Description of "Current Data"
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Table of Contents:
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 24hour average of the flux for
a day.
In calculating the daily average, missing daytime hours are filled in by replicating data
from nonmissing 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 daytime hours are missing no filling is done. Fluxes at a missing hour are
obtained by multiplying the nonmissing fluxes by the ratio of the solar zenith angle
cosine at the missing hour to that at the nonmissing hour. In addition, the daily average
flux, obtained from summing up the hourly values, is multiplied by the ratio of
numericaltoanalytical 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. Nonfilled and noncorrected 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, equalangle)
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 ) = ( Lat1 ) * Reslat + Lat1
20 CONTINUE
DO 30 Lon = 1, Nlon
Glon( Lon ) = ( Lon1 ) * Reslon + Lon1
30 CONTINUE
Here Glat and Glon
are the latitude and longitude coordinates of cellcenters, Reslat
and Reslon are the distances between
cellcenters 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, equalangle)
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 ) = ( Lat1 ) * Reslat + Lat1
20 CONTINUE
DO 30 Lon = 1, Nlon
Glon( Lon ) = ( Lon1 ) * Reslon + Lon1
30 CONTINUE
Here Glat and Glon are
the latitude and longitude coordinates of cellcenters, Reslat
and Reslon are the distances between cellcenters 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 (023), while those for the hourly
average files are local standard time (124).
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 (023), while those for the hourly
average files are local standard time (124).
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
Sample Outputs
FTP access [Top]
To access the current data,
please regist, and get usrname and password, then go to http://dsrs.atmos.umd.edu/DATA/Sda_03/srb/gcip/current/.

Last modified on Mar
28, 2016
