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AMSR-E/Aqua Daily L3 6.25 km 89 GHz Brightness Temperature (Tb) Polar Grids

Summary

The Advanced Microwave Scanning Radiometer - Earth Observing System (AMSR-E) instrument on the NASA EOS Aqua satellite provides global passive microwave measurements of terrestrial, oceanic, and atmospheric variables for the investigation of water and energy cycles.

This Level-3 gridded 89.0 GHz brightness temperature (T_B) product (AE_SI6) at 6.25 km resolution is an intermediate product during processing of AMSR-E Level-3 sea ice products at 12.5 km and 25 km resolution. Values are daily average, daily ascending, and daily descending horizontally and vertically polarized T_Bs mapped to a polar stereographic grid at 6.25 km spatial resolution. Data are stored in HDF-EOS format, and are available via FTP, CD-ROM, 8-mm tape, DLT, or DVD-ROM.

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 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 and version, dates of data you used, publishers (NSIDC), and digital media.

Cavalieri, D., and J. Comiso. 2004, updated daily. AMSR-E/Aqua Daily L3 6.25 km 89 GHz Brightness Temperature (Tb) Polar Grids V001, March to June 2004. Boulder, CO, 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

Donald J. Cavalieri, Josefino C. Comiso, and Thorsten Markus
Laboratory for Hydrospheric Processes
NASA Goddard Space Flight Center

Technical Contact

2. Detailed Data Description

Format

Data are stored in Hierarchical Data Format - Earth Observing System (HDF-EOS) format. Files contain core metadata, product-specific attributes, and the data fields in 2-byte signed integer format, defined in the following table. Data are scaled by 10, and missing data values are indicated by "0." The valid range of T_Bs is approximately 50 to 300 K.

File Naming Convention

The file naming convention is as follows:

AMSR_E_L3_SeaIce6kmX##_yyyymmddhhmm.hdf

where:

X = product maturity code
## = file version number
yyyy = year
mm = month
dd = day
hh = hour
mm = minutes

The valid values for the product maturity code are "b", "v", and "p" for beta, validated, and preliminary, respectively. Beta product maturity indicates use of data calibrated by the Japan Aerospace Exploration Agency (JAXA, Contractor: Mitsubishi Electric Corporation) in producing Level-2A T_Bs. The product maturity is upgraded to "validated" after the science software is tested and the algorithm is validated using the official NASA calibration. Preliminary refers to non-standard, near real-time data available from NSIDC. These data are only available for a limited time, until the corresponding standard product is ingested at NSIDC.

File Size

Each daily granule is approximately 46 MB.

Spatial Coverage

Spatial Coverage Map

Northern Hemisphere

Southern Hemisphere

Spatial Resolution

89 GHz observations are derived from AMSR-E/Aqua L2A Global Swath Spatially-Resampled Brightness Temperatures (Tb) at 56 km, 38 km, 21 km, and 12 km spatial resolution. Data are resampled to a 6.25 km polar stereographic grid.

Projection

T_B grids are in a polar stereographic projection, which specifies a projection plane (i.e., the grid) tangent to the earth at 70 degrees. The planar grid is designed so that the grid cells at 70 degrees latitude are 6.25 km by 6.25 km. For more information on this topic please refer to Pearson (1990) and Snyder (1987).

The polar stereographic projection often assumes that the plane (grid) is tangent to the Earth at the pole. Thus, there is a one-to-one mapping between the Earth's surface and grid (with no distortion) at the pole. Distortion in the grid increases as the latitude decreases because more of the Earth's surface falls into any given grid cell. At the edge of the northern polar grid distortion reaches 31%. The southern polar grid has a maximum distortion of 22%. To minimize the distortion, the projection is true at 70° rather than at the poles. This increases the distortion at the poles by three percent and decreases the distortion at the grid boundaries by the same amount. The latitude of 70° was selected so that little or no distortion would occur in the marginal ice zone. Another result of this assumption is that fewer grid cells will be required as the Earth's surface is more accurately represented.

The polar stereographic formulae for converting between latitude/longitude and X-Y grid coordinates are taken from Snyder (1982). This projection assumes a Hughes ellipsoid with a radius of 3443.992 nautical mi or 6378.273 km and an eccentricity (e) of 0.081816153 (or e**2 = 0.006693883). The structural metadata (StructMetadata.0) built into the HDF-EOS data file lists the squared eccentricity value rounded to four significant digits (0.006694).

Grid Description

Northern Hemisphere: 1216 columns by 1792 rows
Southern Hemisphere: 1264 columns by 1328 rows

The origin of each x, y grid is the pole. The grids' approximate outer boundaries are defined in the following table. "Corner" points are listed; apply values to the polar grids reading clockwise from upper left. Interim rows define boundary midpoints.

Temporal Coverage

See AMSR-E Data Versions for a summary of temporal coverage for different AMSR-E products and algorithms.

Temporal Resolution

After input Level-2A T_Bs are binned into 6.25 km grid cells (see Data Source, the asscending, descending, and daily data are averaged. The daily average is not simply an average of ascending and descending orbits, because a given could have, for example, three measurements from ascending orbits and two from descending orbits. Instead, the daily average is of all the observations for that grid cell. For example, if A = ascending and B = descending:

( (A1 + A2) / 2 + (B1 + B2 + B3) / 3 ) / 2

is not equal to:

(A1 + A2 + B1 + B2 + B3) / 5

However, this biases daytime (ascending) orbits over nighttime (descending).

Error Sources

See the AMSR-E/Aqua L2A Global Swath Spatially-Resampled Brightness Temperatures (Tb) documentation for information about potential errors with constructed T_Bs.

Refer to Aqua Maneuvers for a list of manuevers and orbital anomalies that may potentially affect the quality of data.

Quality Assessment

Each HDF-EOS file contains core metadata with QA metadata flags that are set by the Science Investigator-led Processing System (SIPS) at the Global Hydrology and Climate Center (GHCC) prior to delivery to NSIDC. A separate metadata file (.met file extension) is also delivered to NSIDC with the HDF-EOS file; it contains the same information as the core metadata. Three levels of quality assessment (QA) are conducted with the AMSR-E Level 2 and 3 products: automatic, operational, and science QA. If a product does not fail QA, it is ready to be used for higher-level processing, browse generation, active science QA, archive, and distribution. If a granule fails QA, SIPS does not send the granule to NSIDC until it is reprocessed. Level-3 products that fail QA are never delivered to NSIDC (Conway 2002).

Automatic QA
Out-of-bounds Level-2A T_Bs are screened out before T_Bs are interpolated to the 6.25 km grid.

Operational QA
AMSR-E Level-2A data arriving at GHCC are subject to operational QA prior to processing higher-level products. Operational QA varies by product, but it typically checks for the following criteria in a given file (Conway 2002):

• File is correctly named and sized
• File contains all expected elements
• File is in the expected format
• Required EOS fields of time, latitude, and lLongitude are present and populated
• Structural metadata is correct and complete
• The file is not a duplicate
• The HDF-EOS version number is provided in the global attributes
• The correct number of input files were available and processed

Science QA
AMSR-E Level-2A data arriving at GHCC are also subject to science QA prior to processing higher-level products. If less than 50% of a granule's data is good, the science Q/A flag is marked "suspect" when the granule is delivered to NSIDC. In the SIPS environment, the science QA includes checking the maximum and minimum variable values, and percent of missing data and out-of-bounds data per variable value. At the Science Computing Facility (SCF), also at GHCC, science QA involves reviewing the operational QA files, generating browse images, and performing the following additional automated QA procedures (Conway 2002):

• Historical data comparisons
• Detection of errors in geolocation
• Verification of calibration data
• Trends in calibration data
• Detection of large scatter among data points that should be consistent

Geolocation errors are corrected during Level-2A processing to prevent processing anomalies such as extended execution times and large percentages of out-of-bounds data in the products derived from Level-2A data.

The Team Lead SIPS (TLSIPS) developed tools for use at SIPS and SCF for inspecting the data granules. These tools generate a QA browse image in Portable Network Graphics (PNG) format and a QA summary report in text format for each data granule. Each browse file shows Level-2A and Level 2B data. These are forwarded from RSS to GHCC along with associated granule information, where they are converted to HDF raster images prior to delivery to NSIDC. The QA summary reports are available on the GHCC AMSR-E Web page.

Please refer to AMSR-E Validation Data for information about data used to check the accuracy and precision of AMSR-E observations.

3. Data Access and Tools

Data Access

Please see Ordering AMSR-E Products from NSIDC for a list of order options.

Volume

Each daily granule is approximately 46 MB.

Software and Tools

Please visit NSIDC's HDF-EOS Web site for tools that work with HDF-EOS data. This documentation will be updated as tools become available for AMSR-E data.

Related Data Collections

Sea Ice Products at NSIDC
This site offers a complete summary of sea ice data derived from passive microwave sensors and other sources, and 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.

Sea Ice Trends and Climatologies from SMMR and SSM/I
NSIDC provides a suite of value-added products to aid in investigations of the variability and trends of sea ice cover. These products provide users with information about sea ice extent, total ice covered area, ice persistence, monthly climatologies of sea ice concentrations, and ocean masks.

Sea Ice Remote Sensing at NASA/Goddard Space Flight Center

4. Data Acquisition and Processing

Sensor or Instrument Description

Please refer to the AMSR-E Instrument Description document.

Data Acquisition Methods

Please refer to the AMSR-E Instrument Description document.

Data Source

AMSR-E/Aqua L2A Global Swath Spatially-Resampled Brightness Temperatures (Tb) are gridded to a 6.25 km grid using a "drop-in-the-bucket" approach where the grid cell that contains the center of the observation footprint is given the whole weight of the observation. All valid observations within the extent of the polar grids are binned into grid cells including land observations.

Accordingly, swath data from the 89 GHz channel are mapped onto the 6.25 km polar stereographic grid by converting the geodetic latitude and longitude for the center of each scene station (i.e. observation footprint) into AMSR-E map grid coordinates. Scene station map grid coordinates determine grid cell assignments. Observations falling outside the AMSR-E polar grid are ignored. For each grid cell, T_Bs observed over a 24-hour period (midnight to midnight GMT) are summed and then divided by the total observations to obtain a daily-average T_B value. If no observations fall within a grid cell for a given day, the average T_B is labeled "missing."

Derivation Techniques and Algorithms

Refer to the AMSR-E/Aqua L2A Global Swath Spatially-Resampled Brightness Temperatures (Tb) documentation for details of calculating AMSR-E T_Bs.

Processing Steps

Orbital AMSR-E 6.25 km T_Bs were mapped to a polar stereographic grid in the same manner as the 12.5 km and 25 km AMSR-E sea ice products.

Processing History

See AMSR-E Data Versions for a summary of algorithm changes since the start of mission.

5. References and Related Publications

Cavalieri, D. and J. Comiso. 2000. Algorithm Theoretical Basis Document for the AMSR-E Sea Ice Algorithm, Revised December 1. Landover, MD, USA: Goddard Space Flight Center.  (view pdf)

Cavalieri, D.J., K.M. St. Germain, and C.T. Swift. 1995. Reduction of weather effects in the calculation of sea ice concentration with the DMSP SSM/I. Journal of Glaciology 41(139): 455-464.

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.

Comiso, J., D. Cavalieri, and T. Markus. 2003. Sea ice concentration, ice temperature, and snow depth using AMSR-E data. IEEE Transactions on Geoscience and Remote Sensing 41(2): 243-252.

Comiso, J., and K. Steffen. 2001. Studies of Antarctic sea ice concentrations from satellite data and their applications. Journal of Geophysical Research 106(C12): 31,361-31,385.

Comiso, J.C. 1995. SSM/I ice concentrations using the Bootstrap Algorithm. NASA RP 1380, 50 pp.

Conway, D. 2002. Advanced Microwave Scanning Radiometer - EOS Quality Assurance Plan. Huntsville, AL: Global Hydrology and Climate Center.

Gloersen P. and D.J. Cavalieri. 1986. Reduction of weather effects in the calculation of sea ice concentration from microwave radiances. Journal of Geophysical Research 91(C3):3913-3919.

Kummerow, C. 1993. On the accuracy of the Eddington approximation for radiative transfer in the microwave frequencies. Journal of Geophysical Research 98: 2757-2765.

Markus, T. and D. Cavalieri. 1998. Snow depth distribution over sea ice in the Southern Ocean from satellite passive microwave data. Antarctic Sea Ice: Physical Processes, Interactions, and Variability. Antarctic Research Series 74:19-39. Washington, DC, USA: American Geophysical Union.

Markus, T., and D. Cavalieri. 2000. An enhancement of the NASA Team sea ice algorithm. IEEE Transactions on Geoscience and Remote Sensing 38: 1387-1398.

Pearson, F. 1990. Map projections: Theory and applications. CRC Press. Boca Raton, Florida. 372 pages.

Snyder, J.P. 1987. Map projections - a working manual. U.S. Geological Survey Professional Paper 1395. U.S. Government Printing Office. Washington, D.C. 383 pages.

Snyder, J. P. 1982. Map Projections Used by the U.S. Geological Survey. U.S. Geological Survey Bulletin 1532.

6. Document Information

Acronyms and Abbreviations

The following acronyms and abbreviations are used in this document.

Document Creation Date

March 2004

Document Revision Date

January 2005

Document Review Date

March 2004

Document URL

http://nsidc.org/data/docs/daac/ae_si6_6km_tbs.gd.html