7. Cube interpolation and regridding

Iris provides powerful cube-aware interpolation and regridding functionality, exposed through Iris cube methods. This functionality is provided by building upon existing interpolation schemes implemented by SciPy.

In Iris we refer to the avaliable types of interpolation and regridding as schemes. The following are the interpolation schemes that are currently available in Iris:

The following are the regridding schemes that are currently available in Iris:

7.1. Interpolation

Interpolating a cube is achieved with the interpolate() method. This method expects two arguments:

  1. the sample points to interpolate, and
  2. the second argument being the interpolation scheme to use.

The result is a new cube, interpolated at the sample points.

Sample points must be defined as an iterable of (coord, value(s)) pairs. The coord argument can be either a coordinate name or coordinate instance. The specified coordinate must exist on the cube being interpolated! For example:

  • coordinate names and scalar sample points: [('latitude', 51.48), ('longitude', 0)],
  • a coordinate instance and a scalar sample point: [(cube.coord('latitude'), 51.48)], and
  • a coordinate name and a NumPy array of sample points: [('longitude', np.linspace(-11, 2, 14))]

are all examples of valid sample points.

The values for coordinates that correspond to date/times can be supplied as datetime.datetime or netcdftime.datetime instances, e.g. [('time', datetime.datetime(2009, 11, 19, 10, 30))]).

Let’s take the air temperature cube we’ve seen previously:

>>> air_temp = iris.load_cube(iris.sample_data_path('air_temp.pp'))
>>> print(air_temp)
air_temperature / (K)               (latitude: 73; longitude: 96)
     Dimension coordinates:
          latitude                           x              -
          longitude                          -              x
     Scalar coordinates:
          forecast_period: 6477 hours, bound=(-28083.0, 6477.0) hours
          forecast_reference_time: 1998-03-01 03:00:00
          pressure: 1000.0 hPa
          time: 1998-12-01 00:00:00, bound=(1994-12-01 00:00:00, 1998-12-01 00:00:00)
     Attributes:
          STASH: m01s16i203
          source: Data from Met Office Unified Model
     Cell methods:
          mean within years: time
          mean over years: time

We can interpolate specific values from the coordinates of the cube:

>>> sample_points = [('latitude', 51.48), ('longitude', 0)]
>>> print(air_temp.interpolate(sample_points, iris.analysis.Linear()))
air_temperature / (K)               (scalar cube)
     Scalar coordinates:
          forecast_period: 6477 hours, bound=(-28083.0, 6477.0) hours
          forecast_reference_time: 1998-03-01 03:00:00
          latitude: 51.48 degrees
          longitude: 0 degrees
          pressure: 1000.0 hPa
          time: 1998-12-01 00:00:00, bound=(1994-12-01 00:00:00, 1998-12-01 00:00:00)
     Attributes:
          STASH: m01s16i203
          source: Data from Met Office Unified Model
     Cell methods:
          mean within years: time
          mean over years: time

As we can see, the resulting cube is scalar and has longitude and latitude coordinates with the values defined in our sample points.

It isn’t necessary to specify sample points for every dimension, only those that you wish to interpolate over:

>>> result = air_temp.interpolate([('longitude', 0)], iris.analysis.Linear())
>>> print('Original: ' + air_temp.summary(shorten=True))
Original: air_temperature / (K)               (latitude: 73; longitude: 96)
>>> print('Interpolated: ' + result.summary(shorten=True))
Interpolated: air_temperature / (K)               (latitude: 73)

The sample points for a coordinate can be an array of values. When multiple coordinates are provided with arrays instead of scalar sample points, the coordinates on the resulting cube will be orthogonal:

>>> sample_points = [('longitude', np.linspace(-11, 2, 14)),
...                  ('latitude',  np.linspace(48, 60, 13))]
>>> result = air_temp.interpolate(sample_points, iris.analysis.Linear())
>>> print(result.summary(shorten=True))
air_temperature / (K)               (latitude: 13; longitude: 14)

7.1.1. Interpolating non-horizontal coordinates

Interpolation in Iris is not limited to horizontal-spatial coordinates - any coordinate satisfying the prerequisites of the chosen scheme may be interpolated over.

For instance, the iris.analysis.Linear scheme requires 1D numeric, monotonic, coordinates. Supposing we have a single column cube such as the one defined below:

>>> column = iris.load_cube(iris.sample_data_path('hybrid_height.nc'))[:, 0, 0]
>>> print(column.summary(shorten=True))
air_potential_temperature / (K)     (model_level_number: 15)

This cube has a “hybrid-height” vertical coordinate system, meaning that the vertical coordinate is unevenly spaced in altitude:

>>> print(column.coord('altitude').points)
[  418.69836426   434.57049561   456.79278564   485.3664856    520.29327393
   561.57519531   609.21447754   663.21411133   723.57696533   790.30664062
   863.40722656   942.88232422  1028.73706055  1120.97644043  1219.60510254]

We could regularise the vertical coordinate by defining 10 equally spaced altitude sample points between 400 and 1250 and interpolating our vertical coordinate onto these sample points:

>>> sample_points = [('altitude', np.linspace(400, 1250, 10))]
>>> new_column = column.interpolate(sample_points, iris.analysis.Linear())
>>> print(new_column.summary(shorten=True))
air_potential_temperature / (K)     (model_level_number: 10)

Let’s look at the original data, the interpolation line and the new data in a plot. This will help us to see what is going on:

(Source code)

The red diamonds on the extremes of the altitude values show that we have extrapolated data beyond the range of the original data. In some cases this is desirable but in other cases it is not. For example, this column defines a surface altitude value of 414m, so extrapolating an “air potential temperature” at 400m makes little physical sense in this case.

We can control the extrapolation mode when defining the interpolation scheme. Controlling the extrapolation mode allows us to avoid situations like the above where extrapolating values makes little physical sense.

The extrapolation mode is controlled by the extrapolation_mode keyword. For the available interpolation schemes available in Iris, the extrapolation_mode keyword must be one of:

  • extrapolate – the extrapolation points will be calculated by extending the gradient of the closest two points,
  • error – a ValueError exception will be raised, notifying an attempt to extrapolate,
  • nan – the extrapolation points will be be set to NaN,
  • mask – the extrapolation points will always be masked, even if the source data is not a MaskedArray, or
  • nanmask – if the source data is a MaskedArray the extrapolation points will be masked. Otherwise they will be set to NaN.

Using an extrapolation mode is achieved by constructing an interpolation scheme with the extrapolation mode keyword set as required. The constructed scheme is then passed to the interpolate() method. For example, to mask values that lie beyond the range of the original data:

>>> scheme = iris.analysis.Linear(extrapolation_mode='mask')
>>> new_column = column.interpolate(sample_points, scheme)
>>> print(new_column.coord('altitude').points)
[           nan   494.44451904   588.88891602   683.33325195   777.77783203
   872.222229     966.66674805  1061.11108398  1155.55541992            nan]

7.1.2. Caching an interpolator

If you need to interpolate a cube on multiple sets of sample points you can ‘cache’ an interpolator to be used for each of these interpolations. This can shorten the execution time of your code as the most computationally intensive part of an interpolation is setting up the interpolator.

To cache an interpolator you must set up an interpolator scheme and call the scheme’s interpolator method. The interpolator method takes as arguments:

  1. a cube to be interpolated, and
  2. an iterable of coordinate names or coordinate instances of the coordinates that are to be interpolated over.

For example:

>>> air_temp = iris.load_cube(iris.sample_data_path('air_temp.pp'))
>>> interpolator = iris.analysis.Nearest().interpolator(air_temp, ['latitude', 'longitude'])

When this cached interpolator is called you must pass it an iterable of sample points that have the same form as the iterable of coordinates passed to the constructor. So, to use the cached interpolator defined above:

>>> latitudes = np.linspace(48, 60, 13)
>>> longitudes = np.linspace(-11, 2, 14)
>>> for lat, lon in zip(latitudes, longitudes):
...     result = interpolator([lat, lon])

In each case result will be a cube interpolated from the air_temp cube we passed to interpolator.

Note that you must specify the required extrapolation mode when setting up the cached interpolator. For example:

>>> interpolator = iris.analysis.Nearest(extrapolation_mode='nan').interpolator(cube, coords)

7.2. Regridding

Regridding is conceptually a very similar process to interpolation in Iris. The primary difference is that interpolation is based on sample points, while regridding is based on the horizontal grid of another cube.

Regridding a cube is achieved with the cube.regrid() method. This method expects two arguments:

  1. another cube that defines the target grid onto which the cube should be regridded, and
  2. the regridding scheme to use.

Note

Regridding is a common operation needed to allow comparisons of data on different grids. The powerful mapping functionality provided by cartopy, however, means that regridding is often not necessary if performed just for visualisation purposes.

Let’s load two cubes that have different grids and coordinate systems:

>>> global_air_temp = iris.load_cube(iris.sample_data_path('air_temp.pp'))
>>> rotated_psl = iris.load_cube(iris.sample_data_path('rotated_pole.nc'))

We can visually confirm that they are on different grids by plotting the two cubes:

(Source code, png)

../_images/regridding_plot.png

Let’s regrid the global_air_temp cube onto a rotated pole grid using a linear regridding scheme. To achieve this we pass the rotated_psl cube to the regridder to supply the target grid to regrid the global_air_temp cube onto:

>>> rotated_air_temp = global_air_temp.regrid(rotated_psl, iris.analysis.Linear())

(Source code, png)

../_images/regridded_to_rotated.png

We could regrid the pressure values onto the global grid, but this will involve some form of extrapolation. As with interpolation, we can control the extrapolation mode when defining the regridding scheme.

For the available regridding schemes in Iris, the extrapolation_mode keyword must be one of:

  • extrapolate
    • for Linear the extrapolation points will be calculated by extending the gradient of the closest two points.
    • for Nearest the extrapolation points will take their value from the nearest source point.
  • nan – the extrapolation points will be be set to NaN.

  • error – a ValueError exception will be raised, notifying an attempt to extrapolate.

  • mask – the extrapolation points will always be masked, even if the source data is not a MaskedArray.

  • nanmask – if the source data is a MaskedArray the extrapolation points will be masked. Otherwise they will be set to NaN.

The rotated_psl cube is defined on a limited area rotated pole grid. If we regridded the rotated_psl cube onto the global grid as defined by the global_air_temp cube any linearly extrapolated values would quickly become dominant and highly inaccurate. We can control this behaviour by defining the extrapolation_mode in the constructor of the regridding scheme to mask values that lie outside of the domain of the rotated pole grid:

>>> scheme = iris.analysis.Linear(extrapolation_mode='mask')
>>> global_psl = rotated_psl.regrid(global_air_temp, scheme)

(Source code, png)

../_images/regridded_to_global.png

Notice that although we can still see the approximate shape of the rotated pole grid, the cells have now become rectangular in a plate carrée (equirectangular) projection. The spatial grid of the resulting cube is really global, with a large proportion of the data being masked.

7.2.1. Area-weighted regridding

It is often the case that a point-based regridding scheme (such as iris.analysis.Linear or iris.analysis.Nearest) is not appropriate when you need to conserve quantities when regridding. The iris.analysis.AreaWeighted scheme is less general than Linear or Nearest, but is a conservative regridding scheme, meaning that the area-weighted total is approximately preserved across grids.

With the AreaWeighted regridding scheme, each target grid-box’s data is computed as a weighted mean of all grid-boxes from the source grid. The weighting for any given target grid-box is the area of the intersection with each of the source grid-boxes. This scheme performs well when regridding from a high resolution source grid to a lower resolution target grid, since all source data points will be accounted for in the target grid.

Let’s demonstrate this with the global air temperature cube we saw previously, along with a limited area cube containing total concentration of volcanic ash:

>>> global_air_temp = iris.load_cube(iris.sample_data_path('air_temp.pp'))
>>> print(global_air_temp.summary(shorten=True))
air_temperature / (K)               (latitude: 73; longitude: 96)
>>>
>>> regional_ash = iris.load_cube(iris.sample_data_path('NAME_output.txt'))
>>> regional_ash = regional_ash.collapsed('flight_level', iris.analysis.SUM)
>>> print(regional_ash.summary(shorten=True))
VOLCANIC_ASH_AIR_CONCENTRATION / (g/m3) (latitude: 214; longitude: 584)

One of the key limitations of the AreaWeighted regridding scheme is that the two input grids must be defined in the same coordinate system as each other. Both input grids must also contain monotonic, bounded, 1D spatial coordinates.

Note

The AreaWeighted regridding scheme requires spatial areas, therefore the longitude and latitude coordinates must be bounded. If the longitude and latitude bounds are not defined in the cube we can guess the bounds based on the coordinates’ point values:

>>> global_air_temp.coord('longitude').guess_bounds()
>>> global_air_temp.coord('latitude').guess_bounds()

Using NumPy’s masked array module we can mask any data that falls below a meaningful concentration:

>>> regional_ash.data = np.ma.masked_less(regional_ash.data, 5e-6)

Finally, we can regrid the data using the AreaWeighted regridding scheme:

>>> scheme = iris.analysis.AreaWeighted(mdtol=0.5)
>>> global_ash = regional_ash.regrid(global_air_temp, scheme)
>>> print(global_ash.summary(shorten=True))
VOLCANIC_ASH_AIR_CONCENTRATION / (g/m3) (latitude: 73; longitude: 96)

Note that the AreaWeighted regridding scheme allows us to define a missing data tolerance (mdtol), which specifies the tolerated fraction of masked data in any given target grid-box. If the fraction of masked data within a target grid-box exceeds this value, the data in this target grid-box will be masked in the result.

The fraction of masked data is calculated based on the area of masked source grid-boxes that overlaps with each target grid-box. Defining an mdtol in the AreaWeighted regridding scheme allows fine control of masked data tolerance. It is worth remembering that defining an mdtol of anything other than 1 will prevent the scheme from being fully conservative, as some data will be disregarded if it lies close to masked data.

To visualise the above regrid, let’s plot the original data, along with 3 distinct mdtol values to compare the result:

(Source code, png)

../_images/regridded_to_global_area_weighted.png

7.2.2. Caching a regridder

If you need to regrid multiple cubes with a common source grid onto a common target grid you can ‘cache’ a regridder to be used for each of these regrids. This can shorten the execution time of your code as the most computationally intensive part of a regrid is setting up the regridder.

To cache a regridder you must set up a regridder scheme and call the scheme’s regridder method. The regridder method takes as arguments:

  1. a cube (that is to be regridded) defining the source grid, and
  2. a cube defining the target grid to regrid the source cube to.

For example:

>>> global_air_temp = iris.load_cube(iris.sample_data_path('air_temp.pp'))
>>> rotated_psl = iris.load_cube(iris.sample_data_path('rotated_pole.nc'))
>>> regridder = iris.analysis.Nearest().regridder(global_air_temp, rotated_psl)

When this cached regridder is called you must pass it a cube on the same grid as the source grid cube (in this case global_air_temp) that is to be regridded to the target grid. For example:

>>> for cube in list_of_cubes_on_source_grid:
...     result = regridder(cube)

In each case result will be the input cube regridded to the grid defined by the target grid cube (in this case rotated_psl) that we used to define the cached regridder.