[CF-metadata] Cell methods when there are no coordinates

From: Jonathan Gregory <j.m.gregory>
Date: Fri, 1 Nov 2013 15:21:03 +0000

Dear Mark

In the cell_methods attribute, the entries have the form "name: method", as
you know. The name usually identifies a dimension (see first para of 7.3), and
it doesn't matter if the dimension doesn't have a coordinate variable. I agree
with you that cell_methods is the best way to describe an ensemble statistic,
such as ensemble mean, and there have been previous discussions about doing
this over previous years. I think your second example should look something
like this.

dimensions:
  ensemble = 1 ;
variables:
  int realization(ensemble);
    realization:long_name = "Number of the simulation in the ensemble" ;
  char experiment_id(ensemble,STRINGLENGTH);
    experiment_id:long_name = "Experiment identifier" ;
  char source(ensemble,STRINGLENGTH);
    source:long_name = "Method of production of the data" ;
  char institution(ensemble,STRINGLENGTH);
    institution:long_name = "Institution responsible for the forecast system" ;
  float sh_sd(ensemble, time, latitude, longitude) ;
    sh_sd:units = "1" ;
    sh_sd:standard_name = "specific_humidity" ;
    sh_sd:cell_methods = "leadtime: mean (interval 6 h) ensemble: mean";
    sh_sd:coordinates = "experiment_id source realization institution" ;

CF doesn't give any guidance about the contents of the aux coord vars after
aggregation; the data processing software will have to decide how best to
represent the ranges of the collapsed ensemble axis. For numerical aux coord
vars such as realization above you could provide bounds.

Best wishes

Jonathan
Received on Fri Nov 01 2013 - 09:21:03 GMT