Dear Karl, Jonathan, Jim,
thanks for those comments.
The CMIP6 variable in question is clmisr
with a coordinatte of 16 altitude bins
I'd be happy with Jim's proposed solution, which does not need any change to
the convention, though it may be a bit cryptic: all the examples in the
convention are for cases in which all array values are intended to match one of
the flag_values. Having an array which is a mixture of flags and "normal"
values would be a new usage. We could, perhaps, introduce a consistency
problem: ticket 151 (http://cf-trac.llnl.gov/trac/ticket/151) explains how, for
variables with standard_name "area_type", flag_values and flag_meanings can be
used to encode the data, in which case it is the "flag_meanings" which match
the requirements of the standard name. Here, on the other hand, we want the
special bin to be the exception which is not described by the standard name
(altitude). So .. perhaps it is simpler to introduce a new attribute name?
Concerning Jonathan and Karl's comments, the idea of calling it a
"missing_value" was a mistake I made, but it actually refers to locations where
cloud is detected but the height of the cloud cannot be retrieved.
The current proposal is to have a value of 0.0 in the coordinate and
(-99000.0,0.0) in the bounds of the special value "bin". I imagine these need
to be present, but I think their values are not going to mean anything.
It is certainly possible to do as Karl suggests and place an explanation in the
variable description. Having the special status of the first bin explicitly
flagged in way which can be easily picked up by software brings added value.
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