Polar Satellite Precipitation Data Center

Special Sensor Microwave/Imager (SSM/I) Dataset Description

The SSM/I monthly Rainfall Indices data set contains gridded oceanic monthly total rainfall estimates in millimeters. A probability Distribution Functions (PDF) method (Wilheit et al., 1991) was used to derived the monthly rain indices. The Polar Satellite Precipitation Data Center (PSPDC) is responsible for producing the data set. This data set is a part of the Global Precipitation Climatology Project (GPCP) monthly rainfall products. The period of record for the data set is July 1987 to present with about a 3-month lag. The Defense Mapping Satellite Program (DMSP) F08 SSM/I data were used from July 1987 to December 1991, DMSP F11 SSM/I data used from January 1992 to April 1995, DMSP F13 SSM/I data used from May 1995 to present, DMSP F14 SSM/I data used from May 1997 to present, and DMSP F15 SSM/I data used from January 2000 to present. As additional data set, monthly rainfalls derived from DMSP F10 data (since January 1991) and F11 data (after April 1995) are also available.

The data are on 5° x 5° and 2.5° x 2.5° grids. 5° x 5° grids covers the area from 50°N to 50°S, 0°E to 360°E and 2.5° x 2.5° grids covers the area from 65°N to 65°S, 0°E to 360°E. Calendar month is the unit for this data set. Thus for leap year, February has 29 days. Outputs from the histogram fitting program are also available. They are in compressed ASCII columnar format on the ftp server (snowmelt.gsfc.nasa.gov). The first record is a header record. There are 24 columns in the output data. They are:

1 Row Number 5° x 5° grid 1-20; 2.5° x 2.5° grid 1-52 if row number = 99, end of data set
2 Col Number 5° x 5° grid 1-72; 2.5° x 2.5° grid 1-144
3 Chi Sq Chi square fit values
4 Rain Amount Estimated rainfall (mm/month)
5 p Probability of rain 0.0-1.0
6 r Mean rain rate
7 Sigma Rain Standard dev of rain rate
8 Cut-Off Rain rate cutoff point currently 0.5 mm/hr
9 Sigma0 Standard deviation of non raining TB
10 t0 Average TB of non raining cases
11 m1 1st moment of histogram
12 m2 2nd moment of histogram
13 m3 3rd moment of histogram
14 Total Number of samples in the grid
15 pm1 Fitted 1st moment
16 pm2 Fitted 2nd moment
17 pm3 Fitted 3rd moment
18 fl Freezing level height
19 Daily Rainfall Estimated daily rainfall (mm/day)
20 Max Maximum sample number in a histogram bin
21 Flag2 If=64 simple average of rain rate used
22 Lat Upper latitude of the grid box
23 Lon Left longitude of the grid box


Audit trail information is located after row number 99. This information is used if we need to trace the version of programs used in this process and should be of no interest for users. For those users only interested in monthly rain amounts, a software is provided to generate these values that will match the color charts in the web page.

Monthly rainfall estimates in matrix form are also available. For 5° x 5° data there are 72 x 20 grid boxes and for 2.5° x 2.5 ° there are 144 x 52 grid boxes. For 5° x 5° the a.m. estimates and p.m. estimates are averaged to give mean-value estimates.

Last Update: September 14, 2015
Research Scientist: Dr. Si Gao, AOES/GMU
Please send questions and comments to: Prof. Long S. Chiu