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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Received on Thu Mar 16 2017 - 13:42:45 GMT
