[CF-metadata] new named fields for ocean

[Karl Taylor]

Dear Steve and Jonathan,

[I'm not sure Steve was copied on your email Jonathan, so I have 
attached it in full below.]

Concerning Jonathan's last point in his email below, I too wondered 
whether it wouldn't be more straight forward simply to save the 
temperature of the precipitation arriving at the surface.

I also was surprised that the evaporating water might be at a different 
temperature than the surface it is leaving.  If this is the case, I 
don't really see why the heat flux is given by the formula you 
specified.  As I understand it evaporation is the residual difference 
between the H2O molecules leaving the liquid surface and the H2O vapor 
molecules entering the surface.  Shouldn't the "energy flux" you are 
interested in here depend on these upward and downward fluxes of water 
mass explicitly in this case and not simply on the evaporation rate?

One more general comment:  even if precipitation and evaporating water 
molecules are both assumed to be at the same temperature as the surface 
water, the ocean energy can be changed through the mechanism you are 
attempting to account for (which is in addition to the latent heat 
flux).  I'm concerned, however, that the atmosphere (in current models) 
won't "feel" the energy transfer implied by this additional flux.

Suppose, for example, that evaporation occurs preferentially over warm 
regions of the ocean, and precipitation over cool regions.  Then besides 
the latent heat transfer from ocean to atmosphere (which is accounted 
for by atmos. models and normally assumed to be independent of 
temperature), you will find the ocean loses additional energy (because 
the water molecules leaving the ocean through evaporation are warmer 
than the rain entering it).  This implies that water molecules give up 
heat to the atmosphere (beyond the latent heat released), but I'm not 
sure our current models account for this.  Do you know if this is one of 
the reasons that most coupled AOGCMs fail to conserve energy exactly? 
Or does the atmosphere model actually indirectly account for this gain 
of energy in some way (by use of virtual temperature?).

Best regards,
Karl


Dear Steve

I would prefer keeping the sea_water in sea_water_pressure:
 > > sea_water_pressure_at_the_sea_water_surface
 > > sea_water_pressure_at_sea_floor
It might seem redundant in this context but it corresponds to the standard
name of sea_water_pressure, analogous to air_pressure. That means the
pressure that exists in the medium of sea water. It doesn't imply that it is
caused only by overlying sea water.

 > > I too find this name unappealing.  I suggest the following 
alternatives:
 > >
 > > 
temperature_flux_due_to_rainfall_expressed_as_heat_flux_into_sea_water
 > > --> heat_flux_into_sea_water_due_to_rainfall_mass_flux
 > >
 > > 
temperature_flux_due_to_evaporation_expressed_as_heat_flux_out_of_sea_water
 > > --> heat_flux_out_of_sea_water_due_to_evaporative_mass_flux
 > >
 > > 
temperature_flux_due_to_runoff_expressed_as_heat_flux_into_sea_water -->
 > > heat_flux_into_sea_water_due_to_runoff_mass_flux
 > >
 > > These new names are A/ shorter, B/ more directly what is intended
 > > physically; i.e., a heat flux due to a mass flux.

When we had the earlier discussion, I proposed temperature flux because I
think that temperature multiplied by mass flux is a quantity that people
have to be careful with because of its arbitrary zero. In that respect it
is not like a genuine heat flux. You can't tell what heat is being added
unless you know the temperature and the mass of the water it's being added
to. I thought "temperature flux" would draw attention to the way it was
evaluated. To be more explicit, it is
   product_of_rainfall_temperature_and_rainfall_flux_expressed_as_heat_flux
Without expressed_as_heat_flux it would be in K kg m-2 s-1 rather than W 
m-2.
Maybe it's OK to omit _into_sea_water since rainfall flux is generally 
assumed
to be at the surface, and this quantity exists equally over land. We 
would use
cell_methods to restrict it to the sea part of the gridbox. The definition
should also state what the zero of temperature is.

Why do we want these quantities? Would be clearer and easier to ask for the
rainfall_temperature and the
temperature_of_water_flux_into_sea_water_from_rivers? I'm sorry that I 
hadn't
thought about the evaporation before. I'm not sure what this means: 
isn't the
evaporating water necessarily at surface temperature? The heat carried away
is just the latent heat.

Best wishes

Jonathan
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