[CF-metadata] high cloud amount

[Burkhardt Rockel]

Dear All,

I am looking for a method how to define high, medium, and low clouds  
in CF-conventions. I have found a correspondence (see below) in the  
CF-archive. However, there are still open questions for me:

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?

2) boundaries:
the boundaries of the high/medium/low clouds in our model are (0 hPa,  
400 hPa)/(400 hPa, 800 hPa)/(800 hPa, ps), where ps is the surface  
pressure. The issue here is that ps is forstly not a constant and  
secondly 2D. In the discussion below it is said that one can also use  
character type for the bounds. Thus would then  
("top_of_atmosphere","400hPa")/("400hPa","800hPa")/ 
("800hPa","surface_air_pressure") be an acceptable solution?

Regards
Burkhardt



The help for cloud_area_fraction_in_atmosphere_layer indicates that the
layer must be specified via an associated vertical coordinate.  The
definition of the quantity is incomplete without this information  
which is
what distinguishes the quantities in different variables.

Also note that the long_name attribute is available (and intended)  
for the
common usage names.  If a single variable is used for all 3 layers the
long_name could be something like "high/medium/low cloud fraction".

On Tue, Dec 28, 2004 at 03:00:52PM -0500, V. Balaji wrote:
 > Jonathan Gregory writes:
 >
 > > Dear Balaji
 > >
 > > > What happens when I also want to define "middle" and "low" cloud
 > > > amount? Do I define a single field with 3 Z levels?
 > >
 > >Yes, that's the idea. Alternatively, if you want high, medium and  
low cloud to
 > > be stored in separate data variables, you can have three  
different size-one
 > > dimensions for them, or you can use scalar coordinate variables.
 >
 > I'm not sure how to do them in separate data variables... wouldn't it
 > be confusing to have three variables all bearing the standard name
 > "cloud_area_fraction_in_atmosphere_layer"?
 >
 > > > Is it possible somehow to associate the words "high", "middle"  
and
 > > > "low" with the three layers? This is common parlance.
 > >
 > > Yes, that could be done, by defining an auxiliary coordinate  
variable with
 > > the dimension of Z and of character type, containing the  
descriptive strings.
 > > However, I think we ought to insist that the Z ranges are  
precisely defined,
 > > and not try to standardise what high, medium and low cloud mean.  
These are
 > > vague terms, and without precise Z ranges attached to them, I  
don't think
 > > there is sufficient metadata to allow you to decide whether  
quantities from
 > > different sources are comparable. What do you think?
 >
 > Correct.
 >
 > --
 >
 > V. Balaji                               Office:  +1-609-452-6516
 > Head, Modeling Systems Group, GFDL      Home:    +1-212-253-6662
 > Princeton University                    Email: v.balaji at noaa.gov
 > 
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