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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>>
>> --
>> CICS-NC <http://www.cicsnc.org/> Visit us on
>> Facebook <http://www.facebook.com/cicsnc> *Jim Biard*
>> *Research Scholar*
>> Cooperative Institute for Climate and Satellites NC <http://cicsnc.org/>
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>> NOAA National Centers for Environmental Information
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>> /formerly NOAA?s National Climatic Data Center/
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>>
>>
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Received on Thu Mar 16 2017 - 06:45:58 GMT
