Nan,
That's not true though. When we do monthly climatologies here, we pick
the 15th of each month as the time, and the climatological bounds
capture that the temporal cell covers the 1st to the Nth of each month.
You don't build the climatology on the time variable alone.
Ajay just needs to pick a day, such as Feb 15, for his time variable,
and specify the time and climatological bounds values as:
time = 1955-02-15
climatology = 1955-01-01, 2012-04-01
(expressing the dates relative to the epoch date given in the time
variable units attribute, and expressing the cell method as being 'time:
mean within years time: mean over years'). That is accurate, is properly
convertible from one set of time units to another, and is what CF
prescribes.
Grace and peace,
Jim
On 9/17/15 9:54 AM, Nan Galbraith wrote:
> 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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Received on Thu Sep 17 2015 - 09:37:35 BST
