Opening datasets#

Reading local files#

Hyoga acts as a thin wrapper around a much more powerful Python library called xarray. Xarray is used to open a dataset that can be processed by hyoga. Typically this will be a local netCDF file as in the example below, but it could also be a raster file, an online store or a huge dataset spread across hundreds of files. Whatever the source, xarray does the hard work and provides us with a xarray.Dataset object ready to use with hyoga:

import xarray as xr
ds = xr.open_dataset('yourfile.nc')

Note

Hyoga also provides functions to open datasets with an age coordinate in ka (see Programming interface). However, if possible I recommend to stick with xarray functions for the time being.

Opening online data#

A central functionality for hyoga consists in opening web-available data in custom projections for numerical ice-sheet modelling. Internally, hyoga will download the original data, typically global, store a copy in the cache directory (~/.cache/hyoga/), reproject to the desired modelling domain, and return an xarray.Dataset.

Currently two types of input datasets are supported: bootstrapping and atmospheric, respectively returned by hyoga.open.bootstrap() and hyoga.open.atmosphere(). The bootstrapping data contains bedrock and/or surface topography. The atmosphere data contains a monthly climatology of air temperature and precipitation needed to force a positive degree-day model. This nomenclature follows that introduced by PISM, and the resulting datasets are ready to export as PISM input files using Dataset.to_netcdf().

Warning

Running these for the first time will download and deflate ca. 12 GB data from the web. Broken or partially downloaded files are not handled and will need to be manually deleted from the cache directory (~/.cache/hyoga) in case of interrupted downloads.

PISM example run#

The following EPSG code and coordinate bounds describe a Universal Transverse Mercator (UTM) zone 32 projection covering the European Alps. This is the format for the data used in the Examples gallery.

import hyoga

# UTM-32 projection, WGS 84 datum
crs = 'epsg:32632'

# west, south, east, north bounds
bounds = [150e3, 4820e3, 1050e3, 5420e3]

The following will prepare a simple bootstrapping dataset containing bedrock topography from GEBCO with a horizontal resolution of a 1 km, and save it to a PISM-readable NetCDF file:

ds = hyoga.open.bootstrap(crs=crs, bounds=bounds, resolution=1000)
ds.to_netcdf('boot.nc')

For the atmospheric forcing, we reduce the input air temperature by six degrees to simulate glacial conditions. The resulting file contains 24 high-resolution, monthly temperature and precipitation grid from CHELSA, hence reprojecting the data can take several minutes.

ds = hyoga.open.atmosphere(crs=crs, bounds=bounds, resolution=1000)
ds['air_temp'] -= 6
ds.to_netcdf('atm.nc')

Both input files are now ready to be used in a simple paleo-glacier modelling run on the alps:

mpiexec -n 4 pismr \
    -i boot.nc -bootstrap -o run.nc -ys 0 -ye 100 \
    -atmosphere given,elevation_change \
        -atmosphere.given.periodic \
        -atmosphere_given_file atm.nc \
        -atmosphere_lapse_rate_file atm.nc \
    -surface pdd

Since the input data are global, the crs and bounds can be modified to run this setup anywhere on Earth. The results can be viewed in e.g. ncview, or plotted in hyoga using functionality described in the following pages.

Opening datasets

Page contents

Opening datasets#

Reading local files#

Opening online data#

PISM example run#