Atmos Composition
Modeling
Arkin and Janowiak Global Precipitation Index (GPI)
IR-Based Monthly Rainfall for the GPCP
Online FTP Access to Arkin and Janowiak GPI Data
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.
Sponsor
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.
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
The GPI Precipitation data set also resides online at the Goddard DAAC anonymous FTP site and may be accessed either directly from this document,
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)
