[CF-metadata] statistic indices

Heinke Hoeck heinke.hoeck at zmaw.de
Tue May 27 04:18:47 MDT 2008


Dear Jonathan

> Thanks for your email and sorry for another long delay in this correspondence
> (two months this time). My slowness indicates that although this discussion is
> useful and thought-provoking, I don't really have enough time to pursue it,
> unfortunately; I am holding up progress. 
I am not so optimistic that the trac system would help us.
I have a new idea and may be we must not change the cell methods.
To open the cell method for structering should be a more general approach.
With my last proposal I would strech it to much. We would change the
units of
the physical quantity with these cell methods and I do not fully
reconsider the
consequences. If possible I would like to avoid this.
> Perhaps the best way to take this
> forward, if you would like to, would be for you to make specific proposals as
> a trac ticket for amendment to CF, when you are ready, and I hope that others
> will then comment.
>   
I am not so optimistic that the trac system would help us.
I have a new idea and may be we must not change the cell methods.
To open the cell method for structuring should be a more general approach.
With my last proposal I would stretch it to much. We would change the
units of
the physical quantity with these cell methods and I do not fully
reconsider the
consequences. If possible I would like to avoid this.
> You are, I think, suggesting an addition to cell_methods for a number_of_days
> standard name, to indicate whether the condition is that the quantity (air
> temperature etc.) should be below or above the threshold. That idea has more
> general applicability than these statistical indices, and I think it's a good
> use of cell_methods.
>   
That was not all. We need an indicator function.
> If a standard_name has the phrase "dependent_on_X" (or whatever phrase is
> used) it would mean that it must have a coordinate variable, scalar coordinate
> variable or auxiliary coordinate variable with standard_name of X, to specify
> the condition.
>   

This gives me a new idea:

/* number of days with daily minimum below 0 degC = frost_days = fd
(IPCC AR4 and AR5) */
float n1(lat,lon);
  n1:standard_name="number_of_days_with_variable_below_threshold";
  n1:coordinates="threshold time";
  n1:cell_methods="time: sum over days";
float threshold;
  threshold:standard_name="air_temperature";
  threshold:units="degC";
  threshold:cell_methods="time:minimun within days"
data:
  threshold=0.;

The threshold with the term "below_threshold" describes the indicator
condition.
The term "number_of_days_with" describes the sum over the indicator value.

Is this possible ?
If yes we need also number_of_days_with_variable_above_threshold
I like the term "threshold". This was your or Alison's proposal.

Other examples for this would be:
summer days max>25 degC
tropical nights min>20 degC
ice days max<0 degC
frost days where no snow

with threshold:standard_name="lwe_thickness_of_precipitation_amount"
heavy precipitation >10 mm = r10 (IPCC AR4 and AR5)
very heavy precipitation >20 mm
wet days > 1mm

with threshold:standard_name="wind_speed"
strong breeze days max > 10,5 m/s
strong gale days max > 20,5 m/s
hurricane days max > 32,5 m/s

These are only the simple statistic indices but it is a start.
What do you think ? Could we use the scalar coordinate this way ?

Best wishes

Heinke





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