Description of Dataset Files
-
Data Type - 4-byte floating point
(binary) data (generated on a Windows platform
- see sample program and notes below).
-
Quality Control - Input data source is quality controlled prior to product generation at NOAA/NESDIS.
-
Data Origin - The input data originates from the Defense Meteorological Satellite Program (DMSP), Special Sensor
Microwave/Imager (SSM/I) instrument. A subsampled, 1/3 degree gridded daily archive of this data has been assembled by researchers at the NOAA/NESDIS/Office of Research and Applications. These daily fields are used as input to generate the monthly products.
- Content Elements
- One-Degree Products
The files have the following naming convention:
ppp.yy
where ppp=product type and yy=year (e.g., 87 for 1987).
(Note that data from other than the current year are compressed on the UNIX basedhost and have the .Z extension).
Each field is stored as a 1440 byte record of 4-byte real variables which have been generated on a Windows based computer (see sample program and notes below regarding format issues when using on another platform). Each record contains 360 values corresponding to each degree of longitude, with the first element corresponding to longitude 0.5° East, the second 1.5° East, etc., and the last 0.5° West. The first record contains data centered at latitude 89.5° South, the second record contains data centered at 88.5° South, etc., and the last 89.5° North. Missing data are indicated by a value of -999.0.
The first 180 records in the data set correspond to January of that year, the next 180 is February, etc. The total length of each annual data set is 180*1440*12=3,110,400 bytes.Please note that previous years data have been compressed and have the .Z extension.
The following describes the content of the 1 degree gridded files:
-
cfr.yy - Monthly mean cloud fraction (0 - 1.0)
-
ice.yy - Monthly mean sea-ice cover (0 - 100 percent)
-
lwp.yy - Monthly mean liquid water path (1000 * mm)
-
pfr.yy - Monthly mean rain fraction (0 - 1.0)
-
pre.yy - Monthly rainfall (mm)
-
snw.yy - Monthly mean snow cover fraction (0 - 1.0 percent)
-
ssa.yy - Monthly mean sampling fraction (0 - 1.0)
-
win.yy - Monthly mean surface wind speed (m/sec)
-
wvp.yy - Monthly mean total precipitable water (mm)
A sample segment of FORTRAN code to read the data:
PROGRAM SSMIREAD
CHARACTER*6 FNAME
REAL*4 SSMI(360,180)
C
C OPEN FILE
C
FNAME='PRE.88'
OPEN(UNIT=10,FILE=FNAME,RECL=1440,FORM='UNFORMATTED')
C
C LOOP OVER ENTIRE YEAR OF DATA
C
DO I=1,12
READ(10) SSMI
C CALL YOUR PROCESSING ROUTINE HERE
END DO
C
STOP
END
It should be noted that the datasets are generated in a PC environment. Users working on other platforms will likely have to change the byteorder and/or be faced with chosing "little endian" or "big endian". See the notes section at the bottom for some routines to handle this.
Period of Record
The data set begins in 1987 and is continually updated with current data. The first valid month of data is
July 1987. Due to F8 instrument failure, there is no valid data for December 1987, July 1990 - December 1991. The F8 satellite is used from July 1987 through December 1991. The F11 satellite is used from January
1992 through April 1995. The F13 satellite is used from May 1995 to the present.
-
2.5-Degree Products
The files have the following naming convention:
ppp.mon
where ppp=product type.
Each field is stored as a 576 byte record of 4-byte real variables which have been generated on a personal
computer. Each record contains 144 values corresponding to each 2.5° of longitude, with the first element corresponding to longitude 1.25° East, the second 2.5° East, etc. and the last 1.25° West. The first
record contains data centered at latitude 88.75° South, the second record contains data centered at 86.25° South, etc. and the last 88.75° North. Missing data are indicated by a value of -999.0. Negative values in the pr1 and pr2 rainfall products denote indeterminate rainfall due to snow and ice cover; the absolute value indicates the fraction of ice/snow cover for the month.
The next 72 records in the data set correspond to January 1987, 72 is February 1987, etc. and the last 72 records correspond to December of the current year. The total length of each data set is 72*576*15*(# of years available....in 2002 # of years = 16)= total # of bytes.
The following describes the content of the 2.5° gridded files:
-
cfr.mon - Monthly mean cloud fraction
(0 - 1.0)
-
lwp.mon - Monthly mean liquid water path
(1000*mm)
-
pf1.mon - Monthly mean rain fraction (0 - 1.0) using 85 GHz scattering algorithm over land, scattering/emission
over ocean (ALG#1)
-
pr1.mon - Monthly rainfall (mm) using ALG#1
-
pf2.mon - Monthly mean rain fraction (0 - 1.0) using 37 GHz scattering algorithm over land, emission over ocean
(ALG#2)
-
pr2.mon - Monthly rainfall (mm) using ALG#2
-
ssa.mon - Monthly mean sampling fraction (0 - 1.0)
-
ice.mon - Monthly mean sea-ice cover (0 - 100 percent)
-
snw.mon - Monthly mean snow cover fraction (0 - 1.0 percent)
-
wvp.mon - Monthly mean total precipitable water (mm)
A sample segment of FORTRAN code to read the data:
PROGRAM SSMIREAD
CHARACTER*6 FNAME
REAL*4 SSMI(144,72)
C
C OPEN FILE
C
FNAME='PRE.MON'
OPEN(UNIT=10,FILE=FNAME,RECL=576,FORM='UNFORMATTED')
C
C LOOP OVER ENTIRE TIME SERIES (1987-2002)
C
DO I=1,196
READ(10) SSMI
C CALL YOUR PROCESSING ROUTINE HERE
END DO
C
STOP
END
It should be noted that the datasets are generated in a PC environment. Users working on other platforms; will likely have to change the byteorder and/or be faced with chosing "little endian" or "big endian". See the notes section at the bottom for some routines to handle this.
Period of Record
-
The data set begins in 1987 and is continually updated with current data. The first valid month of data is July 1987. Due to F8 instrument failure, there is no valid data for December 1987. There is no valid data for cfr, lwp, pf1, and pr1 for July 1990 - December 1991.
Notes
The original data is generated in a PC environment. If the data is used in other computer environments, the data may have to be byte-swapped (e.g., byte 1 becomes byte 4, byte 2 becomes byte 3, etc.). Also, some users will also have to be concerned with "little endian"/"big endian" issue depending on your platform. A few helpful application tips:
program format_ssmi
! Description: Reads in one year's worth of SSM/I monthly average sea ice
! concentrations, on a 1x1 degree grid, and writes the data
! to a new file as text for use by other programs.
!
! Usage: format_ssmi <input_file> <output_file>
!
! e.g. format_ssmi ice.87 ice.87.txt
!
! Written by: Steve Phipps 22 July 2002
implicit none
! Define parameters
integer, parameter :: nx=360, ny=180, inunit=10, outunit=11
! Declare local variables
integer :: j, k, record
real, dimension(nx, ny) :: data
real, dimension(nx) :: temp
character(len=40) :: infile, outfile
! Get file names
call getarg(1, infile)
call getarg(2, outfile)
! Open input file for read access
open (unit=inunit, file=infile, status='old',
recl=nx, &
form='unformatted',
action='read', access='direct')
! Open output file for write access
open (unit=outunit, file=outfile, status='new', form='formatted', &
action='write')
! Loop over months
do k = 1, 12
! Read in data for this month
do j = 1, ny
! Set record number
record = ny *(k-1) + j
! Read this record
read (inunit,rec=record) temp
! Add record to data array
data(:, j) = temp
end do
! Write data for this month to output file
write (outunit, *) data
end do
! Close files
close (inunit)
close (outunit)
end program format_ssmi
b. The products are generated from a single satellite with observation times near 6 am/pm local time and contain
the following:
-
F8 satellite: July 1987 - June 1991
-
F11 satellite: January 1992 - April 1995
-
F13 satellite: May 1995 - present
-
There are some missing data periods as noted above.
-
Problem Areas
a. The 2.5 degree pr2 and pf2 products have artifacts along coastlines and the sea-ice boundary. These are due
to the larger footprint sizes of the lower frequency channels (e.g., 19 GHz) used in the retrievals.
b. The 1 degree rainfall product does not make a distinction between "zero" rain and "indeterminate" rain (due
to snow and ice cover), which is indicated as zero rain. It is done within the 2.5 degree products.
-
Data Set Producer
Ralph Ferraro, E/RA2
NOAA/NESDIS/Office of Research and Applications
Hydrology Team
5200 Auth Road, Room 601
Camp Springs, MD 20746
301-763-8251 x147
Ralph.R.Ferraro@noaa.gov
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