Hi Steve, Cecelia:
I agree we should move to a better default calendar, probably
proleptic_gregorian. We are stuck with mixed Gregorian for previous CF
versions.
I think we just need a proposal that says "As of version X, the default
calender is proleptic_gregorian. You should explicitly add the calendar
attribute for clarity. Udunits is no longer considered to be the
reference software for date/time."
It would be nice to add advice to the user about calendar tradeoffs and
how to do historical date matching for modelers, assuming we have useful
things to say.
I wonder if allowing ISO formatted strings in place of / in addition to
the "time since reference date" form might be useful for historical time
matching?
Arguably having calendar reference libraries in Python and Java would be
sufficient, possibly Python is preferable even to one in C.
John
On 12/6/2012 1:45 PM, Cecelia DeLuca wrote:
>
> Hi John and all,
>>
>> Thanks for this information. A few questions:
>>
>> * So you are not supporting "standard Gregorian calendar" even though
>> thats the CF default?
> Correct, the climate modelers that we work with don't use it. AFAIK the
> decision of
> CESM was to reject the CF default as unreasonable for modeling and use
> proleptic
> Gregorian. GFDL might support it, I don't know.
>
> We could probably add standard Gregorian as a new calendar if people on the
> data side needed it. However, we would be happier to join Steve in putting
> a stake in its heart!
>>
>> * Do modelers need to match "historical dates"? If so, what calendar
>> do they use?
> I think the calendar used would depend on what was supported by the
> model and
> configuration, as well as the form of the date. If you wanted to
> represent the date
> of something like a volcanic eruption you could probably make it work
> with most
> of the calendars.
>
>>
>> * Is the time library suitable to be released seperate from the ESMF
>> code, eg as standalone C++?
> The time library can be used apart from other ESMF interfaces, and some
> modeling groups do use it that way. I don't think we'd be willing to
> extract that
> part and support a separate build. We did that years ago and it was a
> pain
> to maintain. (We could try to isolate the documentation though, so users
> didn't have to wade through the whole reference manual to find the API.)
>
> With ESMP(ython), the scope of the interface is much smaller than
> ESMF proper and I think Ryan could set time functions up nicely. It might
> make sense to represent it as a separate python package in that approach.
> Would python work for you, or would you really prefer C?
>
> - Cecelia
>
>>
>> John
>>
>> On 12/5/2012 3:01 PM, Cecelia DeLuca wrote:
>>> Hi John and all,
>>>
>>> ESMF has a mature time management library with calendars that are
>>> commonly
>>> used in climate/weather/related modeling. It supports the following: 360
>>> day,
>>> no leap, Gregorian (proleptic), Julian, Julian day, modified Julian day,
>>> and custom
>>> (including custom calendars that may change the length of days). ESMF,
>>> like CESM,
>>> doesn't support the standard Gregorian calendar because it doesn't make
>>> sense for modeling groups who may need to cross the Julian/Gregorian
>>> weirdness line (we've never gotten a request to support standard
>>> Gregorian from
>>> modelers). ESMF has calendar, time, time interval, clock, and alarm
>>> constructs,
>>> can run time forward or backward, and prints times and time intervals in
>>> ISO8601
>>> format. CESM/CAM, NASA, NOAA, Navy and other codes use it.
>>>
>>> The most complete interface to the time lib is Fortran, and there are
>>> some basic
>>> time functions exposed in C (the lib is actually written in C++).
>>> However, we could
>>> wrap the time functions in Python and include them in ESMP, which is
>>> currently a
>>> set of ESMF regridding functions wrapped in Python. We could probably do
>>> that
>>> in the late/winter spring timeframe, with some guidance on what
>>> functions were
>>> desired and a pass through our prioritization board.
>>>
>>> Best,
>>> -- Cecelia
>>>
>>>
>>> On 12/5/2012 12:25 PM, John Caron wrote:
>>>> Hi all:
>>>>
>>>> Its probably the right thing to do to make gregorian ("Mixed
>>>> Gregorian/Julian calendar") the default calendar for COARDS/CF, for
>>>> backwards compatibility. However, CDM may leave proleptic_gregorian
>>>> (ISO8601) as its default.
>>>>
>>>> And I would strongly suggest that data writers stop using "time since
>>>> 1-1-1". Ive never seen a dataset where "time since 1-1-1" using the
>>>> mixed gregorian calendar was actually needed. If any one has a real
>>>> example, Id like to hear about it.
>>>>
>>>> If you really need "historical accuracy", then put in an ISO8601
>>>> formatted string, and an explicit calendar attribute. CDM handles
>>>> those ok. CF should be upgraded to allow ISO strings also. "time since
>>>> reference date" is not good for very large ranges of time.
>>>>
>>>> Ill just add that udunits never wanted to be a calendaring library,
>>>> and shouldnt be used anymore for that. Im relying on joda-time (and
>>>> its successor threeten) to be the expert in calendering, so i dont
>>>> have to. I think the netcdf-C library now uses some CDAT (?) code for
>>>> its calendaring, but Im sure theres other standard libraries that
>>>> could be used. Anyone have candidate libraries in C or Python for
>>>> robust calendering>
>>>>
>>>> In short, we should rely on clear encoding standards (eg ISO8601) with
>>>> reference software, rather than implementations like udunits that
>>>> eventually go away.
>>>>
>>>> John
>>>>
>>>> PS: I think ill cross post to cf, just to throw some gasoline on the
>>>> fire ;), and maybe some broader viewpoints.
>>>>
>>>> On 12/5/2012 10:24 AM, Don Murray (NOAA Affiliate) wrote:
>>>>> Hi Gerry-
>>>>>
>>>>> On 12/5/12 9:42 AM, Gerry Creager - NOAA Affiliate wrote:
>>>>>> There are other datasets with reference to 1-1-1. I've seen them most
>>>>>> recently in some ocean models.
>>>>>
>>>>> And the ESRL/PSD NCEP reanalysis datasets use it.
>>>>>
>>>>> Don
>>>>>
>>>>>> On Wed, Dec 5, 2012 at 10:16 AM, Don Murray (NOAA Affiliate)
>>>>>> <don.murray at noaa.gov <mailto:don.murray at noaa.gov>> wrote:
>>>>>>
>>>>>> John-
>>>>>>
>>>>>> I meant to send this to support-netcdf-java, but perhaps
>>>>>> others on
>>>>>> the list might have the same problem.
>>>>>>
>>>>>>
>>>>>> On 12/4/12 4:51 PM, John Caron wrote:
>>>>>>
>>>>>> On 12/4/2012 4:09 PM, Don Murray (NOAA Affiliate) wrote:
>>>>>>
>>>>>> Hi-
>>>>>>
>>>>>> I was just trying to access the NOAA/ESRL/PSD Outgoing
>>>>>> Longwave
>>>>>> Radiation (OLR) data using netCDF-Java 4.3 ToolsUI and
>>>>>> noticed that the
>>>>>> times are wrong. If you open:
>>>>>>
>>>>>>
>>>>>> dods://www.esrl.noaa.gov/psd/__thredds/dodsC/Datasets/__uninterp_OLR/olr.day.mean.nc
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> <http://www.esrl.noaa.gov/psd/thredds/dodsC/Datasets/uninterp_OLR/olr.day.mean.nc>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> in the ToolsUI grid viewer, the last time in the file is
>>>>>> shown as
>>>>>> 2012-12-04 00Z. However, the last time in the file is
>>>>>> actually
>>>>>> 2012-12-02 00Z. Here is the time variable in that file:
>>>>>>
>>>>>> double time(time=3989);
>>>>>> :units = "hours since 1-1-1 00:00:0.0";
>>>>>> :long_name = "Time";
>>>>>> :actual_range = 1.7540448E7, 1.763616E7; // double
>>>>>> :delta_t = "0000-00-01 00:00:00";
>>>>>> :avg_period = "0000-00-01 00:00:00";
>>>>>> :standard_name = "time";
>>>>>> :axis = "T";
>>>>>>
>>>>>> netCDF-Java 4.2 and ncdump -t -v time (C version) show
>>>>>> the
>>>>>> correct
>>>>>> date/times.
>>>>>>
>>>>>>
>>>>>> hours from 1-1-1 is rather problematic, since you are
>>>>>> crossing
>>>>>> the
>>>>>> julian/gregorian weirdness line (i think thats the technical
>>>>>> term ;)
>>>>>>
>>>>>> Im guessing the trouble lies here:
>>>>>>
>>>>>> "Default calendar: for udunits, and therefore for CF, the
>>>>>> default
>>>>>> calendar is gregorian ("Mixed Gregorian/Julian calendar").
>>>>>> For
>>>>>> CDM, the
>>>>>> default calendar is proleptic_gregorian (ISO8601 standard).
>>>>>> This
>>>>>> only
>>>>>> matters for dates before 1582."
>>>>>>
>>>>>>
>>>>>> Joda time supports the GJ calendar (Historically accurate
>>>>>> calendar
>>>>>> with Julian followed by Gregorian) which seems it would be
>>>>>> backward
>>>>>> compatible with the CF/udunits. Perhaps that should be the
>>>>>> default
>>>>>> for backward compatibility.
>>>>>>
>>>>>>
>>>>>> I have to say relying uncritically on a calendar
>>>>>> implementation like
>>>>>> udunits is a mistake. putting the reference date
>>>>>> unnecessarily to
>>>>>> include the problem is, um, unnecessary.
>>>>>>
>>>>>>
>>>>>> But it is historically accurate. For climate datasets, this
>>>>>> would
>>>>>> be important.
>>>>>>
>>>>>>
>>>>>> is there any way those files can be updated? specifying the
>>>>>> gregorian
>>>>>> calendar explicitly should do it, but changing to use a
>>>>>> reference date
>>>>>> after 1582 would be much better.
>>>>>>
>>>>>>
>>>>>> How's your FORTRAN? ;-) I'm not sure why this was chosen
>>>>>> originally, but it doesn't seem reasonable to make people change
>>>>>> their datasets.
>>>>>>
>>>>>> Does anyone else on the list know of datasets (other than
>>>>>> climatologies) that might use a reference of 1-1-1 that will be
>>>>>> affected by this change?
>>>>>>
>>>>>>
>>>>>>
>>>>>> BTW, is there an easier way to see human readable dates
>>>>>> through toolsUI
>>>>>> than loading it into the grid viewer (akin to ncdump -t)?
>>>>>>
>>>>>>
>>>>>> open in coordSys tab; in bottom table, select time coord,
>>>>>> right-click
>>>>>> and choose "show values as date"
>>>>>>
>>>>>>
>>>>>> Thanks, that's easier.
>>>>>>
>>>>>>
>>>>>> Don
>>>>>> --
>>>>>> Don Murray
>>>>>> NOAA/ESRL/PSD and CIRES
>>>>>> 303-497-3596 <tel:303-497-3596>
>>>>>> http://www.esrl.noaa.gov/psd/__people/don.murray/
>>>>>> <http://www.esrl.noaa.gov/psd/people/don.murray/>
>>>>>>
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>>>>>>
>>>>>>
>>>>>
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Received on Thu Dec 06 2012 - 14:37:30 GMT
