Hi Jim -
The problem isn't human-readability, though. The problem is that when you
generate a file that has, say, the mean temperature for each month,
sometimes
over a period of years, there are no 'days' in the process. Any data
that represents
February goes into the February bin, whether 28 or 29 days; although
March is always
31 days long, its mid-point is a different number of 'days since' the
beginning of
the year.
In the case of the file Ajay presented, time is a singleton, and its
value represents
the center point of the first 3 months of the years 1955-2012. That
can't be accurately
expressed as a number of days, only as months.
Is there a trac ticket for climatology data? If not, do we need one?
Cheers - Nan
On 9/17/15 9:32 AM, Jim Biard wrote:
> Nan,
>
> The problem is, udunits defines a month as having a specific length of
> year/12 = 30.44 days, so if you use udunits to convert to anything
> else, you won't end up where you think you will. The better practice
> is to use days. It's not as "human readable", but it's the only way to
> do proper conversions between time bases.
>
> Grace and peace,
>
> Jim
>
> On 9/17/15 9:06 AM, Nan Galbraith wrote:
>> While it's true that 'month is not a proper unit of measure',
>> climatologies do in fact use months, not days, in calculating
>> mean values. Adhering to udunits/CF in this regard could make
>> the dates less easily understood.
>>
>> Regards - Nan
>>
>> On 9/11/15 1:34 PM, Karl Taylor wrote:
>>> Dear Ajay,
>>>
>>> Since "month" is not a proper unit of measure, convert your times to
>>> days and use a unit "days since ...".
>>>
>>> Also, it is normally a bad idea to have your base time set to a date
>>> before the switch from Julian to Gregorian calendar. I suggest using
>>> a base time of "1955-01-01" (i.e., the beginning of your
>>> climatological period).
>>>
>>> I think the cell_methods should be:
>>> cell_methods="area: mean depth: mean time: mean within years time:
>>> mean over years";
>>>
>>> The time bounds should be (expressed in date/time format):
>>>
>>> climatology_bounds = "1955-01-01", "2012-04-01"
>>>
>>> and you can choose your time coordinate value as you think most
>>> appropriate, e.g.,
>>> the middle of the season of the first year of the climatology, or
>>> the beginning of the first month of the first year of the
>>> climatology, or
>>> the middle of the season of the middle year of the climatology, or
>>>
>>> ???
>>>
>>>
>>> Hope someone confirms this, as sometimes I make a mistakes.
>>>
>>> Karl
>>>
>>>
>>>
>>>
>>> On 9/11/15 9:54 AM, Ajay Krishnan - NOAA Affiliate wrote:
>>>> Dear CF members,
>>>>
>>>> I would like your input on the way climatological stats are being
>>>> represented in a file that I am working on. I believe that I am not
>>>> using the time and the climatological_bounds properly:
>>>>
>>>> Seasonal SST
>>>>
>>>> Average seasonal temperature (Jan-Mar) for 6 decades (1955-2012)
>>>>
>>>> dimensions:
>>>> time=1;
>>>> nv=2;
>>>> variables:
>>>>
>>>>
>>>> double time(time);
>>>> time:climatology="climatology_bounds";
>>>> time:units="months since 0000-01-01";
>>>> double climatology_bounds(time,nv);
>>>>
>>>>
>>>> double climatology_bounds(time,nv);
>>>>
>>>> climatology_bounds:comment=? This variable defines the bounds of
>>>> the climatological time period for each time? ;
>>>>
>>>> float t_mn(time,lat,lon,depth);
>>>> t_mn:standard_name: ?sea_water_temperature? ;
>>>>
>>>> t_mn:long_name: ?Average of all unflagged interpolated values at
>>>> each standard depth level for sea_water_temperature in each
>>>> grid-square which contain at least one measurement.? ;
>>>>
>>>> t_mn:cell_methods: ?area: mean depth: mean time: mean? ;
>>>>
>>>>
>>>> data: // time coordinates translated to date/time format
>>>> time= ?1.5? ;
>>>>
>>>> climatology_bounds=?0.0?, ?3.0? ;
>>>>
>>>>
>>>> The CF examples are helpful but my case is different where in I
>>>> have just 1-time co-ordinate in my file. In the above case, what is
>>>> the best way to record time and climatology bounds?
>>>>
>>>>
>>>> Thanks,
>>>>
>>>> Ajay
>>>>
>>>>
>>
--
*******************************************************
* Nan Galbraith Information Systems Specialist *
* Upper Ocean Processes Group Mail Stop 29 *
* Woods Hole Oceanographic Institution *
* Woods Hole, MA 02543 (508) 289-2444 *
*******************************************************
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