combining_netcdf_antimeridian.md

Combining two NetCDF files at the antimeridian

File Name: combining_netcdf_antimeridian.md
Date Created: Mar 2019
Owner: CoastWatch
Author: Dale Robinson, CoastWatch West Coast Node

This example shows how to merge two NetCDF files that span the antimeridian into one NetCDF file. This is a common task when working with satellite data in the polar regions. Here we will demonstrate downloading ice concentration from ERDDAP in two sections and then show how to piece them together into one file. This is accomplished with the easy to use NetCDF Operator Tools which are command line programs for working with NetCDF files.

Learn more about NCO and download the package at: http://nco.sourceforge.net/nco.html#Summary

Download the data in NetCDF Format

Will will download data from ERDDAP from Longitudes 176 to -152. To do this we will download two files, one from 176 to 180, and another from -180 to -152.

Western Pacific File Download

• Longitude Range: 176 to 180, and Latitude Range: 50 to 89.99
• The ERDDAP URL for the file download:
  http://coastwatch.pfeg.noaa.gov/erddap/griddap/jplMURSST41.nc?sea_ice_fraction[(2018-02-04T09:00:00Z):1:(2018-02-04T09:00:00Z)][(50):1:(89.99)][(176):1:(180.0)]
• Rename the downloaded file to Wpacific.nc

Eastern Pacific File Download

• Longitude Range: -179.99 to -152, and Latitude Range: 50 to 89.99
• The ERDDAP URL for the file download:
  http://coastwatch.pfeg.noaa.gov/erddap/griddap/jplMURSST41.nc?sea_ice_fraction[(2018-02-04T09:00:00Z):1:(2018-02-04T09:00:00Z)][(50):1:(89.99)][(-179.99):1:(-152)]
• Rename the downloaded file to Epacific.nc

Change the record dimension from Time to Longitude

Temporarily change the record dimension from time to longitude in the two files. The resulting files are named with a "L" ending, which indicates the files use the longitude dimension and makes it easier to follow the work flow.

ncpdq -O -a longitude,time Wpacific.nc Wpacific_L.nc
ncks -O --mk_rec_dmn longitude Wpacific_L.nc Wpacific_L2.nc
ncpdq -O -a longitude,time Epacific.nc Epacific_L.nc
ncks -O --mk_rec_dmn longitude Epacific_L.nc Epacific_L2.nc

Remove references to longitude extent

Remove the valid_min and valid_max from the longitude variable in both files

ncatted -O -a valid_min,longitude,d,f, Wpacific_L2.nc
ncatted -O -a valid_max,longitude,d,f, Wpacific_L2.nc
ncatted -O -a valid_min,longitude,d,f, Epacific_L2.nc
ncatted -O -a valid_max,longitude,d,f, Epacific_L2.nc

Shift the longitude range

Use ncap2 to shift the Eastern Pacific (western hemisphere) longitudes into the range 180 - 360

ncap2 -s "longitude= longitude+360.0f" Epacific_L2.nc Epacific360.nc

Combine the files

Concatenate the Western and Eastern Pacific files along new (longitude) record dimension, naming the resulting file combined_pacific360.nc

ncrcat Wpacific_L2.nc Epacific360.nc combined_pacific360.nc

Update the dimensions

Change the record dimension back to "time" in the combined_pacific360.nc file

ncpdq -O -a time,longitude combined_pacific360.nc combined_pacific360b.nc
ncks -O --mk_rec_dmn time combined_pacific360b.nc combined_pacific360b.nc

Set the Max and Min Ranges

Restore the valid min and valid_max attributes of the longitude variable in the combined_pacific360.nc file, making them 0 and 360, respectively (optional) and delete the old actual_range (optional)

ncatted -O -a valid_min,longitude,c,f,0.0 combined_pacific360b.nc
ncatted -O -a valid_max,longitude,c,f,360.0 combined_pacific360b.nc
ncatted -O -a actual_range,longitude,d,, combined_pacific360b.nc

Clean Up

Optionally, remove the original and intermediary files.

rm Epacific_L.nc Epacific_L2.nc Epacific.nc Epacific360.nc Wpacific_L.nc Wpacific_L2.nc Wpacific.nc combined_pacific360.nc