In language, definitions are based on usage. Julian date, modulo the
year, is a convention that I have been using for decades to do what you
are talking about but I defer to wiser minds.
J.
On 3/16/17 9:42 AM, Jim Biard wrote:
>
> John,
>
> As best as I understand it, Julian day is a term that is grossly
> misused. Julian Day is defined as the elapsed days since January 1,
> 4713 BCE. Lots of people use the term to refer to day-in-year, but
> this doesn't seem to be a proper usage.
>
> Grace and peace,
>
> Jim
>
>
> On 3/16/17 3:36 PM, John Helly wrote:
>> Sorry to jump in here but isn't this just the Julian day?
>>
>> J.
>>
>>
>> On 3/16/17 8:24 AM, Nan Galbraith wrote:
>>> I agree that there's a lot of interest, and I have 2 questions.
>>>
>>> To make the data most useful, shouldn't the time coordinate variable be
>>> Jan 1, and shouldn't the 'days since' (data) variable represent the
>>> yearday
>>> within that year?
>>>
>>> My specific concerns with Jim's approach:
>>>
>>> first_freeze_date:units = "days since 1900-01-01 00:00:00" - This
>>> doesn't seem
>>> to me to provide the most easily used data point, wouldn't the
>>> year-day be more
>>> convenient, for seeing how this value varies over the years?
>>>
>>> And with Antoio's:
>>>
>>> first_freeze_date:coordinates="threshold time"; - I don't see how
>>> threshold,
>>> which is a temperature, can be a coordinate of this variable. Also,
>>> I'd like to know
>>> why setting time:units="days since 2000-6-1"; is preferable to
>>> using 2000-1-1;
>>> doesn't this invite errors in using the time in applications like
>>> matlab and python?
>>>
>>> Actually, the metadata doesn't tell me how to interpret the values
>>> in first_freeze_date -
>>> the short name implies that they're dates, the units implies they're
>>> elapsed days, but
>>> without a reference date to enable decoding.
>>>
>>> Cheers - Nan
>>>
>>>
>>> On 3/16/17 8:45 AM, Jim Biard wrote:
>>>>
>>>> Hi.
>>>>
>>>> There is clearly interest here! I agree that day_in_year is rather
>>>> generic, and there should probably be a more precise term. I'm not
>>>> so sure about the cell_methods that were suggested below. In my
>>>> particular case the values are derived from a daily Tmin product.
>>>> Each value is the date of the first Tmin < 0 C within the time
>>>> bounds. If it was a spell length, such as growing season length,
>>>> then I can see the need for a more climatological cell_method.
>>>>
>>>> We can keep this up and work up some standard_name definitions to
>>>> propose. I'm sure the results will be better if we collaborate
>>>> compared to what I'd do on my own.
>>>>
>>>> Grace and peace,
>>>>
>>>> Jim
>>>>
>>>>
>>>> On 3/16/17 7:23 AM, Antonio S. Cofi?o wrote:
>>>>> Dear all,
>>>>> There is no standard_name for the concept but there are 2
>>>>> different ones which delimit the approach that it could be used as
>>>>> templates for the new one:
>>>>> *time_when_flood_water_falls_below_threshold
>>>>> *(time_when_flood_water_rises_above_threshold and
>>>>> time_of_maximum_flood_depth are also good examples )
>>>>> http://cfconventions.org/Data/cf-standard-names/41/build/cf-standard-name-table.html#time_when_flood_water_falls_below_threshold_tr
>>>>>
>>>>>> The quantity with standard name
>>>>>> *time_when_flood_water_falls_below_threshold*: is the time
>>>>>> elapsed between the breaking of a levee (origin of flood water
>>>>>> simulation) and the instant when the depth falls below a given
>>>>>> threshold for the last time, having already risen to its maximum
>>>>>> depth, at a given point in space. If a threshold is supplied, it
>>>>>> should be specified by associating a coordinate variable or
>>>>>> scalar coordinate variable with the data variable and giving the
>>>>>> coordinate variable a standard name of flood_water_thickness. The
>>>>>> values of the coordinate variable are the threshold values for
>>>>>> the corresponding subarrays of the data variable. If no threshold
>>>>>> is specified, its value is taken to be zero. Flood water is water
>>>>>> that covers land which is normally not covered by water.
>>>>> the problem is the event definition, which is quite different to
>>>>> the one it's been considered here which is more like a
>>>>> climatological statistics. The good thing is the CF already has
>>>>> some good definitions for those climatological statistics, like
>>>>> Example 7.11 on CF1.6 document:
>>>>> http://cfconventions.org/cf-conventions/v1.6.0/cf-conventions.html#extreme-statistics-and-spell-lengths-ex
>>>>>
>>>>>
>>>>> And more convenient definition of this climatological statistics
>>>>> could be:
>>>>> http://cfconventions.org/Data/cf-standard-names/41/build/cf-standard-name-table.html#spell_length_of_days_with_air_temperature_above_threshold_tr
>>>>>
>>>>>> Air temperature is the bulk temperature of the air, not the
>>>>>> surface (skin) temperature. A spell is the number of consecutive
>>>>>> days on which the condition X_below|above_threshold is
>>>>>> satisified. A variable whose standard name has the form
>>>>>> spell_length_of_days_with_X_below|above_threshold *must have a
>>>>>> coordinate variable or scalar coordinate variable with the a
>>>>>> standard name of X to supply the threshold*(s).*It must have a
>>>>>> climatological time variable, and a cell_method entry* for within
>>>>>> days which describes the processing of quantity X before the
>>>>>> threshold is applied. A spell_length_of_days is an intensive
>>>>>> quantity in time, and the cell_methods entry for over days can be
>>>>>> any of the methods listed in Appendix E appropriate for intensive
>>>>>> quantities e.g. "maximum", "minimum" or "mean".
>>>>>
>>>>> And this definition gives a more appropriate way to encode the
>>>>> date of freezing days using a auxiliary coordinate to specify the
>>>>> threshold and use a cell_methods attribute along with the
>>>>> climatology_bounds attribute on time coordinate to specify an
>>>>> statistics over a period.
>>>>>
>>>>> The standard_name should be more like the definition for
>>>>> spell_length_of_days, but removing using 'time' as general instead
>>>>> of days. This what I would suggest with respect to the encoding:
>>>>>
>>>>> variables:
>>>>> float first_freeze_date(lat,lon);
>>>>> first_freeze_date:standard_name="time_when_air_temperature_below_threshold";
>>>>>
>>>>> first_freeze_date:coordinates="threshold time";
>>>>> first_freeze_date:cell_methods="time: minimum within days
>>>>> time: minimum over days";
>>>>> first_freeze_date:units="days";
>>>>> float last_freeze_date(lat,lon);
>>>>> last_freeze_date:standard_name="time_when_air_temperature_below_threshold";
>>>>>
>>>>> last_freeze_date:coordinates="threshold time";
>>>>> last_freeze_date:cell_methods="time: minimum within days time:
>>>>> maximum over days";
>>>>> last_freeze_date:units="days";
>>>>> float threshold;
>>>>> threshold:standard_name="air_temperature";
>>>>> threshold:units="degC";
>>>>> double time;
>>>>> time:climatology="climatology_bounds";
>>>>> time:units="days since 2000-6-1";
>>>>> double climatology_bounds(time,nv);
>>>>> data: // time coordinates translated to date/time string type format
>>>>> time="2008-01-16T00:00";
>>>>> climatology_bounds="2007-08-01T00:00", "2008-05-31T00:00";
>>>>> threshold=0.;
>>>>>
>>>>> The time: minimum over days, on first_freeze_date cell_methods
>>>>> attribute represents the shortest time minimum daily temperature
>>>>> (time: minimum within days) is below threshold.
>>>>> Equivalent for the last_freeze_date, but in this cas represents
>>>>> the longest time (time: maximum over days).
>>>>>
>>>>> Regards
>>>>>
>>>>> Antonio
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> --
>>>>> Antonio S. Cofi?o
>>>>> Associate Professor and Researcher
>>>>> Grupo de Meteorolog?a de Santander
>>>>> Dep. of Applied Mathematics and Computer Sciences
>>>>> Universidad de Cantabria (Spain)
>>>>>
>>>>> Academic Visitor
>>>>> National Centre for Atmospheric Science
>>>>> Department of Meteorology
>>>>> School of Mathematical, Physical and Computational Sciences
>>>>> University of Reading (UK)
>>>>>
>>>>> http://antonio.cofino.es
>>>>> On 15/03/17 18:16, Jim Biard wrote:
>>>>>>
>>>>>> Dan,
>>>>>>
>>>>>> How about that? I'm working on similar products. We haven't even
>>>>>> considered standard names for them.
>>>>>>
>>>>>> I went ahead and used 'days since YYYY-MM-DD 00:00:00' for my
>>>>>> first and last frost dates, since they are valid dates. My files
>>>>>> are structured as (example for first frost date):
>>>>>>
>>>>>> dimensions:
>>>>>> time = UNLIMITED ; // (56 currently)
>>>>>> lon = 960 ;
>>>>>> lat = 490 ;
>>>>>> bnds = 2 ;
>>>>>> variables:
>>>>>> double time(time) ;
>>>>>> time:standard_name = "time" ;
>>>>>> time:long_name = "time" ;
>>>>>> time:axis = "T" ;
>>>>>> time:units = "days since 1900-01-01 00:00:00" ;
>>>>>> time:calendar = "gregorian" ;
>>>>>> time:bounds = "time_bounds" ;
>>>>>> double time_bounds(time, bnds) ;
>>>>>> double lon(lon) ;
>>>>>> lon:standard_name = "longitude" ;
>>>>>> lon:long_name = "longitude" ;
>>>>>> lon:units = "degrees_east" ;
>>>>>> lon:modulo = 360. ;
>>>>>> lon:axis = "X" ;
>>>>>> lon:bounds = "lon_bounds" ;
>>>>>> double lon_bounds(lon, bnds) ;
>>>>>> double lat(lat) ;
>>>>>> lat:standard_name = "latitude" ;
>>>>>> lat:long_name = "latitude" ;
>>>>>> lat:units = "degrees_north" ;
>>>>>> lat:axis = "Y" ;
>>>>>> lat:bounds = "lat_bounds" ;
>>>>>> double lat_bounds(lat, bnds) ;
>>>>>> float first_freeze_date(time, lat, lon) ;
>>>>>> first_freeze_date:_FillValue = 1.e+20f ;
>>>>>> first_freeze_date:missing_value = 1.e+20f ;
>>>>>> first_freeze_date:comment = "Date of the first
>>>>>> day with a minimum temperature at or below 0 degrees C over the
>>>>>> 9 month period starting Aug 1 of each year." ;
>>>>>> first_freeze_date:flag_meanings =
>>>>>> "No_Freeze_Following" ;
>>>>>> first_freeze_date:long_name = "First freeze
>>>>>> date" ;
>>>>>> first_freeze_date:valid_min = 0. ;
>>>>>> first_freeze_date:flag_values = -2. ;
>>>>>> first_freeze_date:units = "days since 1900-01-01
>>>>>> 00:00:00" ;
>>>>>> first_freeze_date:calendar = "standard" ;
>>>>>>
>>>>>> with the time bounds reflecting 1 Aug to 1 May for each year.
>>>>>>
>>>>>> On 3/15/17 1:50 PM, Hollis, Dan wrote:
>>>>>>>
>>>>>>> Hi Jon,
>>>>>>>
>>>>>>> I?d be interested to know how to tackle this problem too. I?ve
>>>>>>> recently been generating some datasets of ?date of first frost?
>>>>>>> and ?date of last frost? and have no idea how to describe them
>>>>>>> in a CF-compliant way.
>>>>>>>
>>>>>>> Jim?s suggestion of ?day_of_year? is better than just ?days?,
>>>>>>> however this doesn?t capture what the ?something? is that has
>>>>>>> happened, nor that is the first/last/Nth occurrence of that
>>>>>>> event. What sort of events are you looking at?
>>>>>>>
>>>>>>> In my application I?m just looking at UK data, hence my ?year?
>>>>>>> runs from 1^st July to 30^th June (to span the N Hemisphere
>>>>>>> winter). It?s easy enough to use the bounds to indicate this,
>>>>>>> but I?m then not sure what values to store in the data array.
>>>>>>> Number of days since 1^st July maybe? Or ordinal date (1^st Jan
>>>>>>> = 1, 31^st Dec = 365)?
>>>>>>>
>>>>>>> Dan
>>>>>>>
>>>>>>> PS I have a whole bunch of other metrics that I?m looking at
>>>>>>> e.g. length of the longest spell, number of spells greater then
>>>>>>> N days etc. These seem even more complicated to describe using
>>>>>>> CF. Something for another post I think...
>>>>>>>
>>>>>>> *From:*CF-metadata [mailto:cf-metadata-bounces at cgd.ucar.edu] *On
>>>>>>> Behalf Of *Jim Biard
>>>>>>> *Sent:* 15 March 2017 16:28
>>>>>>> *To:* cf-metadata at cgd.ucar.edu
>>>>>>> *Subject:* Re: [CF-metadata] Recording "day of year on which
>>>>>>> something happens"
>>>>>>>
>>>>>>> Jon,
>>>>>>>
>>>>>>> I agree that a cell_methods attribute doesn't seem to be
>>>>>>> necessary. A new standard_name like 'day_in_year' or
>>>>>>> 'day_of_year' would likely make things clearer.
>>>>>>>
>>>>>>> Jim
>>>>>>>
>>>>>>> On 3/15/17 11:22 AM, Jon Blower wrote:
>>>>>>>
>>>>>>> Thanks Jim, that?s very helpful. Is cell_methods necessary in
>>>>>>> this case (for the time axis bounds) ? probably not since this
>>>>>>> isn?t a statistical quantity like an average, but a value
>>>>>>> that?s ?representative? of the year.
>>>>>>>
>>>>>>> I seem to remember from a while back that there was a proposal
>>>>>>> to allow time axes to use ?calendar years since X? (as opposed
>>>>>>> to ?years since X?, which uses a fixed-length UDUNITS year),
>>>>>>> which might handle this use case. I have been out of the loop
>>>>>>> for a while, but I can?t find mention of that in the CF spec,
>>>>>>> so maybe that didn?t go through.
>>>>>>>
>>>>>>> I might consider requesting a new standard name ? ?days? is
>>>>>>> good, but I wonder if a more specific one would be helpful.
>>>>>>>
>>>>>>> Best wishes,
>>>>>>> Jon
>>>>>>>
>>>>>>> *From: *CF-metadata <cf-metadata-bounces at cgd.ucar.edu>
>>>>>>> <mailto:cf-metadata-bounces at cgd.ucar.edu> on behalf of Jim
>>>>>>> Biard <jbiard at cicsnc.org> <mailto:jbiard at cicsnc.org>
>>>>>>> *Date: *Tuesday, 14 March 2017 15:12
>>>>>>> *To: *"cf-metadata at cgd.ucar.edu"
>>>>>>> <mailto:cf-metadata at cgd.ucar.edu> <cf-metadata at cgd.ucar.edu>
>>>>>>> <mailto:cf-metadata at cgd.ucar.edu>
>>>>>>> *Subject: *Re: [CF-metadata] Recording "day of year on which
>>>>>>> something happens"
>>>>>>>
>>>>>>> Jon,
>>>>>>>
>>>>>>> 1) I'd use 'days'. It is a valid standard name apart from the
>>>>>>> 'days since date' formalism. It's not perfect, but it's legal.
>>>>>>> You could, alternatively, request a new standard name.
>>>>>>>
>>>>>>> 2) Use a time_bounds variable. I would tend to set the time to
>>>>>>> be July 1 at midnight for each year, and set the bounds for
>>>>>>> each year to Jan 1 of that year and Jan 1 of the next year.
>>>>>>>
>>>>>>> Grace and peace,
>>>>>>>
>>>>>>> Jim
>>>>>>>
>>>>>>> On 3/14/17 10:43 AM, Jon Blower wrote:
>>>>>>>
>>>>>>> Hi all,
>>>>>>>
>>>>>>>
>>>>>>> We need to structure a NetCDF file that will hold a
>>>>>>> variable that represents the day of the year on which an event
>>>>>>> happened (integers from 0 to 366). This value is recorded every
>>>>>>> year for a number of years. I have a couple of questions about
>>>>>>> how best to do this:
>>>>>>>
>>>>>>>
>>>>>>> 1. What is the best standard name to use for the day of
>>>>>>> the year? I didn?t find anything in the standard name table,
>>>>>>> although I might have missed it.
>>>>>>>
>>>>>>>
>>>>>>> 2. What would be the best way to define the time axis?
>>>>>>> Each point along the axis would represent a whole year, rather
>>>>>>> than an instant in time. I could simply pick an arbitrary
>>>>>>> instant (e.g. midnight on 1st Jan) to represent the year, but is
>>>>>>> there a better way?
>>>>>>>
>>>>>>>
>>>>>>> Thanks in advance for any help!
>>>>>>>
>>>>>>>
>>>>>>> Jon
>>>>>>>
>>>>>>>
>>>>>>>
>>>
>>>
>>
>>
>>
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>
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--
John Helly, University of California, San Diego / San Diego Supercomputer Center / Scripps Institution of Oceanography / 760 840 8660 mobile / http://www.sdsc.edu/~hellyj
ORCID ID: orcid.org/0000-0002-3779-0603
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Received on Thu Mar 16 2017 - 13:49:09 GMT
