Special Sensor Microwave/Imager (SSM/I)

Global Gridded Products

Overview

The Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave/Imager(SSM/I) became operational in July 1987 on the F-8 satellite. Subsequent SSM/I's have been flown on the F- 10 (November 1990), F-11 (December 1991), F-12 (August 1994), F-13 (March 1995), F-14 (April 1997) and most recently, F-15 (January 2000) satellites. At present, NESDIS receives data from the F-13 and F-15 satellites.

The SSM/I is a seven channel passive microwave radiometer operating at four frequencies (19,35, 22,235, 37.0, and 85.5 GHz) and dual-polarization (except at 22.235 GHz which is V-polarization only). It should be noted that the SSM/I will be replaced by an advanced sensor, the SSMIS (Special Sensor Microwave Imager Sounder) on the F-16 and F-17 satellites, which were launched in October 2003 and November 2006, respectively. SSMIS data is now available through Comprehensive Large Array-data Stewardship System (CLASS). There should be little impact on the suite of hydrological products described here, as the primary channels used are very similar between the SSM/I and SSMIS.

Scientists at the NOAA/NESDIS Center for Satellite Applicatons and Research (STAR) are actively involved in SSM/I algorithm development and validation, and the application of such algorithms for weather forecasting and analysis, and climate evaluation. Additionally, the SSM/I has been used to develop algorithms for use on the NOAA Advanced Microwave Sounding Units (AMSU), which contain some frequencies similar to those on the SSM/I (e.g., 23.8, 31.4, and 89.0 GHz), but will not have polarization information. Please visit the NOAA/NESDIS/Microwave Surface and Precipitation Products System web site for more information.

The SSM/I products are useful for evaluating the mean climate state, its interannual and seasonal variations, and the detection of anomalies associated with ENSO and regional climatic variations. A time series of the entire SSM/I archive includes data from July 1987 to the present. Monthly average products are produced for precipitation, cloud liquid water, total precipitable water, snow cover, sea-ice cover, and oceanic surface wind speed.

A paper by M. Colton and G. Poe entitled "Intersensor Calibration of DMSP SSM/I's: F-8 to F-14; 1987-97" appears in the January 1999 issue of IEEE Transactions on Geoscience and Remote Sensing and is a good reference on the SSM/I sensor.

A good overview of the products available from the SSM/I can be found in the May 1996 Bulletin of the American Meteorological Society: pages 891-905.

"An Eight Year (1987-1994) Time Series of Rainfall, Clouds, Water Vapor, Snow-cover, and Sea-ice Derived from SSM/I Measurements" by R. Ferraro, F. Weng, N. Grody, and A. Basist"

Copies are also available from Ralph Ferraro (Ralph.R.Ferraro@noaa.gov).

A preview image and a description of the monthly product is available for each of the following:

• Precipitation
• Snow Cover / Sea Ice
• Total Precipitable Water
• Cloud Liquid Water
• Oceanic Surface Wind Speed

To visualize the monthly products and for other interesting information please visit the SSM/I Climate Web Page.

Description of Dataset Files

• Data Type - 4-byte floating point(binary) data.
• Quality Control - Input data source is quality controlled prior to product generation at NOAA/NESDIS.
• Data Origin - The input data originates from the Defense Meteorological Satellite Program (DMSP), Special Sensor Microwave/Imager (SSM/I) instrument. A subsampled, 1/3 degree gridded daily archive of this data has been assembled by researchers at the NOAA/NESDIS/STAR. These daily fields are used as input to generate the monthly products.
• Content Elements
  
  • One-Degree Products

    The files have the following naming convention:

    ppp.yy

    where ppp=product type and yy=year (e.g., 87 for 1987).

    Each field is stored as a 1440 byte record of 4-byte real variables which have been generated on a Windows based computer (see notes below regarding format issues when using on another platform). Each record contains 360 values corresponding to each degree of longitude, with the first element corresponding to longitude 0.5° East, the second 1.5° East, etc., and the last 0.5° West. The first record contains data centered at latitude 89.5° South, the second record contains data centered at 88.5° South, etc., and the last 89.5° North. Missing data are indicated by a value of -999.0.

    The first 180 records in the data set correspond to January of that year, the next 180 is February, etc. The total length of each annual data set is 180*1440*12=3,110,400 bytes.

    The following describes the content of the 1 degree gridded files:

    1. cfr.yy - Monthly mean cloud fraction (0 - 1.0)
    2. ice.yy - Monthly mean sea-ice cover (0 - 100 percent)
    3. lwp.yy - Monthly mean liquid water path (1000 * mm)
    4. pfr.yy - Monthly mean rain fraction (0 - 1.0)
    5. pre.yy - Monthly rainfall (mm)
    6. snw.yy - Monthly mean snow cover fraction (0 - 1.0 percent)
    7. ssa.yy - Monthly mean sampling fraction (0 - 1.0)
    8. win.yy - Monthly mean surface wind speed (m/sec)
    9. wvp.yy - Monthly mean total precipitable water (mm)

    Period of Record is 1989 to present

  • 2.5-Degree Products
    • Monthly Products

    The files have the following naming convention:

    ppp.mon

    where ppp=product type.

    Each field is stored as a 576 byte record of 4-byte real variables which have been generated on a personal computer. Each record contains 144 values corresponding to each 2.5° of longitude, with the first element corresponding to longitude 1.25° East, the second 2.5° East, etc. and the last 1.25° West. The first record contains data centered at latitude 88.75° South, the second record contains data centered at 86.25° South, etc. and the last 88.75° North. Missing data are indicated by a value of -999.0. Negative values in the pr1 and pr2 rainfall products denote indeterminate rainfall due to snow and ice cover; the absolute value indicates the fraction of ice/snow cover for the month.

    The first 72 records in the data set correspond to January 1987, 72 is February 1987, etc. and the last 72 records correspond to December of the current year. The total length of each data set is 72*576*12*(# of years available....in 2002 # of years = 16)= total # of bytes.

    The following describes the content of the 2.5° gridded files:

    1. cfr.mon - Monthly mean cloud fraction (0 - 1.0)
    2. lwp.mon - Monthly mean liquid water path (1000*mm)
    3. pf1.mon - Monthly mean rain fraction (0 - 1.0) using 85 GHz scattering algorithm over land, scattering/emission over ocean (ALG#1)
    4. pr1.mon - Monthly rainfall (mm) using ALG#1
    5. pf2.mon - Monthly mean rain fraction (0 - 1.0) using 37 GHz scattering algorithm over land, emission over ocean (ALG#2)
    6. pr2.mon - Monthly rainfall (mm) using ALG#2
    7. ssa.mon - Monthly mean sampling fraction (0 - 1.0)
    8. ice.mon - Monthly mean sea-ice cover (0 - 100 percent)
    9. snw.mon - Monthly mean snow cover fraction (0 - 1.0 percent)
    10. wvp.mon - Monthly mean total precipitable water (mm)

    Period of Record

    The data set begins in 1987 and is continually updated with current data. The first valid month of data is July 1987. Due to F8 instrument failure, there is no valid data for December 1987. There is no valid data for cfr, lwp, pf1, and pr1 for July 1990 - December 1991.

    • Pentad Products

    The files have the following naming convention:

    pppyy-f13-2.5.pen

    where ppp=product type and yy=year (e.g.,87 for 1987).

    Each field is stored as a 576 byte record of 4-byte real variables which have been generated on a personal computer. Each record contains 144 values corresponding to each 2.5° of longitude, with the first element corresponding to longitude 1.25° East, the second 2.5° East, etc. and the last 1.25° West. The first record contains data centered at latitude 88.75° South, the second record contains data centered at 86.25° South, etc. and the last 88.75° North. Missing data are indicated by a value of -999.0. Negative values in the pr1 and pr2 rainfall products denote indeterminate rainfall due to snow and ice cover; the absolute value indicates the fraction of ice/snow cover for the month.

    The first 72 records in the data set correspond to the first pentad of the year, 72 to the second one, etc. and the last 72 records correspond to the last pentad of December of the given year. The total length of each data set is 72*576*73 (number of pentad in the year)= total # of bytes.

    The following describes the content of the pentad 2.5° gridded files:

    1. cfryy-f13-2.5.pen - Pentad mean cloud fraction (0 - 1.0)>
    2. lwpyy-f13-2.5.pen - Pentad mean liquid water path (1000*mm)
    3. pf1yy-f13-2.5.pen - Pentad mean rain fraction (0 - 1.0) using 85 GHz scattering algorithm over land, scattering/emission over ocean (ALG#1)
    4. pr1yy-f13-2.5.pen - Pentad rainfall (mm) using ALG#1
    5. pf2yy-f13-2.5.pen - Pentad mean rain fraction (0 - 1.0) using 37 GHz scattering algorithm over land, emission over ocean (ALG#2)
    6. pr2yy-f13-2.5.pen - Pentad rainfall (mm) using ALG#2
    7. ssayy-f13-2.5.pen - Pentad mean sampling fraction (0 - 1.0)
    8. iceyy-f13-2.5.pen - Pentad mean sea-ice cover (0 - 100 percent)
    9. snwyy-f13-2.5.pen - Pentad mean snow cover fraction (0 - 1.0 percent)
    10. winyy-f13-2.5.pen - Pentad mean surface wind speed (m/sec)
    11. wvpyy-f13-2.5.pen - Pentad mean total precipitable water (mm)

    Period of Record

    Only the current year is available. Other periods are available under request
    • GPCP products

    Each field is stored as a 576 byte record of 4-byte real variables which have been generated on a personal computer. Each record contains 144 values corresponding to each 2.5° of longitude, with the first element corresponding to longitude 1.25° East, the second 2.5:deg; East, etc. and the last 1.25° West. The first record contains data centered at latitude 88.75° South; the second record contains data centered at 86.25° South, etc. and the last 88.75° North. Missing data are indicated by a value of -999.0. Negative values in the pr1 and pr2 rainfall products denote indeterminate rainfall due to snow and ice cover; the absolute value indicates the fraction of ice/snow cover for the month.

    The following describes the content of the gpcp 2.5° gridded files:

    1. gpcp_nesdis_pr1: Monthly rainfall using 85 GHz scattering algorithm over land, scattering/emission over ocean (ALG#1) for the early morning constellation (F8-F11 and F13)
    2. gpcp_nesdis_pr2: Monthly rainfall using 37 GHz scattering algorithm over land, emission over ocean (ALG#2) for the early morning constellation (F8-F11 and F13)
    3. gpcp_nesdis_f10_pr1: Monthly rainfall using 85 GHz scattering algorithm over land, scattering/emission over ocean (ALG#1) for the mid morning constellation (F10/14 and F15)
    4. gpcp_nesdis_f10_pr2: Monthly rainfall using 37 GHz scattering algorithm over land, emission over ocean (ALG#2) for the mid morning constellation (F10/14 and F15)
    5. gpcp_nesdis_dual_pr1: Monthly rainfall using 85 GHz scattering algorithm over land, scattering/emission over ocean (ALG#1) for the combination of both constellations
    6. gpcp_nesdis_dual_pr2: Monthly rainfall using 37 GHz scattering algorithm over land, emission over ocean (ALG#2) for the combination of both constellations

    For products 1 and 2, the first 72 records in the data set correspond to January 1987, 72 are February 1987, etc. and the last 72 records correspond to December of the current year. The total length of each data set is 72*576*12*(# of years available since 1987) = total # of bytes. For the rest of the products, the first 72 records in the data set correspond to January 1992, 72 are February 1987, etc. and the last 72 records correspond to December of the current year. The total length of each data set is 72*576*12*(# of years available since 1992) = total # of bytes.

    Period of Record

    The data set begins in 1987 and is continually updated with current data. The first valid month of data is July 1987. Due to F8 instrumental failure, there is no valid data for Decemeber 1987. There is no valid data for cfr, lwp, pf1, and pr1 for July 1990 - December 1991 for product 1 and 2. Products 3 to 6 begin in 1992 and are continually updated with current data.
• Notes

A. The original data is generated in little endian byte order. If the data is used in other computer environments (big-endian), the data may have to be byte-swapped. A few helpful application tips:


• If you use IDL, you can utilize the "byteorder" routine to help you in this conversion.

• If you use GRADS, you can utilize the "OPTIONS" feature in your control file and specify "BYTESWAPPED", "LITTLE ENDIAN", etc.

B. The products are generated from a single satellite with observation times near 6 am/pm local time (except for GPCP products) and contain the following:

F8 satellite: July 1987 - June 1991

F11 satellite: January 1992 - April 1995

F13 satellite: May 1995 - present

There are some missing data periods as noted above.

• Problem Areas

A. The 2.5 degree pr2 and pf2 products have artifacts along coastlines and the sea-ice boundary. These are due to the larger footprint sizes of the lower frequency channels (e.g., 19 GHz) used in the retrievals.

B. The 1 degree rainfall product does not make a distinction between "zero" rain and "indeterminate" rain (due to snow and ice cover), which is indicated as zero rain. It is done within the 2.5 degree products.

• Data Set Producer

Data Access

Access to the data:

• 1.0 Degree Monthly Data
• 2.5 Degree Monthly Data
• 2.5 Degree Pentad Data

FILES WILL BE UPDATED AROUND THE 7TH DAY OF THE MONTH.

Questions and Comments

Send questions or comments to the NCDC Satellite Services Group ncdc.satorder@noaa.gov

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