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. Cofiño 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. Cofiño
Associate Professor and Researcher
Grupo de Meteorología 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:[email protected]] *On
Behalf Of *Jim Biard
*Sent:* 15 March 2017 16:28
*To:* [email protected]
*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 <[email protected]>
<mailto:[email protected]> on behalf of Jim
Biard <[email protected]> <mailto:[email protected]>
*Date: *Tuesday, 14 March 2017 15:12
*To: *"[email protected]"
<mailto:[email protected]> <[email protected]>
<mailto:[email protected]>
*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
--
*******************************************************
* Nan Galbraith Information Systems Specialist *
* Upper Ocean Processes Group Mail Stop 29 *
* Woods Hole Oceanographic Institution *
* Woods Hole, MA 02543 (508) 289-2444 *
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