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

Hollis, Dan dan.hollis at metoffice.gov.uk
Wed Mar 15 11:50:07 MDT 2017


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 1st July to 30th 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 1st July maybe? Or ordinal date (1st Jan = 1, 31st 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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