Dear Martin, Thanks for your detailed explanation. I'd like to add a bit more information. These variables are not joint distributions, they are 1D distributions for different ranges of Z. The question is, does "histogram_of_X[_over_Z]" mean that the Z coordinate has to be completely collapsed? It is not clear to that the current definition implies that. If Z is not completely collapsed, you can then end up with a function of the form frequency(lat,lon,X,Z2), where the coordinate Z is only partially collapsed into bins described by Z2. I'm using here Z2 to explicitly show when the Z coordinate represents bins. This would look like a joint histogram, but it is not. I think that your proposal of dropping "_over_Z" from the standard name works for a joint distribution, but not for a collection of 1D distributions along Z, unless there is a way of distinguishing between both cases with the use of attributes.
Another detail is that these histograms provide relative frequencies (values between 0 and 1, not counts), not absolute frequencies. Is that inconsistent with the current definition of histogram in CF? Regards, Alejandro > -----Original Message----- > From: martin.juc...@stfc.ac.uk [mailto:martin.juc...@stfc.ac.uk] > Sent: 12 October 2016 19:05 > To: cf-metadata@cgd.ucar.edu > Cc: Bodas-Salcedo, Alejandro > Subject: Usage of histogram_of_X_over_Z > > Hello, > > There are two standard names of the form histogram_of_..... in the CF Standard > Name list (at version 36): > histogram_of_backscattering_ratio_over_height_above_reference_ellipsoid and > histogram_of_equivalent_reflectivity_factor_over_height_above_reference_ellipsoid > . Both of these where used in CMIP5 and set to be used in CMIP6, but the usage > does not appear to match the standard name desecriptions. > > The possible confusion is over the role of different coordinates. The CF > definitions > say ''"histogram_of_X[_over_Z]" means histogram (i.e. number of counts for > each > range of X) of variations (over Z) of X.' This implies to me that you start > with a > function of Z and possibly other coordinates and end up with a function of X > and the > other coordinates. E.g. if the source data is X(lat,lon,Z), then the > histogram data will > be of the form frequency(lat,lon,X). > > In the two CMIP5/CMIP6 draft variables (cfadLidarsr532, cfadDbze94) using > these > standard names the "Z" coordinate which is included in the standard name > ("height_above_reference_ellipsoid") is one of the coordinates of the > histogram data > variable. Both these variables appear to be joint distributions (frequency of > X and Y > values) over sub-grid variability as a function of latitude, longitude and > time. > > I've been reviewing these existing definitions in some detail because there > are some > new distribution variables in the request and I'd like to make sure that we > have a > consistent approach. > > If we need to described a variable which carries a joint distribution of X > and Y, then > the variable will have to use X and Y as coordinates, so perhaps we can > simplify the > process by leaving them out of the standard name. Similarly the "over_Z" part > of the > name would be better expressed as a cell_methods construct. This line of > reasoning > suggests using a new standard name such as "frequency_distribution" (units > "1"). > The only difficulty is that the frequency distribution might be a function of > the > quantities X and Y (scattering ratio and cloud top height for cfadLidarsr532) > and also > of latitude, longitude and time. There should be some way of distinguishing > the > different roles of these 5 coordinates: is is the distribution of X and Y as > a function of > latitude, longitude and time. I think this could be done conveniently by > introducing a > single new attribute, e.g. "bin_coords: X Y". > > "frequency_distribution" could be used for single or joint distributions. > > My questions to the list are: > (1) am I missing something in my interpretation of the existing > histogram_of_... > names? > (2) if not, is the adoption of a "frequency_distribution" standard name an > appropriate > way forward? > > regards, > Martin > > regards, > Martin _______________________________________________ CF-metadata mailing list CF-metadata@cgd.ucar.edu http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
