Data Description

 

Description of "Current Data"

Table of Contents:

File types
Grid structure
File structure
File size
File name
Unit of measurement
Special data value
Sample Fortran read programs and outputs
Anonymous FTP access

File types

Instantaneous: The instantaneous flux are available at hourly interval. They are reported at the nominal time of satellite observations viz. 00:15 utc, 01:15 utc, 02:15 utc, ..., 23:15 utc from the year 1996 onward.
Hourly average: The hourly average flux is an estimate of the flux averaged for an hour ending at the hour in local standard time. It is obtained from the instantaneous flux by normalizing it to the average solar zenith angle for the hour.
Daily average: The daily average flux is the 24-hour average of the flux for a day.
In calculating the daily average, missing day-time hours are filled in by replicating data from non-missing hours one hour before the missing ones. If the hour one before the missing one is also missing filling is done from the closest hour after the missing one. If all day-time hours are missing no filling is done. Fluxes at a missing hour are obtained by multiplying the non-missing fluxes by the ratio of the solar zenith angle cosine at the missing hour to that at the non-missing hour. In addition, the daily average flux, obtained from summing up the hourly values, is multiplied by the ratio of numerical-to-analytical integral of the daily top of the atmosphere downward flux. The latter procedure is intended to correct for potential inaccuracies in the numerical integration of hourly fluxes.

Non-filled and non-corrected daily averages can be obtained from the hourly averages. [Top]

Grid structure

Before July 1, 2001:

Each file has information for 5661 (0.5 by 0.5 degree, equal-angle) cells (51 latitudes and 111 longitudes). All cells are present, even when data are missing. The cells are written sequentially, proceeding eastward through the latitude zone, then northward to the next latitude zone. The center lat/lon coordinates of the first cell are 25 N, 125 W, and those of the last cell are 50 N, 70 W.

The following Fortran statements could be used to calculate the center coordinates of cells:

 
      INTEGER  Nlat, Nlon, Lat, Lon
      REAL  Lat1, Lon1, Reslat, Reslon
      PARAMETER  ( Nlat=51, Nlon=111 )
      REAL  Glat( Nlon, Nlat ), Glon( Nlon, Nlat )
      
C            ** Calculate latitude and longitude coordinates of
C            ** cell centers

      Reslat = 0.5
      Reslon = 0.5
      Lat1   = 25.0
      Lon1   = -125.0

      DO 20 Lat = 1, Nlat
         Glat( Lat ) = ( Lat-1 ) * Reslat + Lat1
20    CONTINUE       
      DO 30 Lon = 1, Nlon
         Glon( Lon ) = ( Lon-1 ) * Reslon + Lon1
30    CONTINUE
                                   

Here Glat and Glon are the latitude and longitude coordinates of cell-centers, Reslat and Reslon are the distances between cell-centers in latitude and longitude (resolution), Lat1 and Lon1 are the latitude and longitude coordinates of the center of first cell , respectively. All quantities are in degrees. [Top]

Since July 1, 2001:

Each file has information for 7381 (0.5 by 0.5 degree, equal-angle) cells (61 latitudes and 121 longitudes). All cells are present, even when data are missing. The cells are written sequentially, proceeding eastward through the latitude zone, then northward to the next latitude zone. The center lat/lon coordinates of the first cell are 24 N, 126 W, and those of the last cell are 54 N, 66 W.

The following Fortran statements could be used to calculate the center coordinates of cells:

 
      INTEGER  Nlat, Nlon, Lat, Lon
      REAL  Lat1, Lon1, Reslat, Reslon
      PARAMETER  ( Nlat=61, Nlon=121 )
      REAL  Glat( Nlon, Nlat ), Glon( Nlon, Nlat )
      
C            ** Calculate latitude and longitude coordinates of
C            ** cell centers

      Reslat = 0.5
      Reslon = 0.5
      Lat1   = 24.0
      Lon1   = -126.0

      DO 20 Lat = 1, Nlat
         Glat( Lat ) = ( Lat-1 ) * Reslat + Lat1
20    CONTINUE       
      DO 30 Lon = 1, Nlon
         Glon( Lon ) = ( Lon-1 ) * Reslon + Lon1
30    CONTINUE
                                   

Here Glat and Glon are the latitude and longitude coordinates of cell-centers, Reslat and Reslon are the distances between cell-centers in latitude and longitude (resolution), Lat1 and Lon1 are the latitude and longitude coordinates of the center of first cell , respectively. All quantities are in degrees. [Top]

File structure

Before July 1, 2001:

Data for a particular parameter and time resolution for a day are contained in a single file. Instantaneous and hourly average files contain all 24 hours for a day even if data are missing. For the instantaneous files the hours are UTC (0-23), while those for the hourly average files are local standard time (1-24).

In the instantaneous and hourly average files, all cells are written for a given hour of a given day. Writing of data proceeds from the first hour to the last of a day. That is:

    (lon1,lat1,h1)   (lon2,lat1,h1)   ... (lon111,lat1,h1)
         ... 
    (lon1,lat51,h1)  (lon2,lat51,h1)  ... (lon111,lat51,h1)
    (lon1,lat1,h2)   (lon2,lat1,h2)   ... (lon111,lat1,h2)
         ...
         ...
    (lon1,lat51,h24) (lon2,lat51,h24) ... (lon111,lat51,h24)

In the daily average files, data are written as follows:

    (lon1,lat1)  (lon2,lat1)  ... (lon111,lat1)
        ...
    (lon1,lat51) (lon2,lat51) ... (lon111,lat51)

where lon=longitude, lat=latitude, h=hour.

All files are written unformatted with direct access. The logical record length is 444 bytes (111 columns times 4 bytes). The floating point binary data conform to the little endian specification, and were written on a DEC workstation. Here is a sample Fortran statement to open a file  (e.g., 990201sda.i):

        OPEN ( Unit=1, File='990201sda.i', Status='OLD',
   &       Form='UNFORMATTED', Access='DIRECT', Recl=Lrec )
             

where Lrec is the record length (444 for compilers that require bytes for Recl, or 111 for those that need words for Recl). [Top]

Since July 1, 2001:

Data for a particular parameter and time resolution for a day are contained in a single file. Instantaneous and hourly average files contain all 24 hours for a day even if data are missing. For the instantaneous files the hours are UTC (0-23), while those for the hourly average files are local standard time (1-24).

In the instantaneous and hourly average files, all cells are written for a given hour of a given day. Writing of data proceeds from the first hour to the last of a day. That is:

    (lon1,lat1,h1)   (lon2,lat1,h1)   ... (lon121,lat1,h1)
         ... 
    (lon1,lat61,h1)  (lon2,lat61,h1)  ... (lon121,lat61,h1)
    (lon1,lat1,h2)   (lon2,lat1,h2)   ... (lon121,lat1,h2)
         ...
         ...
    (lon1,lat61,h24) (lon2,lat61,h24) ... (lon121,lat61,h24)

In the daily average files, data are written as follows:

    (lon1,lat1)  (lon2,lat1)  ... (lon121,lat1)
        ...
    (lon1,lat61) (lon2,lat61) ... (lon121,lat61)

where lon=longitude, lat=latitude, h=hour.

All files are written unformatted with direct access. The logical record length is 484 bytes (121 columns times 4 bytes). The floating point binary data conform to the little endian specification, and were written on a DEC workstation. Here is a sample Fortran statement to open a file  (e.g.,010701sda.i):

        OPEN ( Unit=1, File='010701sda.i', Status='OLD',
   &       Form='UNFORMATTED', Access='DIRECT', Recl=Lrec )
             

where Lrec is the record length (484 for compilers that require bytes for Recl, or 121 for those that need words for Recl). [Top]

 

Unit of measurement: [Top]

Fluxes are given in Wm-2

Special data value [Top]

Missing data are represented by -999.

File names [Top]

instantaneous : yymmddppp.i
hourly : yymmddppp.h
daily : yymmddppp.d

where

yy : year (two digits),
mm : month (two digits),
dd: day (two digits),
ppp : parameter (three characters), their values are:

sda: surface downward flux,
par: photosynthetically active radiation,
tda: top of atmosphere downward flux,
tua: top of atmosphere upward flux.

All files are compressed using the gzip utility, and all have the extension ".gz" attached to the above names.

 

File size [Top]

File type Uncompressed size (bytes) Compressed size (bytes) (varies, an example is given)
instantaneous 543456 ~65K
hourly 543456 ~65K
daily 22644 ~6.5K

 

Sample Fortran Read Programs [Top]

Note: The floating point binary data conform to the little endian specification, and were written on a DEC workstation. To read the data on a big endian machine, instructions were prepared and placed at the web site:
http://metosrv2.umd.edu/~srb/gcip/faq.htm
instantaneous files (before July 1, 2001)
instantaneous files (since July 1, 2001)
hourly files (before July 1, 2001)
hourly files (since July 1, 2001)
daily average files (before July 1, 2001)
daily average files (since July 1, 2001)

Sample Outputs

instantaneous files
hourly files
daily average files

 

Anonymous FTP access [Top]

To access the current data using anonymous FTP, 'ftp' to metosrv6.umd.edu, then change directory to /pub/srb/gcip/current.

Last modified on Feb 4, 2000

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