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

Jim Biard jbiard at cicsnc.org
Wed Mar 15 12:16:19 MDT 2017


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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> *Research Scholar*
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> *North Carolina State University * <http://ncsu.edu/>
> *NOAA National Centers for Environmental Information * 
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> /formerly NOAA’s National Climatic Data Center/
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Facebook <http://www.facebook.com/cicsnc> 	*Jim Biard*
*Research Scholar*
Cooperative Institute for Climate and Satellites NC <http://cicsnc.org/>
North Carolina State University <http://ncsu.edu/>
NOAA National Centers for Environmental Information <http://ncdc.noaa.gov/>
/formerly NOAA’s National Climatic Data Center/
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