[CF-metadata] cellmethods="lat: lon: mean over land"

[Beate.Geyer at gkss.de]

Dear Jonathan,

one comment to cell_methods="lat: lon: mean over land" of point 1 from 
Elke Keup-Thiel:

We have a similar example: two variables with standard name
surface_snow_melt_amount

SFSSMLT is value local to snow covered area
SSSMLT is value is gridbox mean (multiplied with snow fraction)

as you proposed for 'fractional land coverage' (cell_methods="lat: lon: 
mean over land"
or "lat: lon: mean")
we may use
cell_methods="lat: lon: mean over snow" and cell_methods="lat: lon: mean"

The second case is OK for me, but the first one assumes to know the 
variable 'land' (are_fraction of land) in your case and 'snow' 
(are_fraction of snow covered area) in my case.
 
I think it would be better to define all variables that way that they are 
valid where really existent (snow melt valid where snow, forest fraction 
valid where land ...). In the definition-table of standard names should 
this part of definition be included as well as the hint 
'cell_methods="lat: lon: mean" for gridbox mean' for all affected 
variables. 
e.g. presently we have in standard name table:
surface_snow_melt_amount 
The surface called "surface" means the lower boundary of the atmosphere; 
over sea areas this is taken to be mean sea level. "Amount" means mass per 
unit area. 

This would become:
surface_snow_melt_amount 
The surface called "surface" means the lower boundary of the atmosphere; 
over sea areas this is taken to be mean sea level. "Amount" means mass per 
unit area. Variable is valid for gridbox area fraction covered by snow. 
For total gridbox mean use  cell_methods="lat: lon: mean"


In case the variable exist over land and water we already have specific 
names as:
surface_temperature_where_land

The name
'water_evaporation_flux_from_canopy_where_land' should not be valid any 
more if one accept my definition above...
For the special case of partial canopy cover at partial land area of 
gridbox  your proposal (using cell_methods for specification) may be used:
variables:
  float water_evaporation_flux_from_canopy(lat,lon);
  cellmethods="lat: lon: mean over land"

  float land(lat,lon);
  land:standard_name = "land_area_fraction"
  land:untis = "1"


Regards,
Beate

====================================================
 Dr. Beate Geyer 
 GKSS Research Centre 
 Institute for Coastal Research 
 Max-Planck-Str. 
 D-21502 Geesthacht, Germany 

E-Mail:   Beate.Geyer at gkss.de
Phone:  04152-871871
====================================================
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