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Readme for the Chang Special Sensor Microwave/ Imager (SSM/I) Monthly Rain Indices Data Set

Online Data Access for Chang SSM/I Monthly Rain Indices Data Set

Global Precipitation Climatology Project (GPCP) SSM/I
Original Archive
Future Updates
Data Set Description
Data Characteristics
Data Format
Sample Software
Data Access
Anonymous FTP
Points of Contact


This README file describes the Chang Special Sensor Microwave/ Imager (SSM/I) Monthly Rainfall Indices over the oceans, including information on the file structure, pertinent scientific references, and contact information for obtaining the data.This data set is one of the Global Precipitation Climatology Project (GPCP) products.The data set currently includes rainfall estimates derived from the SSM/I instruments flown aboard the Defense Meteorological Satellite Program (DMSP) F8 and F11 platforms.


The authors wish to thank the Distributed Active Archive Center (Code 610.2) at the Goddard Space Flight Center, Greenbelt, MD, 20771, for distributing the data; and the science investigator, Dr. Alfred Chang of the Hydrological Sciences Branch, Code 974, NASA Goddard Space Flight Center, Greenbelt, MD 20771, for producing these data products. Goddard's contribution to these activities was sponsored by NASA's Mission to Planet Earth program.

Original Archive

This data set was originally archived at the Marshall Space Flight Center.It was moved to the Goddard Space Flight Center Distributed Active Archive Center (GSFC DAAC) in the fall of 1996.

Future Updates

Future updates to this data set are not planned at this time.

Data Set Description

The SSM/I Monthly Rainfall Indices data set contains gridded ocean monthly total rainfall indices in millimeters.A Probability Distribution Function (PDF) method (Wilheit et al. (1991) and Chang et al. (1993)) was used to derive the monthly rain indices.The temporal coverage for the data set is from July 1987 through December 1995.However, no data are available for December 1987, because the instrument was turned off during that time period.Brightness temperatures were obtained from the SSM/I aboard the DMSP F8 platform from July 1987 through December 1991, and from the SSM/I aboard the DMSP F11 from January 1992 through December 1995.

The data were mapped to a 5 degree latitude by 5 degree longitude grid for the area from 50 degree north latitude to 50 degree south latitude, and 180 degree west longitude to 180 degree east longitude.The division of the data into months followed the GPCP convention:each month consists of six pentads (30 days) except August, which consists of seven pentads (35 days).For example, June monthly averages contain data from May 31 to June 29, inclusive.Table 1 defines the GPCP pentad months.

                        Table 1.

Month           Inclusive Dates         Inclusive Julian Days

January         01/01 - 01/30                   001 - 030
February        01/31 - 03/01                   031 - 060 **
March           03/02 - 03/31                   061 - 090
April           04/01 - 04/30                   091 - 120
May             05/01 - 05/30                   121 - 150
June            05/31 - 06/29                   151 - 180
July            06/30 - 07/29                   181 - 210
August          07/30 - 09/02                   211 - 245
September       09/03 - 10/02                   246 - 275
October         10/03 - 11/01                   276 - 305
November        11/02 - 12/01                   306 - 335
December        12/02 - 12/31                   336 - 365

** Note that leap year will have an extra day.

Each grid cell value is the average of the satellite ascending and descending estimates, corrected for beam filling errors.The equation used to calculate the estimates is

(ascending + descending) / 2.0 * 1.5

Values are only computed over oceans; cells over land are set to -10.0. There are also 33 cells set to -10.0 due to island contamination.Any rain retrieval that did not converge for a given cell results in that cell being set to -10.0.

Data Characteristics

Accumulated surface precipitation
july 1987 - december 1995
Monthly means
50° N to 50° S ; 180° W to 180° E
5° x 5°
1.2 Megabytes (ASCII)

Data Format
An informational header consisting of 55 ASCII records precedes the actual rainfall data in the file.Following this header are the monthly grids for the entire period of coverage.Each month starts with an ASCII tag that identifies the year and month of the data.Following each tag is a 72 x 20 gridded monthly rainfall array, where 72 is the number of longitude bands and 20 is the number of latitude bands.The gridded rainfall data are stored as real numbers with 144 rows of data values written out as 8-character fields (See Sample Software below.)

Grid (1,1) is defined as the latitude band 45-50N, longitude band 0-5E.The grids progress as described in Table 2, with the longitude bands incrementing first.

                        Table 2.

Latitude -->    45-50N  40-45N  ...     40-45S  45-50S 

0- 5E           (1,1)   (1,2)           (1,19)  (1,20) 
5-10E           (2,1)   (2,2)           (2,19)  (2,20)
175-180W        (72,1)  (72,2)          (72,19) (72,20)

Sample Software

An example program in FORTRAN to read the Chang SSM/I rainfall indices is shown below:
real precip (72, 20)
      character*50 infile, yyymmm*6, info(55)*80
      data infile / 'GPCP_SSMI_1295_5.0_v23' /
      open (unit=20, file=infile, form='formatted',
     +      access='sequential', status='readonly')
      read (20, '(a)') info
      do 200 irec=1, 150
        read (20, 1000, end=9999) yyymmm, precip       
	  write (6, *) ' Data read for time period ', yyymmm
  200 continue
 1000 format (1x, a6 / (10f8.1))
 9999 write (6, *) ' End of file reached '


Chang, A.T.C., L.S. Chiu, T.T. Wilheit, 1993. Random errors of oceanic rainfall derived from SSM/I using probability distribution functions.Mon. Wea. Rev., 121, 2351-2354.

Wilheit, T.T., A.T.C. Chang, L.S. Chiu, 1991. Retrieval of monthly rainfall indices from microwave radiometric measurements using probability distribution functions.J. Atmos. Oceanic Tech., 8, 118-136.

Data Access Anonymous FTP

The Chang Precipitation data set resides online and may be accessed either directly from this


FTP link Chang SSM/I Derived Rainfall Data

or via anonymous FTP at
password:< your email address >
cd /data/hydrology/precip/chang

Points of Contact

Data Producers:

This data set was produced by the late Dr. Alfred Chang of the Hydrological Sciences Branch (Code 974), NASA Goddard Space Flight Center, Greenbelt, MD 20771

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Last updated: Jul 20, 2010 02:39 PM ET