Let's say I have made a netCDF
file, which has a lot of zeros. I want to apply a sea mask to the data, such that only the regions in the sea are really masked and the data on the land is retained.
The data that I have has a lot of zeros on land (which is correct), but also has a lot of zeros in the sea (which is not correct).
I could have used cdo setmissval,nan input.nc output.nc
but that would have also changed the value on the land to NaN
.
Does someone have any (good) solution to that?
This question is already posed here: Create a NetCDF file with data masked to retain land points only
So basically you can build a land-sea mask using the built in topography function, and then set all the sea points to missing:
cdo -f nc setctomiss,0 -gtc,0 -remapcon,your_data_file.nc -topo seamask.nc
You can now use this to mask your datafile:
cdo mul your_data_file.nc seamask.nc masked_datafile.nc
However, in some circumstances I have found that the remapping process leaves traces of "ocean" data around the edges, in this case to be safer you can use the second method:
Download the netcdf data file for "distance to ocean" at 1km resolution from this thredds server: https://pae-paha.pacioos.hawaii.edu/thredds/ncss/dist2coast_1deg_land/dataset.html
Then you can mask out any points within a certain distance of the ocean to play it safe, at the expense of possibly masking out a small amount of land data.
I remapped the distance file to the target resolution first:
cdo remapbil,your_data.nc distance.nc remap_dist.nc
then mask (e.g. in this case all points within 5km of the coast, sea points are already "missing" in this file) and multiply
cdo mul your_data.nc -gtc,5 remap_dist.nc masked_data.nc
As said, this is a little safer, a little more longwinded, but may mask some land data.