This folder contains some netCDF template files for 3D data on the GEOS-Chem grid (hybrid sigma pressure levels).
The template files contain all the meta-data needed to properly describe the horizontal and vertical coordinates. These
files can be merged with effective 3D data to create CF-compatible netCDF files.

For example, suppose 3D monthly data for an arbitrary year and with incomplete vertical axis description is available in 
file orig3D.geos.4x5.nc. The following commands can then be used to create a netCDF file with properly defined hybrid 
sigma coordinates:

# make sure that grid is same as template:
cdo remapdis,../geos.4x5.grid orig3D.geos.4x5.nc tmp.nc

# set year to 2005
cdo setyear,2005 tmp.nc tmp2.nc
rm -f tmp.nc

# now add 3D array to new file
cp GEOS5_4x5_47L_template_2005_monthly.nc new3D.geos.4x5.nc
ncks -A tmp2.nc new3D.geos.4x5.nc
rm -f tmp2.nc


NOTES:
(1) In order to properly describe the vertical levels, the surface pressure field must be available for every data
time step. Thus, it may be necessary to modify the existing templates so that they cover the desired time steps. To
add a new pressure field (in file new_pressure.nc), use e.g.:

   cp GEOS5_4x5_47L_template_2005_monthly.nc new_template.nc
   ncks -A -v aps new_pressure.nc new_template.nc

(2) It is crucial that the level attribute "formula_terms" refers to the proper variable names. For instance, the 
formula term in the template is:

   lev:formula_terms = "ap: hyam b: hybm ps: aps" ;

In this case, the hybrid a coefficient must have variable name 'hyam', b coefficient must have name 'hybm', and the surface
pressure variable must be named 'aps'.


Christoph Keller, October 2014