[CF-metadata] Recording "day of year on which something happens"

John Helly hellyj at ucsd.edu
Thu Mar 16 13:49:09 MDT 2017


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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