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SSMI and SSMIS netCDF Data Products Table of Contents
These data sets are part of the collection of Special Sensor Microwave/Imager (SSM/I) and Special Sensor Microwave Imager Sounder (SSMIS) data products produced as part of NASA's MEaSUREs Program. Remote Sensing Systems generates SSM/I and SSMIS binary data products using a unified, physically based algorithm to simultaneously retrieve ocean wind speed (at 10 meters), water vapor, cloud water, and rain rate. The SSMIS data have been carefully intercalibrated on the brightness temperature level with the previous SSM/I and therefore extend this important time series of ocean winds, vapor, cloud and rain values. This algorithm is a product of 20 years of refinements, improvements, and verifications. The Global Hydrology Resource Center (GHRC), a NASA science data center managed by the University of Alabama in Huntsville, has reformatted the version 7 binary data into a netCDF data product for each temporal group for each satellite. The version 7 netCDF SSMI/SSMIS collection will be available for F8, F10, F11, F13, F14, F15, F16, and F17 for each temporal aggregation: daily, 3-day, weekly and monthly. The GHRC SSM/I and SSMIS netCDF data sets consist of data derived from observations collected by SSM/I and SSMIS instruments carried onboard the DMSP series of polar orbiting satellites. These satellites are numbered:
*Note: DO NOT use F15 data from 2006-Aug-14 forward for climate research. Since 2006-Aug-14 UTC, 22GHz(V) on SSM/I F15 has been degraded by a RADCAL beacon. More information can be found at http://www.ssmi.com/ssmi/ssmi_F15_RADCAL_beacon_correction.html. InvestigatorFrank Wentz Daily files consist of a single file per day.
3-Day average files consist of a single file with the date being the ending day of the 3-day period.
Weekly average files consist of a single file with the date being the ending day of the one-week period.
Monthly average files consist of a single file with the date being the month (note that the day is not present).
where Data Format The SSM/I and SSMIS RSS version 7 binary data have been reformatted to netCDF Version 4 format (which is based on HDF Version 5). All data is in grid format. There are daily files with 2 grids: one for the ascending and one for the descending passes. There are three files with grids for 3-day, weekly, and monthly averages. All files contain grids for 10-meter surface wind speed, columnar water vapor, columnar cloud liquid water, and rain rate. Within the netCDF file are dimensions; geo, time, and data variables; and attributes. Dimensions are named and will always consist of the following set:
Variables with the fixed names "Latitude" and "Longitude" are defined in all SSM/I netCDF data files. The variable "SST_DTime" is defined only in daily files. "Latitude" and "Longitude" are stored as one-dimensional arrays of 32-bit floating-point values with dimensions named "latitude" and "longitude". Valid latitude values range from -89.875 to +89.875 (South pole to North pole "pixel centers"), and valid longitude values range from +0.125 (just east of Dateline) eastward to 359.875 (just west of Dateline). The following data fields are defined: "SST_DTime" is stored as a two-dimensional array (with dimensions "latitude" by "longitude") of 16-bit signed integers. The scale value of 0.1 is to be applied to the values to produce a time value representing the number of hours since the beginning of the day, GMT, that the daily file represents. The valid range is 0.0 to 24.0 m/s. "10-Meter Surface Wind Speed": A two-dimensional array ("latitude" by "longitude") of 16-bit signed integers. A scale value of 0.2 is to be applied to produce the real values. The valid range of real values is 0.0 to 50.0. "Columnar Water Vapor": A two-dimensional array ("latitude" by "longitude") of 16-bit signed integers. A scale value of 0.3 is to be applied to produce the real values. The valid range of real values is 0.0 to 75.0 kg/m2. "Columnar Cloud Liquid Water": A two-dimensional array ("latitude" by "longitude") of 16-bit signed integers. A scale value of 0.01, then an offset of -0.05 is to be applied to produce the real values. The valid range of real values is -0.05 to 2.45 kg/m2. "Rain Rate": A two dimensional array ("latitude" by "longitude") of 16-bit signed integers containing the data. A scale value of 0.1 is to be applied to produce the real values. The valid range of real values is 0.0 to 25.0 mm/hr. The data values, scaling, and offsets from the RSS binary files were retained in the translation to netCDF so that a direct copy of the values stored in the RSS binary files for the 4 data fields could be made without changing any values. netCDF 4 supports data compression, but requires that the data be tiled into "chunks". All two-dimensional arrays are chunked using 2x90x90 chunk sizes for daily files or 90x90 for the 3-day, weekly, and monthly average files, and compressed. Chunking and compression in netCDF are invisible to the end-user since fields are unchunked and uncompressed as needed as they are read. Tests were performed to determine a reasonable chunk size by doing experiments with square chunk sizes with dimensions that divide evenly into 720 (the size of the "latitude" dimension). Chunk sizes that are too small hurt the compression ratio and make the data slower to read while chunk sizes that are too large could make subsetting inefficient. The 90x90 choice was at the "sweet spot" where going larger had a negligible effect on the compression and the size gives 128 "tiles" of data (8x16) to possibly help with subsetting. netCDF global attributes contain metadata about the file. The following attributes (all character strings) are defined in the SSM/I and SSMIS data files:
Algorithm and Processing Steps These Special Sensor Microwave/Imager (SSM/I) and Special Sensor Microwave Imager Sounder (SSMIS) data products are produced as part of NASA's MEaSUREs Program. Remote Sensing Systems generates SSM/I and SSMIS data products using a unified, physically based algorithm to simultaneously retrieve ocean wind speed (at 10 meters), atmospheric water vapor, cloud liquid water, and rain rate. This algorithm is a product of 20 years of refinements, improvements, and verifications. While the algorithms have evolved over time, a substantial background to the radiative transfer function used to derive the geophysical parameters is described in the following papers: netCDF Format Conversion and Verification A subset of netCDF formatted data files was compared and verified by RSS with regards to the original data content in the binary format. RSS provided GHRC with sample outputs from the binary version for each of the datasets. GHRC validated the netCDF values against the sample output using an internal verification program, Panoply, and the Integrated Data Viewer (IDV), and found netCDF data content to be the same as the binary format. RSS also verified the content. Read SoftwarenetCDF The netCDF library is used to read or write netCDF files. It is available for several languages, including Java, C++, C, FORTRAN, and others. At the time of this writing, the netCDF-Java library is at version 4.2 and the C/C++ libraries are also at version 4.2. Version 4.2 or later is recommended, as earlier versions may not fully support the compression and chunking options used in the SSM/I and SSMIS files. The netCDF and netCDF-Java libraries can be downloaded free of charge from NCSA at http://www.unidata.ucar.edu/downloads/netcdf/. Java JAR files are available that have the dependencies in-place, making a project setup much easier. For other languages, other libraries must be obtained in binary form and installed, or compiled from source code. HDF 5 Since netCDF is based on HDF 5, the HDF version 5 library is required. At the time of this writing, HDF5-1.8.9 is the latest version. This version or later is recommended. HDF can be downloaded free of charge from NCSA at http://www.hdfgroup.org/HDF5/release/obtain5.html. Note that NSCA provides pre-compiled binaries for many platforms or source code if you wish to go through the trouble of customizing a library for your system. SZIP HDF 5 requires the SZIP library to perform compression. SZIP can be downloaded free of charge from the NCSA site at http://www.hdfgroup.org/HDF5/release/obtain5.html. NCSA provides pre-compiled binaries or source code for this package. ZLIB HDF 5 requires the ZLIB library to perform compression. ZLIB can be downloaded free of charge from the NCSA site at http://www.hdfgroup.org/HDF5/release/obtain5.html. NCSA provides pre-compiled binaries or source code for this package. JPEG HDF 5 requires the JPEG library to perform compression. JPEG can be downloaded free of charge from the NCSA site at http://www.hdfgroup.org/HDF5/release/obtain5.html. NCSA provides pre-compiled binaries or source code for this package. More information about the software required and how to link and compile the software can be found in the SSM/I and SSMIS Data in NetCDF User's Guide. There is also a sample read program available at https://ghrc.nsstc.nasa.gov/pub/doc/ssmi_netcdf/ReadNetCDF.c. ToolsThere are a number of freeware packages that can be downloaded to examine and manipulate netCDF files. Many of these can be found at the HDF-EOS web site, http://hdfeos.org/software. Panoply is a cross-platform Java application which plots geo-gridded arrays from netCDF data sets. There are versions specific for Mac OS X and Windows, as well as generic versions for other platforms that support Java 6. Panoply is available at http://www.giss.nasa.gov/tools/panoply/ . The Integrated Data Viewer (IDV) is a Java-based software framework for analyzing and visualizing geoscience data. The IDV is developed at the Unidata Program Center (UPC), part of the University Corporation for Atmospheric Research (UCAR), Boulder, Colorado, which is funded by the National Science Foundation. The software is freely available under the terms of the GNU Lesser General Public License, and is available at http://www.unidata.ucar.edu/software/idv/. GLIDER, http://miningsolutions.itsc.uah.edu/glider/content/glider-features, is a free tool to easily visualize, analyze and mine satellite imagery. GLIDER allows users to visualize and analyze satellite data in its native sensor view. Users can enhance the image by applying different image processing algorithms on the data. GLIDER provides the users with a full suite of pattern recognition and data mining algorithms that can be applied to the satellite imagery to extract thematic information. The suite of algorithms includes both supervised and unsupervised classification algorithms. In addition, users can project satellite imagery and analysis/mining results onto a 3D globe for visualization. GLIDER also allows users to add additional layers to the globe along with the projected image. Users can open multiple views within GLIDER to manage, visualize and analyze many data files all at once. Our data sets are provided through the NASA Earth Science Data and Information System (ESDIS) Project and the Global Hydrology Resource Center (GHRC) Distributed Active Archive Center (DAAC). GHRC DAAC is one of NASA's Earth Observing System Data and Information System (EOSDIS) data centers that are part of the ESDIS project. ESDIS data are not copyrighted; however, in the event that you publish our data or results derived by using our data, we request that you include an acknowledgment within the text of the article and a citation on your reference list. Examples for general acknowledgments, data set citation in a reference listing, and crediting online web images and information can be found at: http://ghrc.nsstc.nasa.gov/uso/citation.html Wentz Frank J. 2013, "SSM/I Version-7 Calibration Report", Remote Sensing Systems, Santa Rosa, CA. Description of Remote Sensing Systems Version-7 Geophysical Retrievals by Hilburn et al., 2010. Contact InformationTo order these data or for further information, please contact:
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