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.
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 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.
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.
|Accumulated surface precipitation
|july 1987 - december 1995
|50° N to 50° S ; 180° W to 180° E
|5° x 5°
|1.2 Megabytes (ASCII)
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.
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)
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
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
Chang SSM/I Derived Rainfall Data
- or via anonymous FTP at
- ftp disc3.nascom.nasa.gov
- password:< your email address >
- cd /data/hydrology/precip/chang
Points of Contact
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