Data Set ID: 
NSIDC-0079

Bootstrap Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I-SSMIS, Version 3

This sea ice concentration data set was derived using measurements from the Scanning Multichannel Microwave Radiometer (SMMR) on the Nimbus-7 satellite and from the Special Sensor Microwave/Imager (SSM/I) sensors on the Defense Meteorological Satellite Program's (DMSP) -F8, -F11, and -F13 satellites. Measurements from the Special Sensor Microwave Imager/Sounder (SSMIS) aboard DMSP-F17 are also included. The data set has been generated using the Advanced Microwave Scanning Radiometer - Earth Observing System (AMSR-E) Bootstrap Algorithm with daily varying tie-points. Daily (every other day prior to July 1987) and monthly data are available for both the north and south polar regions. Data are gridded on the SSM/I polar stereographic grid (25 x 25 km) and provided in two-byte integer format.

Version Summary: 

Changes to this version include:

  • Intercalbration techniques between SMMR and F08, as well as F08 and F11 were changed to match sea ice area rather than sea ice extent.
  • Ocean tie points now change each day (similar to the ice tie points) based on brightness temperatures for that day.
  • The threshold for the lower limit for ice was relaxed to allow retrieval of ice at 10 percent ice concentration.

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    Parameter(s):
    • Sea Ice > Sea Ice Concentration
    Spatial Coverage:
    • N: 90, S: 30, E: 180, W: -180

    • N: -39, S: -90, E: 180, W: -180

    Spatial Resolution:
    • 25 km x 25 km
    Temporal Coverage:
    • 26 October 1978 to 31 December 2017
    Temporal Resolution: 1 day, 1 month
    Data Format(s):
    • Binary
    Platform(s) DMSP 5D-2/F11, DMSP 5D-2/F13, DMSP 5D-2/F8, DMSP 5D-3/F17, Nimbus-7
    Sensor(s): SMMR, SSM/I, SSMIS
    Version: V3
    Data Contributor(s): Josephino 'Joey' Comiso

    Data Citation

    As a condition of using these data, you must cite the use of this data set using the following citation. For more information, see our Use and Copyright Web page.

    Comiso, J. C. 2017. Bootstrap Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I-SSMIS, Version 3. [Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: https://doi.org/10.5067/7Q8HCCWS4I0R. [Date Accessed].

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    Detailed Data Description

    This sea ice concentration data set was derived using measurements from the Scanning Multichannel Microwave Radiometer (SMMR) on the Nimbus-7 satellite and from the Special Sensor Microwave/Imager (SSM/I) sensors on the Defense Meteorological Satellite Program's (DMSP) -F8, -F11, and -F13 satellites. Measurements from the Special Sensor Microwave Imager/Sounder (SSMIS) aboard DMSP-F17 are also included. The data set has been generated using the Advanced Microwave Scanning Radiometer - Earth Observing System (AMSR-E) Bootstrap Algorithm with daily varying tie-points. Daily (every other day prior to July 1987) and monthly data are available for both the north and south polar regions. Data are gridded on the SSM/I polar stereographic grid (25 x 25 km) and provided in two-byte integer format.

    Users should acquire the entire Version 3 data set in order to update their time series. For more information, refer to the Version History section of this document.
    Format

    Data files are stored in the original Goddard Space Flight Center (GSFC) flat binary two-byte integer format, use the little-endian byte order convention, and are scaled by a factor of 10. An individual image of daily or monthly averaged data constitutes a granule in the Bootstrap sea ice concentration time series.

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    File and Directory Structure

    Data are available on the HTTPS site in the https://daacdata.apps.nsidc.org/pub/DATASETS/nsidc0079_gsfc_bootstrap_seaice_v3/ directory. Within this directory there is one main folder, final-gsfc and within the main folder there are two subfolders, north and south. Within the subfolders there is a daily folder and monthly folder. The daily folders are further subdivided into yearly folders. Monthly files are located in the monthly directory with no further subdivision. Figure 1 shows the directory structure.

    Figure 1. File Organization
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    File Naming Convention

    For this Version 3.1 data set, file names were changed from v03 to v3.1, such as bt_20060901_f13_v3.1_n.bin.

    Daily Files

    This section explains the daily file naming convention used for this product with an example.

    Example File Name: bt_20060901_f13_v3.1_n.bin

    bt_YYYYMMDD_SSS_vV.V_H.bin

    Refer to Table 1 for the valid values for the file name variables listed above.

    Where:

    Variable Description
    Table 1. Daily Files Naming Convention
    bt indicates that this file was created using the bootstrap algorithm
    YYYY 4-digit year
    MM 2-digit month
    DD 2-digit day
    SSS 3-character sensor name: 
    n07 for Nimbus-7 SMMR
    f08, f11, or f13 for DMSP SSM/I sensors
    f17 for the DMSP SSMIS sensor
    vV.V 2-digit version number (for example, v3.1)
    H Hemisphere:
    for Northern
    s for Southern
    .bin indicates that this file is in binary format

    Monthly Files

    This section explains the monthly file naming convention used for this product with an example.

    Example File Name: bt_200609_f13_v3.1_n.bin

    bt_YYYYMM_SSS_vV.V_H.bin

    Refer to Table 2 for the valid values for the file name variables listed above.

    Where:

    Variable Description
    Table 2. Monthly Files Naming Convention
    bt indicates that this file was created using the bootstrap algorithm
    YYYY  4-digit year
    MM 2-digit month
    SSS 3-character sensor name (n07 for Nimbus-7 SMMR; f08, f11, or f13 for DMSP SSM/I sensors; f17 for the DMSP SSMIS sensor
    vV.V 2-digit version number (for example, v3.1)
    H Hemisphere (for Northern, for Southern)
    .bin indicates that this file is in binary format
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    File Size

    Northern Hemisphere files are approximately 266 KB. Southern Hemisphere files are approximately 205 KB.

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    Volume

    The entire data set is approximately 7.6 GB.

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    Spatial Coverage

    Data set coverage includes the northern and southern polar regions. For a detailed description, see the SSM/I Polar Spatial Coverage Maps section of the Polar Stereographic Projections and Grids website.

    SSM/I and SSMIS instrument coverage is global, except for circular sectors centered over the North Pole, 311 km in radius, located poleward of 87.2°. These sectors are never measured due to orbit inclination. The 50° scan pattern provided a swath width of 780 km at the Earth's surface. The spatial resolutions at the various frequencies ranged from approximately 27 km at 37 GHz to 148 km at 6.6 GHz. For more details, please refer to the SMMR, SSM/I, and SSMIS Sensors page. The measurement footprint size or effective field of view (FOV) varies by frequency, as shown in the following table.

    Table 3. SSM/I-SSMIS
    Frequency Footprint Size
    19.3 GHz 70 x 45 km
    22.2 GHz 60 x 40 km
    37.0 GHz> 38 x 30 km

    SMMR instrument coverage is global, except for circular sectors centered over the North Pole, approximately 611 km in radius, located poleward of 84.5°. These sectors are never measured due to orbit inclination. The 50° scan pattern provided a swath width of 780 km at the Earth's surface. The spatial resolutions at the various frequencies ranged from approximately 27 km at 37 GHz to 148 km at 6.6 GHz. For more details, please refer to the SMMR, SSM/I, and SSMIS Sensors page. The measurement footprint size varies by frequency, as shown in the following table.

    Table 4. SMMR
    Frequency Footprint Size
    6.6 GHz 148 x 95 km
    10.7 GHz 91 x 59 km
    18.0 GHz 55 x 41 km
    21.0 GHz 46 x 30 km
    37.0 GHz 27 x 18 km
     

    Spatial Resolution

    Sea ice concentrations are provided at a resolution of 25 km.

    Projection and Grid Description

    For more information, see Polar Stereographic Projections and Grids. The grid size varies depending on the region, as shown in the following table.

    Table 5. Grid Size
    Region Columns Rows
    North 304 448
    South 316 332
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    Temporal Coverage

    Bootstrap sea ice concentration data are available for 26 October 1978 through 31 December 2017.

    Temporal Resolution

    SMMR data were collected every other day. (The scanner operated only on alternate days, due to spacecraft power limitations). Major data gaps occurred in August 1982 (4, 8, and 16 August) and 1984 (13 through 23 August) for both polar regions. Monthly means are generated by averaging all available files for each individual month, excluding pixels of missing data. Data were interpolated where missing pixels were present so that no areas of missing data remained.

    SSM/I and SSMIS data are collected daily. Ice concentrations are provided for each day of data and as monthly means. Monthly mean files are generated by averaging all available daily files for each individual month, excluding pixels of missing data. A major data gap in the SSM/I data occurs from 03 December 1987 to 13 January 1988.

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    Parameter or Variable

    The data set consists of sea ice concentration derived from gridded brightness temperatures. Sea ice concentrations range from 0 to 100 percent. Please see the Parameter Range section for more details

    Parameter Range

    Data are stored as two-byte integers representing sea ice concentration values. The sea ice concentration data values are packed into integer format by multiplying the original sea ice concentration values by 10 (divide stored value by 10 to get the percent). The sea ice concentration values range from 0 to 1000. Land is registered as 1200 and the Northern Hemisphere hole (a region of the North Pole that is not measured due to orbit inclination) is registered as 1100. Refer to the Spatial Coverage section.

    Sample Data Record

    The following is a sample data record from the Northern Hemisphere monthly data.

     837  887  924 971  971 992 980 975 974 964 980 965 936 972 968 986
     982  987  978 984  988 965 980 972 981 985 992 988 989 983 993 986
     985  984  982 983  982 981 974 990 982 983 978 977 983 987 981 992
     985  979  978 982  981 969 966 972 977 984 973 976 980 970 972 977
     980  984  980 982  977 973 973 969 968 972 980 951 887 877 928 936
    1200 1200 1200 969 1200 951 956 980 980 980 975 982 982 961 937 863
     791  710  706 706  678

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    Software and Tools

    Fortran tools are provided which extract the sea ice concentration files and geolocate the data. Sample Fortran code is provided for reading and displaying these sea ice concentration data.

    Sample Interactive Data Language (IDL) commands to read and display sea ice grids and latitude/longitude grids are provided via the IDL commands text file. In addition, IDL tools to extract the sea ice concentration files and geolocate the data, as well as masking tools that limit the influence of non-sea ice brightness temperatures, are provided.

    Table 6 lists the tools that can be used with this data set. For a comprehensive list of all polar stereographic tools, see the Polar Stereographic Data Tools web page.

    Table 6. Tools for this Data Set
    Tool Type Tool File Name(s)
    Data Extraction extract_ice.pro
    Geocoordinate 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
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    Data Acquisition and Processing

    Satellite passive microwave sensors provide global radiance measurements that may be used to map, monitor, and study Arctic and Antarctic polar sea ice. The microwave data used for this sea ice concentration data set spans over 25 years, starting with the launch of the SMMR instrument on NASA's Nimbus-7 in 1978 and continuing with the DMSP SSM/I series beginning in 1987.

    Sea ice concentrations are generated using brightness temperatures derived from SMMR and SSM/I radiances. The goal in the creation of the data set is to produce a long-term, consistent data set in which sea ice extent and area differences between the sensors are reduced that could serve as a baseline for future measurements. The Bootstrap data set provides improved consistency between sensors and enhanced removal of weather and land contamination.

    The Bootstrap data set includes gridded daily and monthly averaged sea ice concentrations (every-other-day for the SMMR data) for both the north and south polar regions beginning 26 October 1978 through the most current processing.

    Algorithms designed to estimate sea ice concentration using passive microwave data frequently do not work well over open ocean. When applied, instead of predicting the expected zero value, the algorithms predict sea ice concentration values as high as 35 percent, particularly in regions with excessive winds and stormy weather. A climatological sea surface temperature mask was applied to remove pixels from regions where the ocean surface is above freezing. Also, land contamination (false ice along the coast due to pixels containing a mixture of land and ocean) were removed using a filter adapted from Cho et al. (1996). Even with these quality-control measures, some residual sea ice concentrations remain in the open ocean and along the coast.

    Theory of Measurements

    The SMMR and SSM/I instruments are dual-polarized and multi-frequency microwave radiometers that sense emitted microwave radiation at an incidence angle of about 53° and an altitude of about 800 km. At microwave frequencies, the observed radiance varies linearly with the temperature of the emitting material, according to the Rayleigh-Jeans law, and is usually expressed in terms of brightness temperature. The constant of proportionality in the linear relationship is the emissivity, which provides information about the electrical and physical properties of the material itself. The observed brightness temperatures come primarily from the Earth's surface with some contributions from the atmosphere and outer space. Depending on frequency and polarization, the brightness temperature is affected by changing surface, subsurface, and atmospheric conditions. The multichannel capability allows for the discrimination of different surfaces and atmospheric effects and allows for the development of algorithms that provide a range of geophysical information about the surface.

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    Data Acquisition Methods

    Four sets of satellite data are used to create the Bootstrap sea ice data stream:

    • Nimbus-7 SMMR, data range: 01 November 1978 through 31 July 1987
    • DMSP-F08 SSM/I, data range: 01 August 1987 through 17 December 1991
    • DMSP-F11 SSM/I, data range: 18 December 1991 through 09 May 1995
    • DMSP-F13 SSM/I, data range: 10 May 1995 through 31 December 2007
    • DMSP-F17 SSMIS, data range: 01 January 2008 through the most current processing

    SMMR

    Sea ice concentrations were processed by GSFC using SMMR brightness temperatures. The SMMR brightness temperatures were processed and quality checked at GSFC (Gloersen et al. 1992).

    SSM/I-SSMIS

    Bootstrap sea ice concentrations were processed at GSFC using DMSP SSM/I-SSMIS Daily Polar Gridded Brightness Temperatures from NSIDC. Processing of DMSP-F17 brightness temperatures is ongoing. For more information on the brightness temperatures, see the DMSP SSM/I-SSMIS Daily Polar Gridded Brightness Temperatures documentation.

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    Derivation Techniques and Algorithms

    Sea ice concentrations for this data set were derived from a revised Bootstrap algorithm which uses a different set of tie-points and adjustments. For algorithm background, please see the Enhanced Sea Ice Concentrations from Passive Microwave Data document (Comiso and Nishio 2008). For changes made to this Version 3 data set, see the Positive Trend in the Antarctic Sea Ice Cover and Associated Changes in Surface Temperature document (Comiso, et al. 2017).

    Processing Steps

    Interpolation

    Data were interpolated in areas with missing pixels according to the following steps. First, data were spatially interpolated only for isolated empty pixels. An empty pixel was replaced by the average of four good surrounding pixels, or if four good pixels were not available, then a smaller number of pixels was selected. Second, a time interpolation was applied to the spatially interpolated map. Time interpolation was based on a weighting scheme; the closer the good data were in time, the higher the weighted value. For each empty pixel, the algorithm searched forward in time for a good pixel, and backward in time for a second good pixel. The algorithm determined how many days ahead and behind the two good pixels occurred and calculated the weight of each pixel. A weighted average of the two good pixels was then calculated, and the result was used for the empty pixel. During the SSM/I period, most temporal interpolations were conducted using only adjacent days. During the SMMR period, particularly in 1986 when larger gaps were present, temporal interpolations had separations of more days.

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    Version History

    Table 7 outlines the processing and algorithm history for this product.

    Table 7. Description of Version Changes
    Version Date Description
    V3.1 July 2018 Version 3.1 of the Bootstrap ice concentrations computes sea ice concentration with an open water identifier that is calculated daily in May and October; in version 3.0, this value was calculated using a single monthly value for May and a single monthly value for October.
    V3 November 2017 Changes to this version include:
    • Intercalibration techniques between SMMR and F08, as well as F08 and F11, were changed to match sea ice area rather than sea ice extent
    • Ocean tie points now change each day (similar to the ice tie points) based on brightness temperatures for that day
    • The threshold for the lower limit for ice was relaxed to allow retrieval of ice at 10 percent ice concentration
    V2 September 2007 Changes to this version include:
    • Adjusted tie points to be consistent with the AMSR-E Bootstrap algorithm
    • Adjusted Southern Hemisphere land mask to account for changes in extent and positions of ice shelves
    • Reprocessed entire SMMR-SSM/I time series
    Due to the updates outlined in this version summary, we ask that you acquire the entire Version 3 data set in order to update your time series.
    V1 January 1996 Original version of data. For information regarding Version 1 data processing, see the Bootstrap Sea Ice Concentrations Version 1 Processing Steps web page.
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    Quality Assessment
    A large fraction of the text in this document was supplied by the data provider. It may include qualitative judgments and statements by the provider and does not necessarily represent endorsements or imply that independent assessments have been performed by NSIDC staff.

    Assessment of Version Differences

    For Version 3.1 data, the differences in total Arctic sea ice area and sea ice extent are usually small (less than ± 0.5%), except in the second half of May when daily sea ice areas are often 0.5-1.5% higher and extents are 1-3% higher in V3.1 than in V3.0.

    The staff at the NSIDC DAAC performed an evaluation between Version 2 and Version 3 data to verify any difference between the two versions. Here are their findings:

    • The changes introduced in Version 3 have some effect on the total ice covered area and extent.  These differences have some seasonality and appear to be largely due to the use of dynamic open water tie points.
    • The majority of pixels show no change in concentration value or less than one percent, although a small fraction have larger differences.  Roughly two percent of pixels show concentration changes of more than 10 percent.

    Although most bad data are easily identifiable and removed with little ambiguity, human judgments must be made and applied consistently to the time series data. While efforts were made to minimize errors in judgment and inconsistencies, the resulting data set is not perfect. Nevertheless, the data provider feels that these maps are suitable for some types of research studies, such as basin-scale trend analysis, model inputs, and some regional studies. They are less useful in evaluating conditions on a specific day in a localized region.

    NSIDC staff visually checked the entire set of data files, including file structure, comparisons to existing SMMR- and SSM/I-derived sea ice concentration grids, and information files and examination of data quality.

    Errors and Limitations

    Sensitivity analysis of potential errors associated with the retrieval of sea ice products were discussed previously. For example, see Comiso et al. 1997 and other references associated with the NASA Team Algorithm sea ice products. Sea ice concentration errors are difficult to quantify because sea ice is an evolving ice cover and does not have uniform physical and radiative characteristics throughout the ice pack. The emissivity of sea ice is known to change as the ice cover develops from grease ice to nilas, pancake to young ice, or first year ice to multiyear ice. For definitions of these types of ice, see the NSIDC glossary page. The line AD in the scatter plot that provides the sea ice tie-point in the Bootstrap Algorithm corresponds to 100 percent ice for the relatively thick ice types such as first year or multiyear ice. Areas with 100 percent of thinner ice types are often estimated to have ice concentrations as low as 80 percent due to lower emissivities (Comiso et al. 1992). In the summer, retrievals in the Arctic could have biases caused by meltponding effects (Comiso and Kwok 1996).

    Inside the ice pack, observations from helicopter and aircraft flights typically show very high sea ice concentrations with some 5 to 10 percent of the leads covered by nilas or pancake ice. The frequency distribution of sea ice concentration in highly consolidated ice areas has a standard deviation of about 3 percent, which includes sea ice cover with concentrations of less than 100 percent. Overall, retrieval accuracy is estimated to be approximately 5 to 10 percent except in some unusual cases, such as the presence of a large fraction of thin ice or meltponding within the pixel and the presence of stormy weather conditions, especially near the ice edge. Limited comparative analysis with high resolution instruments and other measurements confirm this, but more extensive validation in a greater number of places over all of the seasons would provide more accurate error assessments.

    For information describing the rough error/differences between each sensor (F8, F11, F13, F17) during the sensor overlap time period, refer to the following documentation: Comiso et al. 2008, Comiso et al. 2009, and Comiso et al. 2010.

    Basic limitations arise from the sensor resolution, temporal coverage, and algorithm assumptions and characteristics. Users should review the information provided on fields of view, temporal sampling, and algorithm characteristics.

    Particular care is needed in interpreting the sea ice concentrations during summer when melt is underway and in regions where new sea ice makes up a substantial part of the sea ice cover. As noted, some residual errors remain due to sensor differences and to weather effects along with mixing of ocean and land area within the sensor field of view.

    No data coverage is available for regions poleward of 84.5° N for SMMR and 87.2° N for SSM/I or SSMIS due to the inclination of the orbits. SMMR data were acquired every other day, while SSM/I data were acquired daily. SSMIS data are also acquired daily.

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    Sensor/Platform Description

    Please refer to the SMMR, SSM/I, and SSMIS Sensors page for details regarding the following sensors and platforms: 

    Sensors

    Scanning Multi-channel Microwave Radiometer (SMMR)
    Special Sensor Microwave Imager (SSM/I)
    Special Sensor Microwave Imager (SSMIS)

    Platforms

    Nimbus 7 Spacecraft System
    Defense Meteorological Satellite Program (DMSP) Satellite F8
    Defense Meteorological Satellite Program (DMSP) Satellite F11
    Defense Meteorological Satellite Program (DMSP) Satellite F13

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    References and Related Publications

    Contacts and Acknowledgments

    Josefino C. Comiso
    Oceans and Ice Branch, Code 971
    NASA Goddard Space Flight Center
    Greenbelt, MD 20771 USA

    Document Information

    DOCUMENT CREATION DATE

    July 2018

    DOCUMENT REVISION DATE

    N/A

    No technical references available for this data set.
    No FAQs or How Tos available for this data set.

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