Dear Beate and Jonathan,
I don't have time to think about this right now, but are your
conclusions consistent with what we decided to do for the AR4? In
particular, we should check the definitions given in:
http://www-pcmdi.llnl.gov/ipcc/standard_output.html
The following variables may be particularly relevent to the discussion
and are carefully defined there:
soil_moisture_content:
water in all phases summed over all soil layers, and averaged over the
land portion of the grid cell (i.e., compute by dividing the total mass
of water contained in the soil layer of the grid cell by the land area
in the grid cell); report as "missing" or 0.0 where the land fraction is 0.
surface_snow_thickness:
this thickness when multiplied by the average area of the grid cell
covered by snow yields the time-mean snow volume. Thus, for time means,
compute as the weighted sum of thickness (averaged over the snow-covered
portion of the grid cell) divided by the sum of the weights, with the
weights equal to the area covered by snow. report as 0.0 in snow-free
regions.
soil_frozen_water_content:
summed over all soil layers, and averaged over the land portion of the
grid cell (i.e., compute by dividing the total mass of frozen water
contained in the soil layer of the grid cell by the land area in the
grid cell); report as "missing" or 0.0 where the land fraction is 0.
surface_runoff_flux:
compute as the total surface runoff leaving the land portion of the grid
cell divided by the land area in the grid cell; report as "missing" or
0.0 where the land fraction is 0.
surface_snow_amount_where_land:
Compute as the mass of surface snow on the land portion of the grid cell
divided by the land area in the grid cell; report as "missing" or 0.0
where the land fraction is 0; exclude snow on vegetation canopy or on
sea ice.
surface_snow_area_fraction_where_land:
fraction of grid cell covered by snow that lies on land; exclude snow
that lies on sea ice.
surface_snow_melt_flux_where_land:
compute as the total surface melt water on the land portion of the grid
cell divided by the land area in the grid cell; report as 0.0 for
snow-free land regions; report as 0.0 or "missing" where the land
fraction is 0.
soil_moisture_content_at_field_capacity:
divide the total water holding capacity of all the soil in the grid cell
by the land area in the grid cell; report as "missing" or 0.0 outside
land areas.
sea_ice_thickness:
this thickness, when multiplied by the average area of the grid cell
covered by sea ice, yields the time-mean sea ice volume. Thus, for time
means, compute as the weighted sum of thickness (averaged over the sea
ice-covered portion of the grid cell) divided by the sum of the weights,
with the weights equal to the area covered by sea-ice; Report as 0.0 in
regions free of sea ice.
water_flux_into_ocean:
precipitation minus evaporation, plus runoff, melting of sea ice and any
water flux correction calculated considering only the ocean-portion of
each grid cell
heat_flux_correction_where_ocean:
if applicable, should be positive down (i.e., added to ocean); the total
flux correction entering the ocean portion of the grid cell should be
divided by the ocean area in the grid cell (in this context, ocean
includes sea ice); report only for a single year and a single run,
assuming this field is the same from year to year and for all runs.
water_evaporation_flux_where_sea_ice:
Compute the average rate that water mass evaporates (or sublimates) from
the sea ice surface (i.e., kg/s) divided by the average area of the grid
cell covered by sea ice. This quantity multiplied both by the average
area covered by sea ice and by the length of the month should yield the
total mass of water evaporated (or sublimated) from the sea ice. Report
as 0.0 in regions free of sea ice.
upward_sea_ice_basal_heat_flux:
Compute the average rate that heat flows up at the base of the sea ice
(i.e., Watts) divided by the average area of the grid cell covered by
sea ice. This quantity multiplied both by the average area covered by
sea ice and by the length of the month should yield the total energy
flowing into the ice from below. Report as 0.0 in regions free of sea ice.
precipitation_flux_onto_canopy:
Compute the average rate that salt mass flows down at the base of the
sea ice (i.e., kg/s) divided by the average area of the grid cell
covered by sea ice. This quantity multiplied both by the average area
covered by sea ice and by the length of the month should yield the total
salt mass flowing into the ocean at the base of the sea ice. Report as
0.0 in regions free of sea ice.
best regards,
Karl
Beate.Geyer at gkss.de wrote:
>
> 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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Received on Wed Feb 15 2006 - 10:26:43 GMT
