Hi,
I am an occasional reader of this list and have a query/proposal about
the CF description of partial cells in (ocean) models with z-
coordinates.
As it stands in the CF convention (as I understand it), when using
partial cells the different volume of cells is accounted for by
including a 3-D array of the size of the full grid to describe the
volume of each cell as well as a cell measure attribute in the variable:
dimensions:
deptht = 301 ;
y = 149 ;
x = 182 ;
variables:
float deptht(deptht) ;
deptht: long_name = ?depth? ;
deptht: units = ?m? ;
float lon(x,y) ;
lon: long_name = ?longitude? ;
lon: units = ?degrees_east? ;
float lat(x,y) ;
lat: long_name = ?latitude? ;
lat: units = ?degrees_north? ;
double volumet(y, x) ;
volumet :units = "m3" ;
volumet :long_name = "volume of grid cell" ;
volumet :standard_name = "cell_volume?;
float votemper(deptht, y, x) ;
votemper:units = "degC" ;
votemper:long_name = "Temperature" ;
votemper:coordinates = "deptht lat lon" ;
votemper:cell_measures = "volume: volumet" ;
votemper:standard_name = "sea_water_potential_temperature" ;
My query/suggestion is about the need for a 3-D array for the thickness
of the partial cell. The information on the thickness of the partial
cell is essentially 2-D and the "volumet" array in the example above is
almost doubling the size of the file. It would be entirely possible to
fully define grid cell volumes using level thickness, grid cell area and
information on the level and thickness of the partial cell. In the below
example the "deptht_bounds" is used to find level thickness while the
grid cell area is given in the 2-D array "areat". These two can be used
for volume of every cell except the partial cells. To deal with the
volume of the partial cells a new cell_measure of
"partial_cell_thickness" could be used. In such a case the thickness and
level of the partial cell can be specifed in a single number. For
example of if the 24th level was the partial cell and it was 0.4 of the
original thickness a "partial_cell_thickness" of 23.4 would specify the
levels and thickness, i.e 23 full levels and 0.4 of the 24th. The
thickness of the partial cell being 0.4 of the thickness determined from
the 24th levels of "deptht_bounds" which would act as a refference. This
in combination with the "areat" array would describe the cell volumes.
e.g.
dimensions:
deptht = 301 ;
y = 149 ;
x = 182 ;
ndepth_bounds = 2 ;
variables:
float deptht(deptht) ;
deptht: long_name = ?depth? ;
deptht: units = ?m? ;
deptht: bounds = ?deptht_bounds" ;
float lon(x,y) ;
lon: long_name = ?longitude? ;
lon: units = ?degrees_east? ;
float lat(x,y) ;
lat: long_name = ?latitude? ;
lat: units = ?degrees_north? ;
double deptht_bounds(deptht, ndepth_bounds) ;
deptht_bounds:units = "m" ;
deptht_bounds:long_name = "depth boundaries of cells" ;
double areat(y, x) ;
areat:units = "m2" ;
areat:long_name = "area of grid cell" ;
I'm aware that this would require some extra processing to calculate a
3-D array of cell volumes whereas keeping the 3-D volume array is
foolproof, but the calculation is simple and it would greatly reduce the
over head if many files were stored at higher vertical resolutions as in
the above example.
I'd be keen to hear people opinions on this or possible alternatives if
I've missed something about possible ways to deal with this that are
already used.
Regards
--
Dan Bernie
HadGEM Physical Processes and Coupling Scientist
Met Office Hadley Centre, FitzRoy Road, Exeter, EX1 3PB
Tel: +44 (0)1392 884862 Fax: +44 (0)1392 885681
E-mail: dan.bernie at metoffice.gov.uk http://www.metoffice.gov.uk
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Received on Tue Jan 29 2008 - 02:19:47 GMT
