Dear Dan I don't think I have understood this. What is the field of precipitation amount? The F(x) would actually be a 3D data variable, I think, F(x,lat,lon), which gives the probability that precipitation is less than x at (lat,lon). For this field, x is a 1D coord variable, not a field.
Best wishes Jonathan > Yes, that is what I had in mind. What slightly concerns me is that I would > effectively end up storing the precipitation amount twice: > > - once as a data variable in its own right > > - once as an auxilliary coordinate, with F(x) as the data variable > > Duplication, especially within the same data archive, seems like something to > be avoided if possible, hence my idea to have F(x) as the auxilliary > coordinate variable and precipitation amount as the data variable and not > store F(x) as a separate data variable. Do you think that it would be > preferable/acceptable to store the precipitation values twice? > > Regards, > > Dan > > > -----Original Message----- > From: CF-metadata [mailto:[email protected]] On Behalf Of > Jonathan Gregory > Sent: 04 September 2014 14:14 > To: [email protected] > Subject: [CF-metadata] Return periods > > Dear Dan > > I agree with you that it would be better to store F(x) than to use your sign > convention for return periods. However it would be fine to split the return > periods into the two tails in different data variables and give them distinct > standard names. We have some standard names for such things e.g. > > spell_length_of_days_with_lwe_thickness_of_precipitation_amount_above_threshold > and you could propose suitable ones. > > If you store F(x), I think it would be a data variable, not a coordinate or > ancillary variable, and it should have a standard name. I believe the guidance > you quote is about probability distribution functions rather than cumulative > (probability) distribution functions. Following a similar approach, however, > we could have a standard name such as > cumulative_distribution_function_of_precipitation_amount > for F(x), where x is precipitation_amount, which would be a coordinate. Is > that what you have in mind? > > Cheers > > Jonathan > > > ----- Forwarded message from "Hollis, Dan" <[email protected]> ----- > > > Dear all, > > > > Here is another question related to migrating our UK climate grids to > > NetCDF. > > > > As well as grids of the monthly rainfall total (in mm) we also generate > > grids of the estimated return period of the rainfall total (in years). > > Currently these two quantities are stored in separate files (with only the > > file name and location to tell us they are related). I've been trying to > > think how to store the return period information using CF-NetCDF and would > > be grateful for advice. > > > > Some further details: > > > > Our existing grids contain the return period in years i.e. if the return > > period for a particular grid point is N years then this means that we > > estimate that the rainfall total for that grid point will be exceeded on > > average once every N years. This is equivalent to saying that each year > > there is a probability of 1/N of exceeding that rainfall amount i.e. the > > cummulative distribution function, F(x) = 1 - 1/N. For example, if N = 10 > > then F(x) = 0.9. Additionally, as we are also interested in droughts, we > > have adopted our own convention of using negative values to refer to the > > left (dry) tail of the rainfall distribution. For example N = -10 is used > > to mean that F(x) = 0.1 i.e. we estimate that rainfall amounts *less* than > > the observed value will occur once every 10 years on average. > > > > This use of positive and negative values to indicate return periods > > relating to the right (wet) and left (dry) tails is convenient but > > unconventional. My initial thought is that we should store F(x) itself and > > only convert to return period for the purposes of presentation e.g. > > creating maps. > > > > So, how to store F(x)? The main problem is that the value to which the > > return period relates (i.e. the rainfall amount) varies from one grid point > > to another. Two possibilities occur to me, both of which involve storing > > F(x) alongside the rainfall total: > > > > - Store F(x) as an auxilliary coordinate > > > > - Store F(x) as ancillary data > > > > It's not clear to me whether one is better than the other, or even whether > > either approach is valid. > > > > The other question is what to call the F(x) values. The guidance for > > ancillary data says to use standard name modifiers to indicate the > > relationship, but there doesn't seem to be anything suitable for describing > > F(x). > > > > The other thing I've looked at is the guidance for constructing standard > > names. I can't seem to locate this on the current CF web site so I've > > refered to the archived copy available here: > > > > https://web.archive.org/web/20130728212039/http://cf-pcmdi.llnl.gov/documents/cf-standard-names/guidelines > > > > The section on transformations includes > > 'probability_distribution_of_X[_over_Z]' in the list, however it's unclear > > to me whether this is what I need, or even how I might use it in other > > circumstances. The notes state: > > > > "probability distribution (i.e. a number in the range 0.0-1.0 for each > > range of X) of variations (over Z) of X. The data variable should have an > > axis for X." > > > > The reference to 'each range of X' is the bit I find confusing. Is the idea > > to store F(X1), F(X2), F(X3) etc, or is it intended to be F(X2) - F(X1), > > F(X3) - F(X2), F(X4) - F(X3) etc? The former doesn't quite fit the > > description, but the latter has the problem that the number of ranges (= > > the number of data values) will be one less than the number of X values. I > > can't see any existing names that use this transformation to use as a guide. > > > > If anyone can help that would be much appreciated. > > > > Thanks, > > > > Dan > > > > > > Dan Hollis Climatologist > > Met Office Hadley Centre FitzRoy Road Exeter Devon EX1 3PB > > United Kingdom > > Tel: +44 (0)1392 886780 Fax: +44 (0)1392 885681 > > E-mail: [email protected] Website: http://www.metoffice.gov.uk > > For UK climate and past weather information, visit > > http://www.metoffice.gov.uk/climate > > > > > > > _______________________________________________ > > CF-metadata mailing list > > [email protected] > > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata > > > ----- End forwarded message ----- > _______________________________________________ > CF-metadata mailing list > [email protected] > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata ----- End forwarded message ----- _______________________________________________ CF-metadata mailing list [email protected] http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
