[CF-metadata] new standard names for atmospheric dynamics

[Jonathan Gregory]

Dear Martin

> longname = 'air mass in grid box'
> proposed standard_name:  air_mass            units: kg

This could be just air_mass (or would mass_of_air be better, since "air mass"
is a meteorological term). The fact that's in a gridbox is implied by its being
a gridded quantity, and the cell_methods can indicate it's the sum in the box
- an extensive quantity. There is a stdname sea_ice_mass following this idea.

> longname = 'volume of grid box'
> proposed standard_name:  grid_box_volume     units: m3

cell_volume has been discussed before.

> 
> longname = 'height of grid box'
> proposed standard_name:  grid_box_height     units: m

cell_thickness is currently under consideration for ocean and would apply
equally to atmos.

> longname = 'upper tropopause level index'
> proposed standard_name: upper_tropopause_level_index    units: index
> longname = 'lower tropopause level index'note: this is a diagnostic using the WMO criterion. proposed standard_name: lower_tropopause_level_index    units: index
> Note: The tropopause is not always unequivocally defined in this way, this is why we defined an upper and lower level index. The tropopause height is already defined as "tropopause_altitude", but in units of m.

Could you formulate these as model_level_number_at_X? There are already
several standard names of that form.

> longname='precipitation formation rate'
> proposed standard_name:  precipitation_formation_flux    units: kg m-2 s-1
> or:                      tendency_of_precipitation_flux_due_to_formation
> 
> longname='precipitation evaporation rate'
> proposed standard_name:  precipitation_evaporation_flux    units: kg m-2 s-1
> or:                      tendency_of_precipitation_flux_due_to_evaporation

It doesn't seem quite right for it to be a tendency of precipitation flux.
Since the precip flux is already kg m-2 s-1, a tendency would be kg m-2 s-2.
Perhaps it is a tendency of the mass of precipitation, or the mass content or
concentration of precipitation, or maybe a vertical divergence of the flux?

Best wishes

Jonathan