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AMSR-E/Aqua Daily Gridded Brightness Temperatures

Summary

This document pertains to two data sets: AMSR-E/Aqua Daily EASE-Grid Brightness Temperatures (NSIDC-0301) and AMSR-E/Aqua Daily Global Quarter-Degree Gridded Brightness Temperatures (NISDC-0302).

The Advanced Microwave Scanning Radiometer - Earth Observing System (AMSR-E) instrument on the NASA EOS Aqua satellite provides global passive microwave measurements of the Earth. NSIDC produces AMSR-E gridded brightness temperature data by interpolating AMSR-E data (6.9 GHz, 10.7 GHz, 18.7 GHz, 23.8 GHz, 36.5 GHz, and 89.0 GHz) to the output grids from swath space using an inverse-distance squared method. AMSR-E/Aqua L2A Global Swath Spatially-Resampled Brightness Temperatures (AE_L2A) input source data are used.

These data are provided in three EASE-Grid projections (north and south Lambert azimuthal and global cylindrical) at 25 km resolution, and in one global cylindrical, equidistant latitude-longitude projection at 0.25° (quarter-degree) resolution. For both data sets, spatial coverage is global, data are daily, coverage begins 19 June 2002, and processing is ongoing.

Data are provided via FTP as they become available. Brightness temperature data (in tenths of kelvins) are two-byte unsigned integers and time files are two-byte signed integers.

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.

The following examples show how to cite the use of these data sets in a publication. The citation to use depends on which data set you use. List the principal investigators, year of data set release, data set title and version (if applicable), dates of data you used (for example, 01 March to 30 June 2004), publisher information (NSIDC), and indicate digital media distribution.

Knowles, K. W., M. H. Savoie, R. L. Armstrong, and M. J. Brodzik. 2006, updated current year. AMSR-E/Aqua daily EASE-Grid brightness temperatures, [list dates of data used]. Boulder, Colorado USA: National Snow and Ice Data Center. Digital media.

Knowles, K. W., M. H. Savoie, R. L. Armstrong, and M. J. Brodzik. 2006, updated current year. AMSR-E/Aqua daily global quarter-degree gridded brightness temperatures, [list dates of data used]. 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

Investigators

Ken Knowles, Matthew Savoie, Richard Armstrong, and Mary Jo Brodzik
National Snow and Ice Data Center (NSIDC)
Cooperative Institute for Research in Environmental Sciences (CIRES)
University of Colorado
Boulder, Colorado USA

Technical Contact

Acknowledgements

We thank the members of the product team at the NSIDC DAAC and Steven Chan of the Jet Propulsion Laboratory for his comments and review.

2. Detailed Data Description

The AMSR-E/Aqua Daily EASE-Grid Brightness Temperatures (T_b)consist of gridded data in one of three projections (all with 25 km resolution): the Northern Hemisphere EASE-Grid, Southern Hemisphere EASE-Grid, and full global EASE-Grid. The AMSR-E/Aqua Daily Global Quarter-Degree Gridded Brightness Temperatures are gridded to one global cylindrical, equidistant latitude-longitude projection at 0.25° (quarter-degree) resolution.

These data are gridded from swath format using an inverse-distance squared interpolation. The source data are Level-2A swath brightness temperatures from the AMSR-E/Aqua L2A Global Swath Spatially-Resampled Brightness Temperatures (AE_L2A) data set.

Format

Files contain flat binary (unformatted) grid arrays, one grid per file. Files are compressed for delivery.

There are 24 brightness temperature data files per day for a given projection: separate data files for each of the 12 channels and the two pass directions per channel. Data are 2-byte unsigned integers, little-endian byte-order, representing temperatures in tenths of kelvins. Data values range from 650 to 3200, with the value 0 indicating missing data.

There are two time files per day for a given projection: ascending and descending passes. Data are 2-byte signed integers, little-endian byte-order, indicating time of data acquisition as minutes since midnight (0:00 UTC) of the date of the enclosing file. The values in the time files range from 0 to 1440, with the value -32768 indicating missing data.

Geolocation files containing latitude and longitude coordinates are also available. Please see the Software and Tools section for more information.

File and Directory Structure

Data files are organized on the FTP site according to projection type, instrument (AMSR-E), specific grid, and year of data. The directory structure on the FTP site is illustrated in Figure 1. In this illustration, the year directories are representative placeholders; on the FTP site, there are multiple such directories, each named for a 4-digit year, such as 2002 or 2006.

Figure 1. FTP Directory Structure

File Naming Convention

Files are compressed (zipped) for delivery, with a .gz extension added to the file names described below.

EASE-Grid Files

The EASE-Grid brightness temperature data files are named according to the following convention and as described in Table 1:

ID2rx-AMSRE-aayyyydddp.vnn.ccc

where:

For example, ID2r3-AMSRE-SL2005135D.v01.89V.

The EASE-Grid time files follow the same naming convention, except they end in the extension .TIM instead of the channel indicator. For example, ID2r3-AMSRE-SL2005135D.v01.TIM

Quarter Degree Files

The quarter degree brightness temperature data files are named according to the following convention and as described in Table 2.

ID2r1-AMSRE-D.25yyyydddp.vnn.ccc

where:

For example, ID2r1-AMSRE-D.252005135D.v01.89V.

Time files follow the same naming convention, except they end with the extension .TIM instead of the channel indicator. For example, ID2r1-AMSRE-D.252005135D.v01.TIM.

Version Numbers

The version number in the file names (vnn) indicates the version of the gridded brightness temperature data set, as described in Table 3. This version number also reflects the level of AMSR-E/Aqua L2A Global Swath Spatially-Resampled Brightness Temperatures (AE_L2A) source data used.

Users should always work with the latest (highest version number) gridded brightness temperature data available for a given date.

When a new version becomes available, any previously-released data (lower version numbers) will be reprocessed to the latest version (highest version number). The new data are staged to the FTP site as they become available, replacing the earlier version data for the same date. For example, as v03 north and south azimuthal EASE-Grid data become available on the FTP site, they replace the previously available v01 or v02 data for the same date. (Note: The global cylindrical EASE-Grid and global quarter-degree gridded brightness temperature data were initially released as version v03.)

Please see AMSR-E Data Versions for more information about the AE_L2A source data versions.

Resolution Number

The rx resolution number in the file name indicates the resolution number (resolution 1 or resolution 3) of the brightness temperatures extracted from the AE_L2A source data. See the Processing Steps section for more information about this resolution number.

File Size

File size varies by projection, as shown in Table 4. The sizes shown are approximations for uncompressed files; files are distributed with approximately 60% compression.

Spatial Coverage

Southernmost Latitude: 90.0° S
Northernmost Latitude: 90.0° N
Westernmost Longitude: 180.0° W
Easternmost Longitude: 180.0° E

EASE-Grid

These data are provided in three projections with different areas of coverage: Northern Hemisphere, Southern Hemisphere, and global. The north and south grids are 721 by 721 and the global grid is 1383 by 586. The global grid does not cover the extreme polar regions. The EASE-Grid resolution is 25 km.

Please see EASE-Grid: A Versatile Set of Equal-Area Projections and Grids for the EASE-Grid description, including the exact grid extent (in section EASE-Grid Family of Grid Definitions) for each projection. The descriptions for NL (north), SL (south) and ML (global) grids apply to this data set.

Quarter-Degree Grid

Quarter-degree data provide full global coverage, with a 0.25° latitude and longitude resolution. These data are in one global cylindrical, equidistant latitude-longitude projection. The quarter-degree data are gridded with 1440 rows and 720 columns.

Temporal Coverage

For specific processing dates, see the Temporal Coverage row of the Overview Table at the top of this document. The dates available may differ for north and south azimuthal EASE-Grid, global cylindrical EASE-Grid, and global quarter-degree data. Processing is ongoing and new or reprocessed data are staged to the FTP site as they become available. See the Version Numbers section for more information about new versions and reprocessed data.

NSIDC provides daily data files for days that are incomplete; but in some cases, there is no data at all for a particular day. Table 5 lists missing data files for data that has been processed.

Note: NSIDC has recently noticed corrupt files in the AE_L2A input source data which has increased the number of missing files in this data set. We want to make sure we provide the most accurate products possible. When data are missing due to this file corruption problem, we will backfill missing dates when the corruption issue has been resolved. Please refer to Table 5 for a list of known missing dates for processed data.

Parameter or Variable

These data are interpolated brightness temperatures, in tenths of kelvins. Data values range from 650 (representing 65.0 K) to 3200 (representing 320.0 K). The value 0 indicates missing data.

Sample Images

The following sample images show AMSR-E 36 GHz, horizontally-polarized brightness temperatures from descending passes for December 31, 2005, gridded to the three different EASE-Grids.

North	South	Global

Error Sources

See the Error Sources section of the AMSR-E/Aqua L2A Global Swath Spatially-Resampled Brightness Temperatures documentation for more information about known problems with the data.

Quality Assessment

Extensive quality control procedures are followed in the production of the AE_L2A source data. See the Quality Assessment section of the AE_L2A documentation for more information. The gridding procedure assumes that source data brightness temperatures are legitimate, and performs no filtering of input data based on quality flags.

3. Data Access and Tools

Data Access

Data are available via FTP as processing is completed. Use the following links to access the desired data.

• EASE-Grid data
• Quarter-degree data

Access to these data sets is unrestricted, but users are encouraged to register. Registered users will receive e-mail notification about any product updates or changes. Users can also request the complete data set or a long time series of data by contacting NSIDC User Services.

Volume

The total data set volume per day includes 24 brightness temperature data files (12 channels, two pass directions per channel) and two time files (two pass directions), for each projection.

The volumes shown in Table 6 are approximations for uncompressed files; files are distributed with approximately 60% compression.

Software and Tools

Geolocation files are available via NSIDC's Web site. See NSIDC'S EASE-Grid Geolocation Tools for EASE-Grid data and Latitude-Longitude Grids for quarter-degree data.

Researchers whose needs are not met by these data sets can create customized brightness temperature grids with NSIDC's AMSR-E Swath-to-Grid Toolkit (part of the Passive Microwave Swath Data Tools).

If you use the IDL tools distributed with either DMSP SSM/I Pathfinder Daily EASE-Grid Brightness Temperatures or Nimbus-7 SMMR Pathfinder Daily EASE-Grid Brightness Temperatures, you may use those same tools with these data sets.

Related Data Collections

• DMSP SSM/I Pathfinder Daily EASE-Grid Brightness Temperatures
• Nimbus-7 SMMR Pathfinder Daily EASE-Grid Brightness Temperatures

For more information about related projects, please see NOAA/NASA Polar Pathfinder Data. For more information on the NSIDC EASE-Grid, geolocation tools, and other related products available in EASE-Grid, please see All About EASE-Grid.

4. Data Acquisition and Processing

Please see Instrument Description: Advanced Microwave Scanning Radiometer (AMSR-E) for a description of the instrument and information about data acquisition.

Data Source

The source data set is AMSR-E/Aqua L2A Global Swath Spatially-Resampled Brightness Temperatures (AE_L2A), which contains Level-2A swath data.

Derivation Techniques and Algorithms

Since its launch in 2002, the AE_L2A AMSR-E Level-2A source data has been made available as a beta-level data set (in seven successive versions). Beginning in 2006, some AE_L2A source data were made available as validation-level data, with the entire data set expected to be reprocessed as "validated" on a rolling basis in the future.

In order to simplify the many versions used as input to the gridded data, and to provide gridded data to users as quickly as possible, NSIDC is issuing these AMSR-E gridded data sets using simplified data set version numbers. Any gridded v01 data will have been produced from beta-level AE_L2A data, while gridded v02 data will have been produced from validation-level AE_L2A data. Gridded v01 data will be replaced by gridded v02 data when AE_L2A data that are designated "validated" become available in the future. We encourage users to register in order to receive e-mail notification about future product changes, including updated versions.

Processing Steps

The AMSR-E Level-2A swath brightness temperature source data (AE_L2A) are converted to gridded brightness temperature data as described in the steps below.

1. For the global EASE-Grid and quarter-degree latitude-longitude grid, extract resolution 1 brightness temperatures from the AE_L2A file. Resolution 1 data include Level-2A 6.9, 10.7, 18.7, 23.8, 36.5, and 89.0 GHz brightness temperatures, all resampled to the 75x43 km (6.9 GHz) antenna pattern.
    
2. For the north and south EASE-Grids, extract resolution 3 brightness temperatures from the AE_L2A file. Resolution 3 data include the following.
• Level-2A 23.8, 36.5 and 89.0 GHz brightness temperatures, resampled to the 27x16 km (18.7 and 23.8 GHz) antenna pattern
• Level-2A 10.7 GHz brightness temperatures, resampled to the 51x29 km (10.7 GHz) antenna pattern
• Level-2A 6.9 GHz brightness temperatures, resampled to the 75x43 km (6.9 GHz) antenna pattern
• Level-1B 18.7 GHz brightness temperatures, not resampled
   
3. Extract the times and locations of the extracted brightness temperatures.
    
4. Segregate the data into 24-hour periods, midnight to midnight UTC.
    
5. For each grid, separate the data into ascending and descending pass directions (this is done relative to the sensor footprint, not the satellite).
    
6. Eliminate samples with temperatures less than 65 K and greater than 320 K.
    
7. Eliminate the first 14 samples of each scan.
    
8. For a given grid cell, select the orbit with local time nearest to the local equator crossing time for the satellite. Record this time in the time file.
    
9. For each channel, calculate the brightness temperature value for the grid cell as a weighted average of a 2x2 kernel of nearest-neighbor input samples from the selected orbit within 17.5 km of the cell center. Weights are equal to the inverse of the distance (cell center to sample center) squared. Cells with total weights less than the threshold value (1.0) are set to the missing data value (0).
    
10. Save and compress the gridded brightness temperatures and time data.

5. References and Related Publications

Ashcroft, P. and F. Wentz. 2000. Algorithm Theoretical Basis Document for the AMSR Level-2A Algorithm, Revised 03 November. Santa Rosa, CA, USA: Remote Sensing Systems.

Brodzik, M. J. and K. W. Knowles. 2002. "EASE-Grid: a versatile set of equal-area projections and grids" in M. Goodchild (Ed.) Discrete Global Grids. Santa Barbara, CA, USA: National Center for Geographic Information & Analysis.

Knowles, Kenneth W. 1993. Points, pixels, grids, and cells -- a mapping and gridding primer. Unpublished report to the National Snow and Ice Data Center, Boulder, Colorado USA.

Knowles, Kenneth W. Intercomparison of Resampling Methods for SMMR Pathfinder in EASE-Grid Format. Unpublished report to geospatialmethods.org, Boulder, Colorado USA.

Table 7 lists related documents available on NSIDC's Web site.

6. Document Information

Acronyms

Table 8 lists acronyms used in this document.

Document Creation Date

December 2006

Document Revision Dates

March 2008
June 2007

Document URL

http://nsidc.org/data/docs/daac/nsidc0301_amsre_gridded_tb.gd.html