_Derived_From     :NIMBUS-7 SMMR

Sea_Ice_Climatology_Month:10 

Sea_Ice_Files_Used       :GODDARD.19781026.N07 GODDARD.19781028.N07 

 

The following statistics files were derived from v03 data (through 2005), and were not updated with v03.1 data:

Monthly Probability of Occurrence Files: percentage of time in the given month that each pixel was either snow or sea ice. Data are 1-byte values, ranging from 0 to 100 percent.

Monthly Average Extent Files: binary value for each pixel, indicating 50 percent or greater probability of occurrence of snow or sea ice. Data are 1-byte values of 0 (probability of occurrence was less than 50 percent) or 1 (probability of occurrence was greater than or equal to 50 percent).

Monthly Variance Files: monthly variance in snow or sea ice probability of occurrence for each pixel, calculated as:

where:

i = 1966 to 2005 for snow or 1978 to 2005 for ice
n = number of years
= snow or sea ice probability of occurrence for data in this month for the period of record
= snow or sea ice probability of occurrence for data in this month for year i

Data are 1-byte values, ranging from 0 to 26, representing variances of 0.0 (least variance, i.e. always or never snow or ice covered) to 0.26 (most variance).

Sample Data Record

Click the thumbnails below for larger images.

Sample image of snow cover and sea ice extent data for one week. See Northern Hemisphere EASE-Grid Weekly Snow Cover and Sea Ice Extent browse animations for other images.

12-month average probability of occurrence for sea ice data.

12-month average probability of occurrence for snow cover data.

12-month average of sea ice data.

Quality Assessment

NSIDC observed and manually inspected every file in this data set, and performed spot checks against the original hand-drawn NOAA/NESDIS snow charts and the original sea ice data.

Please see the Quality Control Summary for more information about quality control procedures at NSIDC.

3.Data Access and Tools

Data Access

Data are available via FTP. Also see Northern Hemisphere EASE-Grid Weekly Snow Cover and Sea Ice Extent browse animations.

Software and Tools

Tools for accessing this data are available via FTP in the pub/DATASETS/snow_and_sea_ice/nsidc0046v03/tools directory. In this directory are C and Fortran source code for conversion from (lat,lon) to (x,y) coordinates and miscellaneous supplementary files, including the Nl EASE-Grid land-ocean-coastline-ice (LOCI) mask and latitude/longitude files for the Nl EASE-Grid.

Related Data Collections

• Digitized NOAA/NESDIS Northern Hemisphere Snow Extent data, revised by D. Robinson, Rutgers University
• DMSP SSM/I Daily and Monthly Polar Gridded Sea Ice Concentrations
• DMSP SSM/I Daily Polar Gridded Brightness Temperatures
• DMSP SSM/I Pathfinder Daily EASE-Grid Brightness Temperatures
• Global Monthly EASE-Grid Snow Water Equivalent Climatology
• Nimbus-7 SMMR Polar Gridded Radiances and Sea Ice Concentrations
• NOAA/NESDIS Weekly Northern Hemisphere Snow Charts
• Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I Passive Microwave Data
• Sea Ice Index

For a list of other related sea ice products, please visit Sea Ice Products at NSIDC. This site offers a complete summary of sea ice data derived from passive microwave sensors and other sources. It is useful for users who want to compare characteristics of various sea ice products to understand their similarities and differences. This site also provides links to tools for passive microwave data and a list of other sea ice resources.

4.Data Acquisition and Processing

Sensor or Instrument Description

Refer to the following for more information:

• SMMR sensor document
• SSM/I sensor document
• DMSP F8 platform document
• DMSP F11 platform document
• DMSP F13 platform document
• Nimbus-7 platform document

Refer to the NOAA Satellite Active Archive for information about the Geostationary Operational Environmental Satellite (GOES).

Data Acquisition Methods

Snow data were derived from digitized versions of manual interpretations of Advanced Very High Resolution Radiometer (AVHRR), GOES, and other visible-band satellite data. Sea ice extent data for Version 3 were derived from the Goddard time series data (through 2003), extended to 2005 using NRTSI. Sea ice extent data for Version 3.1 were derived from the latest available Goddard time series data, extended to 2007 as described below in Data Version History.

Derivation Techniques and Algorithms

The majority of data values were derived via nearest-neighbor interpolation from the respective source grids. Pixels with "QC" data values were determined by more complex algorithms as described in the Quality Control Summary.

Processing Steps

1. Define an EASE-Grid Northern Hemisphere land/ocean/permanent ice mask.
2. Identify source land mask mismatches with the EASE-Grid Northern Hemisphere land mask.
3. Define EASE-Grid sea ice climatology mask.
4. Combine sea ice data for the given week and regrid sea ice from source grid (polar stereographic projection) to EASE-Grid via nearest-neighbor interpolation.
5. Set "one-neighbor" category mismatch pixels to QC sea ice if at least one of the surrounding eight source grid pixels was sea ice.
6. Set "two-neighbor" category mismatch pixels to QC sea ice if at least one of the surrounding twenty-four source grid pixels was sea ice.
7. Set the "others" category mismatch pixels to unclassifiable water.
8. Set all regridded open ocean pixels above 83 degrees north latitude to QC sea ice.
9. Set all regridded sea ice pixels outside the defined sea ice climatology mask area to QC ocean.
10. Regrid digitized snow extent data from source (polar stereographic projection) grid to EASE-Grid via nearest-neighbor interpolation.
11. Set "lower latitude snow neighbor" category mismatch pixels to QC snow if at least one of the adjacent lower latitude source grid pixels was snow.
12. Set "any snow neighbor" category mismatch pixels to QC snow if any of the adjacent source grid pixels were snow.

The EASE-Grid land-ocean-coastline-ice (LOCI) mask was derived from the Boston University version of global 1 km land cover from MODIS 2001, version 4, courtesy of K. Knowles, National Snow and Ice Data Center, Boulder, CO.

NSIDC used American National Standards Institute (ANSI) C and Research Systems' Interactive Data Language (IDL) Version 6.0.3 to develop software for this data set.

Data Version History

Version 3 extended the time series from 2001 to 5 June 2005.

Update 3.1 replaces Version 3 files beginning with 27 December 2003 and extends the time series to 24 June 2007. At the time NSIDC processed the Version 3.1 update, we had received new data in the Goddard time series, extending it beyond 2003. We decided to issue Version 3.1 data using this latest update from Goddard, supplemented by the preliminary data as described in the Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I Passive Microwave Data, and near real-time data as described in the Near Real-Time DMSP SSM/I Daily Polar Gridded Sea Ice Concentrations. See the table below for the specific dates and sea ice sources. This information is also indicated in the metadata record for each file, in metadata/snowice_metadata.v03.1.txt.

This update in the sea ice data results in both v03 and v03.1 files for the period 27 December 2003 to 05 June 2005. Users already making use of the original v03 data would not likely see significant changes in the new data because the processing differences are usually small. See Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I Passive Microwave Data: "Final and Preliminary Data" and "Filling Data Gaps" and Near Real-Time DMSP SSM/I Daily Polar Gridded Sea Ice Concentrations: "Quality Assessment" for details in deciding whether re-analysis may be necessary.

New users of this data should ignore v03 files and use v03.1 files, for the overlap period 27 December 2003 to 05 June 2005.

The sea ice data used for Version 3 and the Version 3.1 update benefits from more extensive quality-control procedures that greatly reduce the number of pixels misclassified as sea ice due to weather effects and/or coastal contamination. The sea ice data used for Version 3 and the Version 3.1 update are also calibrated to provide a consistent long-term time series of sea ice extent, beginning with SMMR and extending through the SSM/I era.

NSIDC believes that these changes warrant replacing the previous versions with Version 3 and Version 3.1 data, if users are interested in the sea ice parameter. If users are only interested in the snow extent parameter, the only changes to the Version 2 data are in pixels at the land/ocean/permanent ice boundaries.

The following table summarizes the cumulative differences between versions of this product.

Limitations of the Data

These data are most appropriate for large area studies. Users should note that source grid snow pixels range in size from approximately 16,000 km² to 42,000 km², and source grid ice concentration pixels range in size from approximately 485 km² to 664 km².

Known Problems with the Data

Pixels with values of "unclassifiable water" are undetermined because of mismatches between the various source and target land masks.

During data set quality control procedures, NSIDC noted a NOAA pixel at the western end of Baffin Island that became snow-free beginning with the week of 31 May to 6 June 1999, and that remained snow-free for the remainder of the current time series (through May 2005). This irregularity is currently under investigation by D. Robinson et al. at Rutgers University. Registered users of this data product will be notified if the investigation warrants a correction to any files.

5.References and Related Publications

Brodzik, M. J. 1997. EASE-Grid: A Versatile Set of Equal-Area Projections and Grids. Boulder, CO, USA: National Snow and Ice Data Center.

Cavalieri, D. J., J. Crawford, M. Drinkwater, W. J. Emery, D. T. Eppler, L. D. Farmer, M. Goodberlet, R. Jentz, A. Milman, C. Morris, R. Onstott, A. Schweiger, R. Shuchman, K. Steffen, C. T. Swift, C. Wackerman, and R. L. Weaver. 1992. NASA sea ice validation program for the DMSP SSM/I: final report. NASA Technical Memorandum 104559. 126 pp.

Cavalieri, D. J., P. Gloersen, and W. J. Campbell. 1984. Determination of sea ice parameters with the NIMBUS-7 SMMR. Journal of Geophysical Research 89(D4):5355-5369.

Dewey, K. F. and R. Heim, Jr. 1981. Satellite observations of variations in Northern Hemisphere seasonal snow cover. NOAA Technical Report NESS 87. 83 pp.

Dewey, K. F. and R. Heim Jr. 1982. New data base: a digital archive of Northern Hemisphere snow cover. Presented at Western Snow Conference, April 19-23. Reno, NV, 9 pp.

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.

Johannessen, O. M., M. W. Miles, E. Bjørgo. 1996. Global sea ice monitoring from microwave satellites. In IGARSS '96. 1996 International Geoscience and Remote Sensing Symposium. Remote Sensing for a Sustainable Future, ed. T. I. Stein, Vol. II: 932-934. Piscataway, NJ: Institute of Electrical and Electronics Engineers, Inc.

Knowles, K. 1993. Points, Pixels, Grids, and Cells: A Mapping and Gridding Primer. Boulder, CO, USA: National Snow and Ice Data Center.

Robinson, D. A., A. Bamzai, and B. Ramsay. 2001. Evaluating Northern Hemisphere snow cover during the satellite era: variations in extent and associations with temperature. Proceedings of the 12th Symposium on Global Change and Climate Variations. Albuquerque, NM, American Meteorological Society, 36-39.

Robinson, D. A. 2000. Weekly Northern Hemisphere snow maps: 1966-1999. 12th Conference on Applied Climatology, Asheville, NC, American Meteorological Society, 12-15.

Robinson, D. A. and A. Frei. 2000. Seasonal variability of northern hemisphere snow extent using visible satellite data. Professional Geographer 51: 307-314.

Robinson, D. A., J. D. Tarpley and B. Ramsay. 1999. Transition from NOAA weekly to daily hemispheric snow charts. Proceedings of the 10th Symposium on Global Change, Dallas, TX, American Meteorological Society, 487-490.

Robinson, D. A., K. F. Dewey, and R. R. Heim, Jr. 1993. Global snow cover monitoring: an update. Bulletin of the American Meteorological Society 74:1689-1696.

Swift, C. T. and D. J. Cavalieri. 1985. Passive microwave remote sensing for sea ice research. EOS 66(49):1210-1212.

6.Document Information

Acronyms and Abbreviations

The following acronyms and abbreviations are used in this document.

Document Creation Date

March 2002

Document Revision Date

September 2007

Document URL

http://nsidc.org/data/docs/daac/nsidc0046_nh_ease_snow_seaice.gd.html