cf.Field.regridc¶

Field.
regridc
(dst, axes=None, method=None, use_src_mask=True, use_dst_mask=False, fracfield=False, axis_order=None, ignore_degenerate=True, return_operator=False, check_coordinates=False, min_weight=None, weights_file=None, inplace=False, i=False, _compute_field_mass=None)[source]¶ Regrid the field to a new Cartesian grid.
Regridding is the process of interpolating the field data values while preserving the qualities of the original data, and the metadata of the unaffected axes. The metadata for the regridded axes are taken from the dst parameter.
Between one and three axes may be simultaneously regridded in Cartesian space.
Coordinates
The source and destination grids of the regridding must both be defined by equivalent coordinates, which must be 1d dimension coordinates. These are automatically detected from the field being regridded and the specification of the destination grid given by the dst parameter.
Masked cells
By default, the data mask of the source data is taken into account during the regridding process, but the destination grid mask is not. This behaviour may be changed with the use_src_mask and use_dst_mask parameters.
In general the source data may be arbitrarily masked, meaning that the mask for the regridding axes may vary along the nonregridding axes. The exceptions to this are for secondorder conservative, patch recovery regridding, and nearest source to destination methods, for which the mask of the regridding axes must be the same across all nonregridding axes. In these special cases an exception will be raised if the source data mask does not meet this requirement.
Implementation
The interpolation is carried out using regridding weights calculated by the
esmpy
package, a Python interface to the Earth System Modeling Framework regridding utility (ESMF, https://earthsystemmodeling.org/regrid). Outside ofesmpy
, these weights are then modified for masked cells (if required) and the regridded data are created as the dot product of the weights with the source data. (Note that whilst theesmpy
package is also able to create the regridded data from its weights, this feature can’t be integrated with thedask
framework that underpins the field’s data.)Logging
Whether
esmpy
logging is enabled or not is determined bycf.regrid_logging
. If it is enabled then logging takes place after every call. By default logging is disabled.See also
 Parameters
 dst:
Field
,Domain
,RegridOperator
or sequence ofDimensionCoordinate
The definition of the destination grid on which to regrid the field’s data. One of:
Field
: The grid is defined by the coordinates of the field construct’s domain.Domain
: The grid is defined by the coordinates of the domain construct.Sequence of
DimensionCoordinate
: The grid is defined by between one and three 1d dimension coordinate constructs that define the coordinates of the destination grid. The order of the coordinate constructs must match the order of source field regridding axes defined by the src_axes or axes parameter.RegridOperator
: The grid is defined by a regrid operator that has been returned by a previous call with the return_operator parameter set to True.Unlike the other options, for which the regrid weights need to be calculated, the regrid operator already contains the weights. Therefore, for cases where multiple fields with the same source grids need to be regridded to the same destination grid, using a regrid operator can give performance improvements by avoiding having to calculate the weights for each source field. Note that for the other types of dst parameter, the calculation of the regrid weights is not a lazy operation.
Note
The source grid of the regrid operator is immediately checked for compatibility with the grid of the source field. By default only the computationally cheap tests are performed (checking that the coordinate system, cyclicity and grid shape are the same), with the grid coordinates not being checked. The coordinates check will be carried out, however, if the check_coordinates parameter is True.
 method:
str
orNone
, optional Specify the regridding interpolation method. This parameter must be set unless dst is a
RegridOperator
, when the method is ignored.The method parameter may be one of the following:
'linear'
: Bilinear interpolation.'bilinear'
: Deprecated alias for'linear'
.'conservative_1st'
: First order conservative interpolation. Preserves the area integral of the data across the interpolation from source to destination. It uses the proportion of the area of the overlapping source and destination cells to determine appropriate weights.'conservative'
: Alias for'conservative_1st'
'conservative_2nd'
: Secondorder conservative interpolation. As with first order conservative interpolation, preserves the area integral of the field between source and destination using a weighted sum, with weights based on the proportionate area of intersection. In addition the secondorder conservative method takes the source gradient into account, so it yields a smoother destination field that typically better matches the source data.'patch'
Patch recovery interpolation. A second degree 2d polynomial regridding method, which uses a least squares algorithm to calculate the polynomials. This method typically results in better approximations to values and derivatives when compared to bilinear interpolation.'nearest_stod'
: Nearest neighbour interpolation for which each destination point is mapped to the closest source point. Useful for extrapolation of categorical data. Some destination cells may be unmapped.'nearest_dtos'
: Nearest neighbour interpolation for which each source point is mapped to the destination point. Useful for extrapolation of categorical data. All destination cells will be mapped.None
: This is the default and can only be used when dst is aRegridOperator
.
 use_src_mask:
bool
, optional By default the mask of the source field is taken into account during the regridding process. The only possible exception to this is when the nearest source to destination regridding method (
'nearest_stod'
) is being used. In this case, if use_src_mask is False then each destination point is mapped to the closest source point, whether or not it is masked (see the method parameter for details).Ignored if dst is a
RegridOperator
. use_dst_mask:
bool
, optional If dst is a
Field
and use_dst_mask is False (the default) then the mask of data on the destination grid is not taken into account when performing regridding. If use_dst_mask is True then any masked cells in the dst field construct are transferred to the result. If dst has more dimensions than are being regridded, then the mask of the destination grid is taken as the subspace defined by index0
of all the nonregridding dimensions.Ignored if dst is not a
Field
. src_axes: sequence, optional
Define the source grid axes to be regridded. The sequence of between one and three values identify unique domain axes by passing each axis description to a call of the source field construct’s
domain_axis
method. For example, for a value of'ncdim%x'
, the domain axis construct returned byf.domain_axis('ncdim%x')
is selected.Must have the same number of values as the dst_axes parameter, if set, and the source and destination regridding axes must be specified in the same order. See the axes parameter.
Ignored if dst is a
RegridOperator
. Parameter example:
['T']
 Parameter example:
[1, 0]
New in version 3.14.0.
 dst_axes:
sequence
, optional When the destination grid is defined by a
Field
orDomain
, define the destination grid axes to be regridded. The sequence of between one and three values identify unique domain axes by passing each axis description to a call of the destination field or domain construct’sdomain_axis
method. For example, for a value of'ncdim%x'
, the domain axis construct returned byg.domain_axis('ncdim%x')
is selected.Must have the same number of values as the src_axes parameter, if set, and the source and destination regridding axes must be specified in the same order. See the axes parameter.
Ignored if dst is a
RegridOperator
. Parameter example:
['T']
 Parameter example:
[1, 0]
New in version 3.14.0.
 axes: optional
Define the axes to be regridded for the source grid and, if dst is a
Field
orDomain
, the destination grid. The axes parameter is a convenience that may be used to replace src_axes and dst_axes when they would contain identical sequences. It may also be used in place of src_axes if dst_axes is not required. ignore_degenerate:
bool
, optional For conservative regridding methods, if True (the default) then degenerate cells (those for which enough vertices collapse to leave a cell as either a line or a point) are skipped, not producing a result. Otherwise an error will be produced if degenerate cells are found, that will be present in the
esmpy
log files.For all other regridding methods, degenerate cells are always skipped, regardless of the value of ignore_degenerate.
Ignored if dst is a
RegridOperator
. return_operator:
bool
, optional If True then do not perform the regridding, rather return the
RegridOperator
instance that defines the regridding operation, and which can be used in subsequent calls. See the dst parameter for details.New in version 3.10.0.
 check_coordinates:
bool
, optional If True and dst is a `RegridOperator`then the source grid coordinates defined by the operator are checked for compatibility against those of the source field. By default this check is not carried out. See the dst parameter for details.
Ignored unless dst is a
RegridOperator
.New in version 3.14.0.
 min_weight: float, optional
A very small nonnegative number. By default min_weight is
2.5 * np.finfo("float64").eps
, i.e.5.551115123125783e16
. It is used during linear and firstorder conservative regridding when adjusting the weights matrix to account for the data mask. It is ignored for all other regrid methods, or if data being regridded has no missing values.In some cases (described below) for which weights might only be nonzero as a result of rounding errors, the min_weight parameter controls whether or a not cell in the regridded field is masked.
The default value has been chosen empirically as the smallest value that produces the same masks as
esmpy
for the use cases defined in the cf test suite.Define
w_ji
as the multiplicative weight that defines how much ofVs_i
(the value in source grid celli
) contributes toVd_j
(the value in destination grid cellj
).Linear regridding
Destination grid cell
j
will only be masked if a) it is masked in the destination grid definition; or b)w_ji >= min_weight
for those masked source grid cellsi
for whichw_ji > 0
.Conservative firstorder regridding
Destination grid cell
j
will only be masked if a) it is masked in the destination grid definition; or b) the sum ofw_ji
for all nonmasked source grid cellsi
is strictly less than min_weight.New in version 3.14.0.
 weights_file:
str
orNone
, optional Provide a netCDF file that contains, or will contain, the regridding weights. If
None
(the default) then the weights are computed in memory for regridding between the source and destination grids, and no file is created.If set to a file path that does not exist then the weights will be computed and also written to that file.
If set to a file path that already exists then the weights will be read from this file, instead of being computed.
Note
No checks are performed on an existing file to ensure that the weights are appropriate for the source field and the values of the keyword parameters. Inappropriate weights will produce incorrect results.
However, when regridding using weights from a file, ensuring that the source field has the same shape over the regridding axes, and the parameter settings are the same as those used when the weights file was created, will ensure correct results.
A netCDF regridding weights file created directly by ESMF has the same structure and variable names (
S
,row
, andcol
for the weights, destination/row indices, and source/col indices respectively), so may be provided as a weights_file, noting that no checks will be applied to it.Performance
The computation of the weights can be much more costly than the regridding itself, in which case reading precalculated weights can improve performance.
New in version 3.15.2.
 inplace:
bool
, optional If True then do the operation inplace and return
None
. axis_order: sequence, optional
Deprecated at version 3.14.0.
 fracfield:
bool
, optional Deprecated at version 3.14.0.
 _compute_field_mass:
dict
, optional Deprecated at version 3.14.0.
 i: deprecated at version 3.0.0
Use the inplace parameter instead.
 dst:
 Returns
Field
orNone
orRegridOperator
The regridded field construct; or
None
if the operation was inplace; or the regridding operator if return_operator is True.
Examples
>>> src, dst = cf.example_fields(1, 0) >>> print(src) Field: air_temperature (ncvar%ta)  Data : air_temperature(atmosphere_hybrid_height_coordinate(1), grid_latitude(10), grid_longitude(9)) K Cell methods : grid_latitude(10): grid_longitude(9): mean where land (interval: 0.1 degrees) time(1): maximum Field ancils : air_temperature standard_error(grid_latitude(10), grid_longitude(9)) = [[0.76, ..., 0.32]] K Dimension coords: atmosphere_hybrid_height_coordinate(1) = [1.5] : grid_latitude(10) = [2.2, ..., 1.76] degrees : grid_longitude(9) = [4.7, ..., 1.18] degrees : time(1) = [20190101 00:00:00] Auxiliary coords: latitude(grid_latitude(10), grid_longitude(9)) = [[53.941, ..., 50.225]] degrees_N : longitude(grid_longitude(9), grid_latitude(10)) = [[2.004, ..., 8.156]] degrees_E : long_name=Grid latitude name(grid_latitude(10)) = [, ..., kappa] Cell measures : measure:area(grid_longitude(9), grid_latitude(10)) = [[2391.9657, ..., 2392.6009]] km2 Coord references: grid_mapping_name:rotated_latitude_longitude : standard_name:atmosphere_hybrid_height_coordinate Domain ancils : ncvar%a(atmosphere_hybrid_height_coordinate(1)) = [10.0] m : ncvar%b(atmosphere_hybrid_height_coordinate(1)) = [20.0] : surface_altitude(grid_latitude(10), grid_longitude(9)) = [[0.0, ..., 270.0]] m >>> print(dst) Field: specific_humidity (ncvar%q)  Data : specific_humidity(latitude(5), longitude(8)) 1 Cell methods : area: mean Dimension coords: latitude(5) = [75.0, ..., 75.0] degrees_north : longitude(8) = [22.5, ..., 337.5] degrees_east : time(1) = [20190101 00:00:00] >>> x = src.regridc(dst, method='linear', axes=['Y']) >>> print(x) Field: air_temperature (ncvar%ta)  Data : air_temperature(atmosphere_hybrid_height_coordinate(1), latitude(5), grid_longitude(9)) K Cell methods : latitude(5): grid_longitude(9): mean where land (interval: 0.1 degrees) time(1): maximum Dimension coords: atmosphere_hybrid_height_coordinate(1) = [1.5] : latitude(5) = [75.0, ..., 75.0] degrees_north : grid_longitude(9) = [4.7, ..., 1.18] degrees : time(1) = [20190101 00:00:00] Coord references: standard_name:atmosphere_hybrid_height_coordinate Domain ancils : ncvar%a(atmosphere_hybrid_height_coordinate(1)) = [10.0] m : ncvar%b(atmosphere_hybrid_height_coordinate(1)) = [20.0] : surface_altitude(latitude(5), grid_longitude(9)) = [[, ..., ]] m
>>> r = src.regridc(dst, method='linear', axes=['Y'], return_operator=True) >>> y = src.regridc(r) >>> y.equals(x) True