Dear Martin,
You are right, those definitions are not correct.
> From your reply I understand now that these are univariate distributions giving the
> frequency of different radar reflectivities in different height bands. Coming from
> radar/lidar instruments (or an emulator of these instruments), there are multiple
> observations in each GCM-scale height band. Presumably, there are also multiple
> profiles in the GCM-scale grid square, so that we have a frequency distribution over
> sub-grid scale variability in the vertical and the horizontal? Or is it actually evaluated
> at a spatial point?
>
There is a sub-grid distribution of vertical profiles from which they are constructed.
The definition that you propose seems accurate to me. Thanks again for your time spent clarifying this.
Regards,
Alejandro
> -----Original Message-----
> From: CF-metadata [mailto:cf-metadata-bounces at cgd.ucar.edu] On Behalf Of
> martin.juckes at stfc.ac.uk
> Sent: 13 October 2016 13:05
> To: cf-metadata at cgd.ucar.edu
> Subject: [CF-metadata] Usage of histogram_of_X_over_Z
>
> Dear Alejandro,
>
> The two CMIP variables which I'm talking about are cfadDbze94 currently defined
> as "CFAD (Cloud Frequency Altitude Diagrams) are joint height - radar reflectivity
> (or lidar scattering ratio) distributions." and cfadLidarsr532, which has the same
> definition. If they are not joint distributions we clearly have a problem with these
> definitions.
>
> From your reply I understand now that these are univariate distributions giving the
> frequency of different radar reflectivities in different height bands. Coming from
> radar/lidar instruments (or an emulator of these instruments), there are multiple
> observations in each GCM-scale height band. Presumably, there are also multiple
> profiles in the GCM-scale grid square, so that we have a frequency distribution over
> sub-grid scale variability in the vertical and the horizontal? Or is it actually evaluated
> at a spatial point?
>
> If this is the case, you are right and we just need to correct the definitions in the
> CMIP tables (though there is still a case for introducing a frequencs_distribution for
> other variables, but that should ne another thread). I would favour a slightly more
> verbose and explicit definition, e.g.
> "CFAD (Cloud Frequency Altitude Diagrams) are frequency distributions of radar
> reflectivity (or lidar scattering ratio) as a function of altitude. cfadDbze94 is defined
> as the simulated relative frequency of radar reflectivity in sampling volumes defined
> by altitude bins and model grid cells."
>
> Note that I'm using "altitude" rather than "height" to match the standard names: in
> the CF Convention, "altitude" means height above the geoid, and "height" means
> height above the surface.
>
> Is that an accurate definition?
>
> regards,
> Martin
>
>
> 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.juckes at
> stfc.ac.uk<http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata>
> [mailto:martin.juckes at stfc.ac.uk<http://mailman.cgd.ucar.edu/mailman/listinfo/cf-
> metadata>]
> > Sent: 12 October 2016 19:05
> > To: cf-metadata at cgd.ucar.edu<http://mailman.cgd.ucar.edu/mailman/listinfo/cf-
> metadata>
> > 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 at cgd.ucar.edu
> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
Received on Thu Oct 13 2016 - 06:39:16 BST
