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Near-Real-Time DMSP SSM/I-SSMIS Daily Polar Gridded Sea Ice Concentrations

Summary

This data set provides a near-real-time (NRT) map of sea ice concentrations for both the Northern and Southern Hemispheres. The near-real-time passive microwave brightness temperature data are acquired with the Special Sensor Microwave Imager/Sounder (SSMIS) on board the Defense Meteorological Satellite Program (DMSP) F17 satellite. The SSMIS instrument is the next generation Special Sensor Microwave/Imager (SSM/I) instrument. SSMIS data are received daily from the Comprehensive Large Array-data Stewardship System (CLASS) at the National Oceanic and Atmospheric Administration (NOAA) and are gridded onto a polar stereographic grid. Investigators generate sea ice concentrations from these data using the NASA Team algorithm.

These data are stored as one-byte flat scaled binary arrays with one byte per pixel, at a resolution of 25 km. For each data file, a corresponding PNG image file is provided. These data are available via FTP for the most recent three to six months and are retained on the FTP site until NSIDC's Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I Passive Microwave Data (preliminary data) become available for the same time period.

These data are primarily meant to provide a best estimate of current ice and snow conditions based on information and algorithms available at the time the data are acquired. Near-real-time products are not intended for operational use in assessing sea ice conditions for navigation and should be used with caution in extending the sea ice time series in NSIDC's Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I Passive Microwave Data.

Citing These Data

The following example shows how to cite the use of this data set in a publication. For more information, see our Use and Copyright Web page.

Please use caution if using this data set to extend the sea ice time series in NSIDC's Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I Passive Microwave Data. Any such use should be explicitly stated within any publication, presentation, or other application. The following example shows how to cite the use of this data set in a publication. List the principal investigators, year of data set release, data set title, dates of data used (for example, 01 January to 15 March 2006), publishers (NSIDC), and digital media.

Maslanik, J. and J. Stroeve. 1999, updated daily. Near-Real-Time DMSP SSM/I-SSMIS Daily Polar Gridded Sea Ice Concentrations. [indicate subset used]. Boulder, Colorado USA: NASA DAAC at the National Snow and Ice Data Center.

Overview Table

Category Description
Data format One-byte flat scaled binary (preceded by a 300-byte header); a PNG image is provided with each data file.

Note: The data format information in this document represents the data in its native format as it is archived at NSIDC. If you have downloaded the data using Polaris, please consult the 00README file located in the tar file for information on the data format operations that were performed on this data set.

Spatial coverage and resolution North and south polar regions; 25 km resolution
Temporal coverage and resolution Most recent three to six months; daily
Tools for accessing data Software and tools to display and extract the data are available via FTP.
Grid type and size See Polar Stereographic Projections and Grids. Grid size varies by region:
North: 304 columns x 448 rows
South: 316 columns x 332 rows
Data range 0 to 250 (fractional coverage scaled by 250; percentage scaled by 2.5)
File naming convention nt_yyyymmdd_fxx_nrt_R.bin
File size File size varies by region:
North: 136492 bytes
South: 105212 bytes
Parameter Sea ice concentration
Procedures for obtaining data Data are available via FTP.

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

Investigators

Jim Maslanik
University of Colorado
Boulder, Colorado USA

Julienne Stroeve, Ken Knowles, Walt Meier
National Snow and Ice Data Center (NSIDC)
Boulder, Colorado USA

Donald J. Cavalieri
NASA Goddard Space Flight Center (GSFC)
Greenbelt, Maryland USA

Technical Contact

NSIDC User Services
National Snow and Ice Data Center
CIRES, 449 UCB
University of Colorado
Boulder, CO 80309-0449  USA
phone: +1 303.492.6199
fax: +1 303.492.2468
form: Contact NSIDC User Services
e-mail: nsidc@nsidc.org

2. Detailed Data Description

Note: The data format information in this document represents the data in its native format as it is archived at NSIDC. If you have downloaded the data using Polaris, please consult the 00README file located in the tar file for information on the data format operations that were performed on this data set.

This near-real-time sea ice concentration (NRTSI) data set is created using SSMIS brightness temperatures received from NOAA's CLASS in order to provide the product within one to two days following data acquisition. Prior to the availabliity of SSMIS data from the DMSP-F17 satellite, SSM/I data received from NASA's Marshall Space Flight Center (MSFC) were used to generate sea ice concentrations for this data set. See the Quality Assessment section for more information.

The NRTSI data processing is as close as possible to that used for NSIDC's Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I Passive Microwave Data (SMMR-SSM/I Sea Ice). Table 1 compares some of the major processing features for these two data sets.

Table 1. Major Processing Features of NRTSI and SMMR-SSM/I Sea Ice Data
Processing NRTSI SMMR-SSM/I Sea Ice
Weather filter Same Same
Sea ice algorithm Same Same
Land-to-ocean spillover correction Same Same
Brightness temperature tie points Same Same
Residual weather effects and associated subjective filtering Maximum ice extent climatology Sea surface temperature-based climatology
Missing data Flagged as missing Filled by spatial or temporal interpolation

For more information about processing, see the Data Acquisition and Processing section in the documentation for NSIDC's Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I Passive Microwave Data.

Format

The NRTSI files are stored as flat, scaled binary data with one byte per pixel. The file format consists of a 300-byte descriptive header followed by a two-dimensional array of one-byte values containing the data. For each data file, a corresponding graphics file in PNG format is provided.

For more information about the data format, see the Format section in the documentation for NSIDC's Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I Passive Microwave Data.

File and Directory Structure

The directory structure on the FTP site is shown in Figure 1. These data are in the near-real-time directory; data for the north polar region are in the north directory, and data for the south polar region are in the south directory. Browse images (PNG files) are in the browse directory.


Figure 1. FTP Directory Structure

File Naming Convention

The binary data files and browse image files are named according to the following convention and as described in Table 2:

Binary: nt_YYYYMMDD_fxx_nrt_R.bin
Images: nt_YYYYMMDD_fxx_nrt_R.png

where:

Table 2. File Naming Convention Variable Description
Variable Description
nt Indicates this was created with the NASA Team algorithm
YYYY 4-digit year
MM 2-digit month
DD 2-digit day
fxx Indicates which DMSP satellite the data came from (f13: DMSP-F13; f15: DMSP-F15; f17: DMSP-F17)

Note:
DMSP-F13 and -F15 data should be used for intercomparison purposes only. F13 data are currently retained in the FTP directory. F15 data are available upon request.
nrt Indicates these are near-real-time data
R Region (n: north; s: south)
.bin Identifies this is a binary data file
.png Identifies this is a browse image file (.PNG format)

For example, nt_20090115_f17_nrt_n.bin holds data for the north polar region on 15 January 2009 from DMSP-F17.

File Size

File size varies by region:
North: 136492 bytes
South: 105212 bytes

Spatial Coverage

North and south polar regions (defined by the Polar Stereographic Projections and Grids spatial coverage map).

Spatial Resolution

The spatial resolution for all channels is 25 km.

Projection

Data are in a polar stereographic projection. For more information on this projection, see the Polar Stereographic Projections and Grids Web page.

Grid Description

Grid size varies by region:
North: 304 columns x 448 rows
South: 316 columns x 332 rows

Temporal Coverage

The data are available for the most recent three to six months.

Temporal Resolution

The temporal resolution is daily.

Parameter or Variable

For more information, see the Parameter or Variable section in the documentation for NSIDC's Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I Passive Microwave Data.

Parameter Description

Sea ice concentration represents an areal coverage of sea ice. For a given grid cell, the parameter provides an estimate of the fractional amount of sea ice covering that cell, with the remainder of the area consisting of open ocean. Land areas are coded with a land mask value.

Parameter Range

The sea ice concentration floating-point values (fractional coverage ranging from 0.0 to 1.0) are multiplied by a scaling factor of 250. To convert to the fractional range of 0.0 to 1.0, divide the scaled data in the file by 250. To convert to percentage values, divide the scaled data in the file by 2.5. Data files may contain integers from 0 to 255, as described in the Table 3.

Table 3. Data Values and their Meaning
Data Value Meaning
0 - 250 Sea ice concentration (fractional coverage scaled by 250)
251 Circular mask used in the Arctic to cover the irregularly-shaped data gap around the pole (caused by the orbit inclination and instrument swath)
252 Unused
253 Coastlines
254 Superimposed land mask
255 Missing data

Quality Assessment

This NRTSI data set is created using brightness temperature data in order to provide the product within one to two days following data acquisition. These brightness temperatures do not receive the same level of quality control as the DMSP SSM/I-SSMIS Daily Polar Gridded Brightness Temperatures product created at NSIDC.

Additionally, these NRTSI data may:

3. Data Access and Tools

Data Access

Data are available via FTP.

Software and Tools

Software tools are available via direct FTP or via the Polar Stereographic Data Tools Web page.

Software and tools for reading and displaying the files are located in the tools directory on the FTP site. Software includes IDL and Matlab routines to ingest and read sea ice concentration data. Masks and overlays are also provided.

Tools

Table4 lists the tools that can be used with this data set. For a comprehensive list of all polar stereographic tools and for more information, see the Polar Stereographic Data Tools Web page.

Table 4. Tools for this Data Set
Tool Type Tool File Name(s) or Description
Data Extraction extract_ice.pro
Data Display dataviewer.tar.gz
Geocoordinate dataviewer.tar.gz
locate.for
mapll.for and mapxy.for
psn25lats_v3.dat and pss25lats_v3.dat
psn25lons_v3.dat and pss25lons_v3.dat
Pixel-Area psn25area_v3.dat and pss25area_v3.dat
Land Masks gsfc_25n.msk and gsfc_25s.msk
coast_25n.msk and coast_25s.msk
ltln_25n.msk and ltln_25s.msk
pole_n.msk
Region Masks region_n.msk and region_s.msk
Ocean Masks Includes monthly ocean masks and maximum extent masks for the Northern (n) and Southern (s) Hemispheres

Related Data Collections

Please see Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I Passive Microwave Data for general information about these data and the data processing.

Please see Near-Real-Time SSM/I-SSMIS EASE-Grid Daily Global Ice Concentration and Snow Extent for more information about how the near-real-time data were acquired and processed.

4. Data Acquisition and Processing

Near-real-time passive microwave brightness temperatures from the DMSP-F17 SSMIS instrument are received daily from CLASS and gridded onto the SSM/I polar stereographic grid. NSIDC generates sea ice concentrations from these brightness temperature data using the NASA Team Sea Ice Algorithm. These data are updated daily and are retained on the FTP site until the Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I Passive Microwave Data (preliminary data) become available for the same time period. Roughly the most recent three to six months of near-real-time sea ice concentration data are available.

The DMSP-F13 satellite that has been central to our sea ice products for the past several years is nearing the end of its mission and is no longer a reliable resource for our sea ice products. As is standard data practice, we have transitioned to a newer sensor. As of 02 June 2009, NSIDC has switched its SSM/I processing stream from the DMSP-F13 satellite to SSMIS data from the DMSP-F17 satellite. For data continuity, F17 data have been acquired and processed back to 01 April 2008.

One year earlier, on 02 June 2008, NSIDC switched its SSM/I processing stream from the DMSP-F13 satellite to the DMSP-F15 satellite due to a failing recorder on F13. For continuity, F15 data were acquired and processed from 01 January 2008 until 25 February 2009. On 16 February 2009, however, NSIDC switched its SSM/I processing stream back to the DMSP-F13 satellite due to an issue with the DMSP-F15 SSM/I 22 GHz frequency brightness temperature fields. NSIDC continued to produce the F13 products until further degradation of the SSM/I instrument on 11 May 2009. Processed F13 data are available for the 01 July 2008 through 11 May 2009 time period.

However, although you may choose to use data from either the F13 or F15 satellites, NSIDC recommends using these data for intercomparison purposes only. SSMIS data from the DMSP-F17 satellite is the current near-real-time product. Regarding the F13 and F15 data, you should be aware of the following differences in the satellite data:

To correct for the sensor differences, NSIDC conducted preliminary inter-calibration between F13 and F15 using an overlap period of 01 January 2007 to 31 December 2007. These data should retain reasonable consistency, though differences of approximately 50,000 sq km may be possible in daily total extents. For the latest transition to F17 SSMIS data, NSIDC has conducted preliminary inter-calibration between F13 and F17 using an overlap period of 01 April 2008 to 31 March 2009. A direct intercomparison of the year of daily sea ice extent (F13-F17) yielded an average daily difference of approximately 2800 sq km with an average magnitude of difference of approximately 28,000 sq km.

The near-real-time data are intended to facilitate time-sensitive research dependent upon precise detection of seasonal polar sea ice formation and break up. These data are primarily meant to provide a best estimate of current ice and snow conditions based on information and algorithms available at the time the data are acquired. Near-real-time products are not intended for operational use in assessing sea ice conditions for navigation and should be used with caution in extending the sea ice time series. For historical SMMR and SSM/I sea ice concentration data, please see Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I Passive Microwave Data.

For more information, see the Data Acquisition and Processing section in the documentation for the Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I Passive Microwave Data.

Sensor or Instrument Description

For a detailed description of the instruments, refer to the Special Sensor Microwave Imager (SSM/I) and Special Sensor Microwave Imager/Sounder (SSMIS) Web pages. For detailed platform descriptions, refer to the DMSP Satellite F13, F15, and F17 Web pages.

5. References and Related Publications

References include:

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.

Cavalieri, D. J., C. I. Parkinson, P. Gloersen, and H. J. Zwally. 1997. Arctic and Antarctic Sea Ice Concentrations from Multichannel Passive-Microwave Satellite Data Sets: October 1978 to December 1996, User's Guide. NASA Technical Memorandum 104647. 17 pages.

Table 5 lists related documents available on NSIDC's Web site.

Table 5. Related Documents
Document Description URL
Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I Passive Microwave Data General information about these data, details about the data processing, and a complete list of references. http://nsidc.org/data/docs/daac/nsidc0051_gsfc_seaice.gd.html
Defense Meteorological Satellite Program (DMSP) Satellite F13 Platform description http://nsidc.org/data/docs/daac/f13_platform.gd.html
Defense Meteorological Satellite Program (DMSP) Satellite F15 Platform description http://nsidc.org/data/docs/daac/f15_platform.gd.html
Defense Meteorological Satellite Program (DMSP) Satellite F17 Platform description http://nsidc.org/data/docs/daac/f17_platform.gd.html
Special Sensor Microwave Imager (SSM/I) Instrument description http://nsidc.org/data/docs/daac/ssmi_instrument.gd.html
NASA Team Sea Ice Algorithm Algorithm description http://nsidc.org/data/docs/daac/nasateam/index.html
Polar Stereographic Projections and Grids Grid definition and details http://nsidc.org/data/grids/ps_grid.html
Near-Real-Time SSM/I EASE-Grid Daily Global Ice Concentration and Snow Extent Information about how the near-real-time data were acquired and processed. http://nsidc.org//data/nise1.html

6. Document Information

Acronyms

Table 6 lists acronyms used in this document.

Table 6. Acronyms
ANSI American National Standards Institute
ASCII American Standard Code for Information Interchange
DMSP Defense Meteorological Satellite Program
FTP File Transfer Protocol
GHRC Global Hydrology Resource Center
GSFC Goddard Space Flight Center
MSFC Marshall Space Flight Center
NASA National Aeronautics and Space Administration
NRT Near-Real-Time
NRTSI Near-Real-Time Sea Ice concentration
NSIDC National Snow and Ice Data Center
PNG Portable Network Graphics
SMMR Scanning Multichannel Microwave Radiometer
SSM/I Special Sensor Microwave/Imager
SSMIS Special Sensor Microwave Imager/Sounder
URL Uniform Resource Locator

Document Revision Date

June 2009
January 2009
May 2008
April 2001
October 2006

Document URL

http://nsidc.org/data/docs/daac/nsidc0081_ssmi_nrt_seaice.gd.html