[CF-metadata] high cloud amount

[Jonathan Gregory]

Dear Burkhardt

>  1) cell_methods:
> > cloud cover over several layers is computed by using
> > maximum_random_overlap which is very common in atmospheric models.
> > However, this is not a valid CF-convention. What shall I do?
> 
> I believe that this would be reported as follows (assuming cloud fraction 
> as a function of layer is named "cl" and the vertical coordinate is "plev":
> 
> cl: cell_methods = "plev: sum (assuming maximum random overlap)"
> 
> You may place any text you want within parentheses following the method.

I think this is a good suggestion of Karl's.

> I'm pretty sure CF doesn't permit character type data for bounds; character 
> data can be used as "labels and alternative coordinates", but I'm pretty 
> sure only numerical values should be used for the bounds.
> 
> One option might be to choose a "surface" pressure well below ground (e.g. 
> 1100 hPa).  Clearly there are no clouds below ground, so the low cloud 
> cover is unaffected by the lower limit.  Another option might be to choose 
> a more standard approx. surface pressure (e.g., 1000 hPa), but then explain 
> in a "comment" attribute associated with the variable (cl) that the low 
> layer cloud is for a layer extending from 800 hPa to the surface.
> 
> I can't think of any software actually making use of the bounds attribute 
> (in the case you bring up), except to perhaps label a plot.  If that is the 
> case, then you might simply define an "alternative coordinate" as a string 
> variable, which might have values something like:
> 
> "high (0-400 hPa)", "medium (400-800 hPa)", "low (800 hPa - surface)"

I agree with these points as well. I would add, however, that the bounds might
be used (apart from to label a plot) by software that wanted to identify which
layer to use for some analysis purpose. To do this, numeric bounds would be
more convenient than strings, so I prefer the first option of choosing a value
for the surface pressure. Perhaps a way to indicate that the value is special
might be to add a valid_max attribute and set it equal to that value.

Best wishes

Jonathan