Nan,
That's not true though. When we do monthly climatologies here, we pick
the 15th of each month as the time, and the climatological bounds
capture that the temporal cell covers the 1st to the Nth of each month.
You don't build the climatology on the time variable alone.
Ajay just needs to pick a day, such as Feb 15, for his time variable,
and specify the time and climatological bounds values as:
time = 1955-02-15
climatology = 1955-01-01, 2012-04-01
(expressing the dates relative to the epoch date given in the time
variable units attribute, and expressing the cell method as being 'time:
mean within years time: mean over years'). That is accurate, is properly
convertible from one set of time units to another, and is what CF
prescribes.
Grace and peace,
Jim
On 9/17/15 9:54 AM, Nan Galbraith wrote:
Hi Jim -
The problem isn't human-readability, though. The problem is that when you
generate a file that has, say, the mean temperature for each month,
sometimes
over a period of years, there are no 'days' in the process. Any data
that represents
February goes into the February bin, whether 28 or 29 days; although
March is always
31 days long, its mid-point is a different number of 'days since' the
beginning of
the year.
In the case of the file Ajay presented, time is a singleton, and its
value represents
the center point of the first 3 months of the years 1955-2012. That
can't be accurately
expressed as a number of days, only as months.
Is there a trac ticket for climatology data? If not, do we need one?
Cheers - Nan
On 9/17/15 9:32 AM, Jim Biard wrote:
Nan,
The problem is, udunits defines a month as having a specific length
of year/12 = 30.44 days, so if you use udunits to convert to anything
else, you won't end up where you think you will. The better practice
is to use days. It's not as "human readable", but it's the only way
to do proper conversions between time bases.
Grace and peace,
Jim
On 9/17/15 9:06 AM, Nan Galbraith wrote:
While it's true that 'month is not a proper unit of measure',
climatologies do in fact use months, not days, in calculating
mean values. Adhering to udunits/CF in this regard could make
the dates less easily understood.
Regards - Nan
On 9/11/15 1:34 PM, Karl Taylor wrote:
Dear Ajay,
Since "month" is not a proper unit of measure, convert your times
to days and use a unit "days since ...".
Also, it is normally a bad idea to have your base time set to a
date before the switch from Julian to Gregorian calendar. I suggest
using a base time of "1955-01-01" (i.e., the beginning of your
climatological period).
I think the cell_methods should be:
cell_methods="area: mean depth: mean time: mean within years time:
mean over years";
The time bounds should be (expressed in date/time format):
climatology_bounds = "1955-01-01", "2012-04-01"
and you can choose your time coordinate value as you think most
appropriate, e.g.,
the middle of the season of the first year of the climatology, or
the beginning of the first month of the first year of the
climatology, or
the middle of the season of the middle year of the climatology, or
???
Hope someone confirms this, as sometimes I make a mistakes.
Karl
On 9/11/15 9:54 AM, Ajay Krishnan - NOAA Affiliate wrote:
Dear CF members,
I would like your input on the way climatological stats are being
represented in a file that I am working on. I believe that I am
not using the time and the climatological_bounds properly:
Seasonal SST
Average seasonal temperature (Jan-Mar) for 6 decades (1955-2012)
dimensions:
time=1;
nv=2;
variables:
double time(time);
time:climatology="climatology_bounds";
time:units="months since 0000-01-01";
double climatology_bounds(time,nv);
double climatology_bounds(time,nv);
climatology_bounds:comment=” This variable defines the bounds of
the climatological time period for each time” ;
float t_mn(time,lat,lon,depth);
t_mn:standard_name: “sea_water_temperature” ;
t_mn:long_name: “Average of all unflagged interpolated values at
each standard depth level for sea_water_temperature in each
grid-square which contain at least one measurement.” ;
t_mn:cell_methods: “area: mean depth: mean time: mean” ;
data: // time coordinates translated to date/time format
time= “1.5” ;
climatology_bounds=”0.0”, “3.0” ;
The CF examples are helpful but my case is different where in I
have just 1-time co-ordinate in my file. In the above case, what
is the best way to record time and climatology bounds?
Thanks,
Ajay
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