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Arkin and Janowiak Global Precipitation Index (GPI)

IR-Based Monthly Rainfall for the GPCP

Online FTP Access to Arkin and Janowiak GPI Data
Summary
Sponsor
Original Archive
Future Updates
Data Set Description
Data Characteristics
Data Format

 

Sample Software
References
Data Access
Anonymous FTP
Points of Contact

Summary

This README file describes the Arkin and Janowiak GOES Precipitation Index (GPI) data, 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 GPI data are infrared-based monthly rainfall estimates, which form an intermediate product of the GPCP, a World Meteorological Organization/World Climate Research Program (WMO/WCRP) effort.

The distribution of this data set is being funded by NASA's Earth Science Enterprise progra m.The data are not copyrighted; however, we request that when you publish data or results using these data, please acknowledge as follows:

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 investigators, Drs. Phillip Arkin and John Janowiak of the Climate Analysis Center, NOAA, Washington, D.C., 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

The Goddard DAAC will update this data set as new data are processed and made available by the data producers.

Data Set Description

The monthly rainfall estimates are based on infrared radiometer measurements which form an intermediate product of the WMO/WCRP effort (See Janowiak and Arkin (1991) for a general description of the method).The rainfall data file consists of 2.5 degree latitude by 2.5 degree longitude gridded monthly accumulations of estimated rainfall.Richards and Arkin (1981) determined that this spatial resolution yielded the highest correlation between fractional coverage of "cold" clouds and observed rainfall. The primary source of information over the tropical oceans is infrared (IR) data from polar-orbiting and geostationary satellites.The estimates are generated using a simple cloud top temperature-thresholding algorithm.This method of using satellite-observed cloud top brightness temperatures to infer areas of deep convection was demonstrated by Arkin (1979) using data from the Global Atmosphere Research Program - Atlantic Tropical Experiment (GATE) (See Hudlow and Patterson (1979)).The estimates are valid for the tropics and the warm season extratropics, and will overestimate rainfall in regions of persistent, thin, high cirrus.

A reduction correction has been applied to the GPI estimates produced from geostationary IR coverage at large zenith angles (e.g., 33 degrees).A systematic bias was found by comparing GPI estimates computed separately from each geostationary satellite's IR data in overlap regions.The satellite with a larger zenith angle yielded a larger amount of clouds colder than the GPI's temperature threshold of 235 K, compared with the amount of clouds derived from satellites with smaller zenith angles.This correction at least partially removed this bias.

Data Characteristics

Accumulated surface precipitation
mm/day
January 1986 - December 1995
Monthly means
Global
2.5 degree x 2.5 degree
One
4.4 Megabytes (ASCII)
gpi_mth_sat-merged_mmday_asc_0186-1295

Data Format
Each rainfall field within the file begins with a four-digit integer month identifier (e.g., 8601 = January 1986), followed by a 144 by 32 array of monthly rainfall estimates (in mm/day).Each value represents the estimated areal average rainfall for a 2.5 degree by 2.5 degree grid cell.Missing values are flagged by "9999".The data grid ranges in latitude from 40 degree north to 40 degree south and in longitude from 180 degree west to 180 degree east.The array structure is given in Table 1.

Examples:

Element (1, 1) contains the mean for the grid cell centered at 38.75 degree north latitude and 1.25 degree east longitude.

Element (144, 32) contains the mean for the grid cell centered at 38.75 degree south latitude and 1.25 degree west longitude.

                                Table 1.


40N   --------------------------       ----------------------
      |       |       |       |         |         |         |
      | (1,1) | (2,1) | (3,1) |  . . .  | (143,1) | (144,1) |
      |       |       |       |         |         |         |
37.5N --------------------------       ----------------------
      |       |       |       |         |         |         |
      | (1,2) | (2,2) | (3,2) |  . . .  | (143,2) | (144,2) |
      |       |       |       |         |         |         |
      --------------------------       ----------------------
                                   .
                                   .
37.5S --------------------------       ----------------------
      |       |       |       |         |         |         |
      | (1,32 | (2,32 | (3,32)|  . . .  | (143,32)| (144,32)|
      |       |       |       |         |         |         |
40S   --------------------------       ----------------------
      |       |       |       |         |         |         |
      0      2.5E     5E     7.5E       5W       2.5W       0

Sample Software

An example program in FORTRAN to read the GPI data set is shown below:

 

      real precip (144, 32)
      character*50 infile
      data infile / 'gpi_mth_sat-merged_mmday_asc_0186-1295' /
      open (unit=20, file=infile, form='formatted',
     +      access='sequential', status='readonly')
      do 100 irec=1, 150
        read (20, 1000, end=9999) iyymth, precip
        write (6, *) 'Data read for time period ', iyymth
 100  continue
 1000 format (i4 / (144f8.2))
 9999 write (6, *) 'End of file reached.'
      stop
      end

References

Arkin, P.A., 1979.The relationship between the fractional coverage of high cloud and rainfall accumulations during GATE over the B-array.Mon. Wea. Rev., 107, 1382-1387.

Hudlow, M.D., and V. Patterson, 1979.GATE radar rainfall atlas.NOAA Spec. Rep., Center for Environ. Assessment Serv., NOAA, 155 pp.

Janowiak, J.E. and P.A. Arkin, 1991.Rainfall variations in the tropics during 1986-1989, as estimated from observations of cloud-top temperatures.J. Geophys. Res., 96, Supplement, 3359-3373.

Richards, F., and P.A. Arkin, 1981.On the relationship between satellite-observed cloud cover and precipitation. Mon. Wea. Rev., 5, 1081-1093.

Data Access

Anonymous FTP

The GPI Precipitation data set also resides online at the Goddard DAAC anonymous FTP site and may be accessed either directly from this document,

FTP data link Arkin and Janowiak GPI Data

 

or via anonymous FTP at
ftp daac.gsfc.nasa.gov
login:anonymous
password:< your internet address >
cd http://disc.sci.gsfc.nasa.gov/data/hydrology/precip/arkin/gpcp_gpi

Points of Contact

Data Producers:

 

The producers of this data set can be contacted as follows:
Dr. Phillip Arkin
Office of Global Programs
Oceanic and Atmospheric Research
National Oceanic and Atmospheric Administration
Washington, D.C.
Email:parkin@sgi17.wwb.noaa.gov
301-763-8317 (voice)
301-763-8434 (fax)
Dr. John E. Janowiak
Climate Analysis Center
National Meteorological Center/National Weather Service
National Oceanic and Atmospheric Administration
Washington, D.C.
Email:jsqr@sgi37.wwb.noaa.gov
301-763-8227 (voice)
301-763-8395 (fax)
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Last updated: Jul 20, 2010 02:38 PM ET
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