[CF-metadata] 'months since' and 'years since' time units

From: Bärring Lars <Lars.Barring>
Date: Thu, 18 Oct 2018 08:29:50 +0000

Hi,

I have have come to think about this from a somewhat different perspective. For some analyses, as well as when calculating certain derived climatological statistics (aka climate indices), using datasets based on different calendars the problem becomes obvious.

In the model world of a 365-day GCM one year _is_ 365 days, and in a 360_day GCM a year _is_ 360 days. In the case of a gregorian/standard calendar GCM I am not sure whether it is 365.25 or 365.242198781 but this is in practice less of a problem.

For datasets based non-standard calendars imposing the current UDUNITS definition of a year leads to complications that require workarounds if one is interested in for example the time elapsed until something happens or the duration of some (long-lasting) events. One way to partly mitigate these issues would be to use the time unit of years_since_START or months_since_START, but this is warned against in the CF Conventions and may software tools do not support it .

The fundamental issue is the inconsistency between the GCM year and the UDUNITS year. So I would like to call on the wisdom of this list to see whether the CF Convention could include a modification to the definition of a year and month:

* standard calendar (no change)
1 day = 84600 seconds
1 year = 365.242198781 days
1 month = 365.242198781 / 12 days

* 365_day calendar
1 day = 84600 seconds
1 year = 365 days
1 month = 365 / 12 days

* 360_day calendar
1 day = 84600 seconds
1 year = 360 days
1 month = 360 / 12 days

That is, the seconds per day ratio is not changed thus maintaining the consistency to other SI units. And, for the 360_day calendar month follows the suggestion by Ryan and Jeffrey.


Kind regards,
Lars

--
Lars B?rring
FDr, Forskare
PhD, Research Scientist
SMHI  /  Swedish Meteorological and Hydrological Institute
Rossby Centre
SE - 601 76 NORRK?PING
Tel / Phone: +46 (0)11 495 8604
Fax: +46 (0)11 495 8001
Bes?ksadress / Visiting address: Folkborgsv?gen 17
________________________________
Fr?n: CF-metadata [cf-metadata-bounces at cgd.ucar.edu] f?r Ryan Abernathey [ryan.abernathey at gmail.com]
Skickat: den 17 oktober 2018 21:22
Till: whitaker.jeffrey at gmail.com
Kopia: cf-metadata at cgd.ucar.edu
?mne: Re: [CF-metadata] 'months since' and 'years since' time units
Hi everyone,
I am that user, and I'm new to this mailing list. Thank you all for your work on CF conventions. It's such a valuable effort!
I want to note that this was inspired by the proliferation of datasets in the wild that use "month" as their units. For example, nearly all of the IRI Data Library does this, in conjunction with a 3"60_day" calendar (example: https://iridl.ldeo.columbia.edu/SOURCES/.NOAA/.NCEP-NCAR/.CDAS-1/.MONTHLY/.Diagnostic/.surface/.temp/).
My impression from talking to data providers is that no one is using "360_day" calendar and "months" as units, and then expecting "months" to be interpreted as 365.242198781/12 days. They all expect it to be interpreted as 30 days. While there are various workarounds that can be used at different levels of the software stack, the best solution, IMHO, is to explicitly allow in CF conventions what Jeff proposed: "months and years be interpreted as calendar months and years for those calendars where they have a fixed length". I don't think this will break existing applications.
Thanks,
Ryan
On Wed, Oct 17, 2018 at 3:06 PM Jeffrey Whitaker <whitaker.jeffrey at gmail.com<mailto:whitaker.jeffrey at gmail.com>> wrote:
Hi:  I'm a developer of the 'cftime' python package (https://github.com/Unidata/cftime).  A user submitted a pull request (https://github.com/Unidata/cftime/pull/69) that implements support for a 30-day calendar month time unit for the '360_day' CF calendar.  Although using a 'month' time unit is a tricky proposition in general, for this calendar it seems straightforward since every month has the same length.  However, in the discussion of the pull request it was pointed out that CF expects  that "the value of the units attribute is a string that can be recognized by UNIDATA?s Udunits package", and that UDUNITS defines a month as 365.242198781/12 days.    My question is this - is it reasonable for our python package to make an exception to this rule for the 360_day calendar?  More generally, can months and years be interpreted as calendar months and years for those calendars where they have a fixed length, or will this deviate from the existing CF conventions and break existing applications?
Regards, Jeff
--
Jeffrey S. Whitaker
NOAA/OAR/PSD  R/PSD1
325 Broadway, Boulder, CO, 80305-3328
Phone: (303)497-6313
FAX: (303)497-6449
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Received on Thu Oct 18 2018 - 02:29:50 BST