We use an I/O library layered on top of the netCDF library. With this
library we can read and write netCDF files with time axes with several
conventions, but internally, our EPS I/O library always returns to the
calling routines a two-integer array in which the first integer is the
"true Julian Day Number" with units of days, and the second integer is
the number of milliseconds since midnight of the true Julian Day. This
two-integer time word represents time periods spanning centuries with
millisecond accuracy. Some other oceanographic institutions are also
using this representation of time in netCDF files.
Our EPS I/O library includes a complete set of routines for
manipulation, calculation, and character string representation of our
standard two-integer representation of time. The representation of the
time axis within the netCDF file can be of several types, for both read
and write, including the UDUNITS standard, the double integer array
which is used internally by the EPS routines, and some other time
representations which are supported for compatibility with other
in-house software packages. Time axes can be written or read in either
real or integer format. However, regardless of the format of the time
axis in the netCDF data file itself, the values used internally by the
EPS library will be the double integer array.
NOTE: We are using the astronomer's True Julian Day (eg, May 23, 1968 is
2,400,000), which should not be confused with the "year-day" (eg, Feb 2
is year-day 33). The "year-day" is frequently called julian day
(incorrectly) by oceanographers and meteorologists. Our two-integer
time word (word1=True Julian Day, word2=milliseconds since midnight of
the True Julian Day) allows millisecond accuracy for time periods
extending over centuries.
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Nancy N. Soreide Phone: 206-526-6728
NOAA/PMEL/OCRD FAX: 206-526-6744
7600 Sand Point Way NE OMNET: TAO.PMEL
Seattle, WA 98115 Internet: nns@pmel.noaa.gov