>Jonathan Gregory j.m.gregory at reading.ac.uk
>Sat Jan 5 06:48:34 MST 2013
>
>Dear John
>
>The same section 7.4 of the CF standard describes the CF convention for this,
>which uses the bounds of the time coordinates to encode the range of years
>to which the data applies e.g. Examples 7.8 and 7.9.
>
>Is this what you mean?
Thanks for the pointer Jonathan, but my issue is with how to set the
units and values for climatological time in a sensible way.
For ocean hydrographic data we have observations as far back as early
1900s, but there are very ver few data unti the 1940s, and the data
density obviously increases dramatically in recent times. To me, the
climatology_bounds in Example 7.8 at
http://cf-pcmdi.llnl.gov/documents/cf-conventions/1.4/cf-conventions.html#climatological-statistics
tend to imply a uniform average over time for the given interval,
whereas here we have whatever data we could get, and it's highly
unevenly distributed in time. Moreover, we use a weighted least
squares method over time so the average for "February" actually has
(down-weighted) contributions from January and March. We might also
have derived the time variability from a harmonic fit in time, in
which case ALL data enter into every month.
But by main issue is that in Example 7.8 the "time" data entered in
the file is still described as having units "days since 1960-1-1"
which really isn't so. It is equally logically "days since 1991-1-1".
In reality it's just "days since Jan 1 of any year".
This convention doesn't prevent the practice of assigning time values thus:
double time(time);
time:units="days since 1960-1-1";
data:
time = 15 45 75 ...
double time(time);
time:units="days since 1959-1-1";
data:
time = 380 410 440 ...
udunits would consider these times to be identical, but for a monthly
climatology starting the counting at 380 is just nuts.
So it seems a standard for expressing climatology time needs some thought.
Thanks, John.
--
John L Wilkin | jwilkin at rutgers.edu | ph: +1-609-630-0559
web: marine.rutgers.edu/~wilkin | cal: tinyurl.com/jwilkincalendar
Received on Mon Jan 07 2013 - 10:37:36 GMT
