SSMI and SSMIS netCDF Data Products
Table of Contents
Introduction
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.
Instrument Description
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:
Satellite |
Start Date |
End Date |
F08 SSM/I |
Jul 1987 |
Dec 1991 |
F10 SSM/I |
Dec 1990 |
Nov 1997 |
F11 SSM/I |
Dec 1991 |
May 2000 |
F13 SSM/I |
May 1995 |
Nov 2009 |
F14 SSM/I |
May 1997 |
Aug 2008 |
*F15 SSM/I |
Dec 1999 |
Dec 2011 (V6 binary data from 2012 forward available from RSS) |
F16 SSM/I |
Oct 2003 |
present |
F17 SSMIS |
Dec 2006 |
present |
*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.
Investigator
Frank Wentz Remote Sensing Systems 444 Tenth Street,
Suite 200 Santa Rosa, CA 95401
File Naming Convention
Daily files consist of a single file per day.
fnn_S_yyyymmddv7.nc
3-Day average files consist of a single file with the date
being the ending day of the 3-day period.
fnn_S_yyyymmddv7_d3d.nc
Weekly average files consist of a single file with the
date being the ending day of the one-week period.
fnn_S_yyyymmddv7_wk.nc
Monthly average files consist of a single file with the
date being the month (note that the day is not present).
fnn_S_yyyymmv7.nc
where
nn is the satellite number (08-17) S is the satellite name (ssmi, ssmis)
yyyy is the 4-digit year mm is the 2-digit month dd is the 2-digit day of
month nc is netCDF format
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:
"Latitude" is the number of horizontal lines in the data
grids (always 720) "Longitude" is the number of vertical lines in the
data grids (always 1440) "Time" is the number of passes (daily files
only, always 2 if present)
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:
"Title": The name of the data set. " Institution":
The institution(s) involved in producing the data files. "Source":
The source of the data in the files. "History": History information,
including provenance (providing documentation of where and when the
files were produced). "References": A URL to documentation containing
a description of the data. "Comment": Notes about the data,
containing a more lengthy description of the history, source, and data
file contents. "Scale": A summary of the scale values that are to be
applied to each variable within the file to produce the real
values. "Value": A summary of special values used that are to be
interpreted in a special way. For SSM/I and SSMIS, such values are
outside the valid range and are flag values. "SatID": The satellite
ID, of the form "DMSP-Fnn", where nn is the satellite
number. "SensorID": The sensor ID, which is "SSM/I" or
"SSMIS". "identifier_product_DOI": the Digital Object Identifier
(DOI) for the data file. "PassDirection": summary of the values used
for pass direction (1 for ascending, 2 for
descending). "NumberOfPasses": the number of passes stored in the
data file (2 for daily files, 1 for others). "ChunkSize": the
dimension of the square tile size used. 90 was chosen, resulting in a
chunk size of 2x90x90 for daily files with 2 passes, and 90x90 for other
files. "Conventions": the CF (Climate and Forecast) convention
version number for the CF conventions used for the data file.
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 Software
netCDF
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 ftp://ghrc.nsstc.nasa.gov/pub/doc/ssmi_netcdf/ReadNetCDF.c. Tools
There 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.
Citation
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
References
Wentz F. J.
1997, "A well-calibrated ocean algorithm for SSM/I", J. Geophys. Res.,
Vol. 102, No. C4, pg. 8703-8718.
Wentz Frank J. 2013, "SSM/I Version-7 Calibration Report", Remote Sensing Systems, Santa Rosa, CA.
Wentz, Frank
J. and Roy W. Spencer, May 1, 1998, "SSM/I Rain Retrievals within a
Unified All-Weather Ocean Algorithm", Journal of the Atmospheric Sciences,
Vol. 55, pg. 1613-1627.
Wentz,
Frank J. and Thomas Meissner, 2000, "AMSR Ocean Algorithm, Version 2",
report number 121599A-1, Remote Sensing Systems, Santa Rosa, CA, 66
pp.
Wentz,
Frank J. and Thomas Meissner, 2007, "Supplement 1 Algorithm Theoretical
Basis Document for AMSR-E Ocean Algorithms", Remote Sensing Systems, Santa
Rosa, CA.
Description
of Remote Sensing Systems Version-7 Geophysical Retrievals by Hilburn et
al., 2010. Contact Information
To order these data or for further information, please
contact:
Global Hydrology Resource Center User Services 320
Sparkman Drive Huntsville, AL 35805 Phone: 256-961-7932 E-mail:
support-ghrc@earthdata.nasa.gov Web:
http://ghrc.nsstc.nasa.gov/
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