Dear Martin When these names were introduced, we took the view that the application of the threshold is an operation which creates a new geophysical quantity. In that respect it is analogous to the transformations mentioned in the standard name guidelines, such as differentiation, integration, and calculating a histogram or probability density function. Hence this operation does not have to be described by the cell_methods, which makes it simpler. I think the number_of_ days data are quantities that have simple time bounds that specify the time interval within which the number of days is calculated. They are extensive in time i.e. they apply to the time interval as a whole, not to a point in time. Hence the cell_methods should be just "time: sum". In fact this would be the default for an extensive quantity, but CF recommends that cell_methods should always be explicitly stated.
I hope this looks sensible to you, or at least acceptable. Best wishes Jonathan ----- Forwarded message from [email protected] ----- > I just wanted to check on the usage of the standard name > "number_of_days_with_air_temperature_below_threshold" for "frost day" and > "ice day" counts. The definition of this standard name implies, I think, that > cell_methods should be specified as "time: minimum within days time: sum over > days" for frost days and "time: maximum within days time: sum over days". > This usage would convey some of the relevant information, but I'm not sure > it is consistent with the usage of cell_methods described in the CF standard > itself. In the standard it is implied that a variable with > cell_methods="time: maximum within days time: sum over days" would be the sum > of the daily maxima. Here there is a thresholding operation in between which > is omitted from the description only because "threshold" is not an available > operation in table e.1. > > If "threshold" was added to table e.1 of the standard I suppose that > "number_of_days_with_air_temperature_below_threshold" would become redundant > and we could use "air_temperature" with cell_methods="time: minimum within > days time: threshold within days time: sum over days". This would also allow > us to deal with other indices (dry days, windy days) without creating new > standard names. An additional attribute would be needed to describe the > threshold in some way. _______________________________________________ CF-metadata mailing list [email protected] http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
