This icon takes data from a GRIB source and performs a variety of operations on it, including spectral to grid conversion, regridding using a large variety of powerful and flexible interpolation techniques, nabla operators and special consideration of wind fields.

This module is designed with re-use in mind. The first time a particular interpolation is performed, it might take some time to compute, but it will create cache files that can be re-used, meaning that the same interpolation will be much faster on subsequent runs.

The macro/python language equivalent is regrid().



The Regrid Editor

Input definition

Input data may be specified either by giving a path in the Source parameter or by giving a GRIB-based data object in the Data parameter. Note that you should specify either Source or Data, not both.

Drop a MARS Retrieval or a GRIB file icon inside this icon field. In Python or Macro, supply a Fieldset object.

Output definition

Grid Definition Mode

Select a method for specifying the output grid.

Grid Definition Mode: Grid

Grid

Supply a grid definition as described here: grid - keyword in MARS/Dissemination request.

Examples of valid grid definitions:

GUIPython / MacroResult
1/1[1, 1] or "1/1'"A regular lat/lon grid with 1x1 degree point spacing
0.25/0.25[0.25, 0.25] or "0.25/0.25"A regular lat/lon grid with 0.25x0.25 degree point spacing
O1280"O1280"An octahedral reduced Gaussian grid, octahedral with 1280 latitude lines between the pole and equator
N640"N640"An 'original' reduced Gaussian grid, with 640 latitude lines between the pole and equator
F400"F400"A regular Gaussian grid, with 400 latitude lines between the pole and equator

This parameter can be left empty to preserve the grid properties (regular/reduced lat/lon or Gaussian) while performing other kinds of post-processing (changing bits per value, calculation of gradients, etc.). 

Grid Definition Mode: Template

This mode configures regridding to output the same grid as the provided template, using one of the following parameters:

Grid Definition Mode: Lambert Conformal or Lambert Azimuthal Equal Area

These projections require setting several parameters, named following the convention in their descriptions:

Most of these parameters are required and do not have default values, meaning that they must be filled in. The parameters are:

Here are examples of generating Lambert grids.


regrid_lambert_conformal = mv.regrid(
    grid_definition_mode = "lambert_conformal",
    first_point          = [50.88,-1.66],
    dx_in_metres         = 2500,
    dy_in_metres         = 2500,
    nx                   = 739,
    ny                   = 949,
    lad_in_degrees       = 63,
    lov_in_degrees       = 15,
    data                 = t_2m_rgg
    )



regrid_lambert_azimuthal = mv.regrid(
    grid_definition_mode         = "lambert_azimuthal_equal_area",
    first_point                  = [50.88,-1.66],
    dx_in_metres                 = 2500,
    dy_in_metres                 = 2500,
    nx                           = 739,
    ny                           = 949,
    standard_parallel_in_degrees = 0,
    central_longitude_in_degrees = 0,
    data                         = t_2m_rgg
    )


Grid Definition Mode: Filter

This mode is similar to Template (see above), except the template is set to the input grid (the resulting output grid is the same as the input grid); In addition, interpolation k-nearest neighbours acts as a filter to the input values.

Wind processing

Wind Processing

Activates processing that is particular to wind fields. Winds are represented by its vector Cartesian components u/v (gridded) or U/V (spectral) and, typically, they are archived as (spectral) vorticity/divergence (vo/d.) The relation between the spectral and gridded wind components is u = U / cos(latitude) and v = V / cos(latitude).  

It is up to the user to specify if the input consists of wind fields. Set this appropriatelly in order to perform the correct processing.

Possible options are:

Wind Processing: U/V to u/v

Converts pairs of Cartesian components vector fields U/V (spectral) to u/v (gridded.) Assumes that the input come in pairs of alternating U/V. This option is required if regridding wind fields on/to a rotated grid.

Wind Processing: vo/d to u/v

Converts pairs of vorticity and divergence fields into U/V (spectral) or u/v (gridded.) Assumes that the input come in pairs of alternating vo/d. The resulting values will be scaled by the cosine of their latitudes (as mentioned above).

Wind Processing: Off

No special processing is performed (default). It is assumed there are no wind fields in the input, each processed is treated individually.

Spectral to grid inverse transform

If the input files are spectral, the following parameters are used to fine-tune the conversion to grid points.

Truncation

Spherical harmonics truncation, as described here: truncation - keyword in MARS/Dissemination request.

When the output is spectral, defines the output intended truncation; When the output is gridded, defines the intermediate truncation before spectral inverse transform to gridded space. Possible values are Automatic, None or a number describing the spectral truncation to be applied.

Intgrid

Intermediate grid when performing spectral inverse transform to gridded space, as described intgrid - keyword in MARS/Dissemination request.

Possible values are:


Extra processing

Area

Supply a grid definition as described here: area - keyword in MARS/Dissemination request (swapping north/south).

Specifies the geographical area that the output fields will cover, the default being for the whole globe. Enter lat/lon in degree bounds of an area separated by a "/" (south/west/north/east), or in Macro or Python provide a list, e.g. [south, west, north, east]; alternatively, use the assist button to define the area graphically.

For example, this set of parameters generates the following output data:


t01 = mv.regrid(
    grid     = [0.1,0.1],
    area     = [31,-17,64,38],
    data     = t_2m
    )

mv.plot(t01)


Frame

Specifies the width of a frame within a given sub-area, as described here frame - keyword in MARS/Dissemination request.

The width of the frame is specified as an (integer) number of grid points inwards from a given area. The following plots show a sub-area with Frame=10.

Rotation

Position of the South Pole of the intended rotated grid as lat/lon in degree, as described here: rotation - keyword in MARS/Dissemination request.

This is applicable to regular lat/lon or regular/reduced Gaussian grids. Enter lat/lon in degree, or in Macro or Python, enter [lat, lon]; alternatively, use the  assist button to select the point graphically.

Nabla

Activates a nabla (differential) operator processing on the fields. Possible functions are:

Nabla Poles Missing Values

QQQ. Possible values are On and Off.

Interpolation methods and parameters

Interpolation

Specifies the type of interpolation to be used on the fields. The default is Automatic, which selects either Linear or Nearest Neighbour based on an internal table of known parameters. If the parameter is unknown, the default will be Linear. The possible interpolation methods are:

Nearest Method

Available for any of the 'nearest' interpolation methods; Supports Interpolation K-Nearest Neighbours or Nearest LSM. Possible values are:

  • Distance
input points with radius (option Distance) of output point
  • Nclosest
n-closest input points (option Nclosest) to output point (default 4)
  • Distance and nclosest
input points respecting Distance ∩ Nclosest
  • Distance or nclosest
input points respecting Distance U Nclosest
  • Nclosest or nearest
n-closest input points (option Nclosest), if all are at the same distance (within option Distance Tolerance) return all points within that distance (robust interpolation of pole values)
  • Nearest neighbour with lowest index
nearest input point, if at the same distance to other points (option Nclosest) chosen by lowest index 
  • Sample
Sample of n-closest points (option Nclosest) out of input points with radius (option Distance) of output point
  • Sorted sample
as above, sorted by distance

Associated options supporting Nearest Method (described above):

Distance Weighting

Only available if Interpolation is K Nearest Neighbours. General way on how to interpolate input neighbouring point values to output points, including the Inverse Distance Weighting (IDW) class methods (see Wikipedia), which operates over input points returned by Nearest Method. Possible values are:

  • Climate Filter
filter for processing topographic data (see IFS documentation, Part IV: Physical Processes, 11.3.1 Smoothing operator)
  • Inverse Distance Weighting
IDW of the form distance-1; If input points match output point, only that point's value is used for output
  • Inverse Distance Weighting Squared
IDW of the form (1 + distance2)-1
  • Shepard
IDW of the form distance-p (option Distance Weighting Shepard Power, default 2.)
  • Gaussian
IDW of the form exp(- distance2 / (2 σ)2) (option Distance Weighting Gaussian Stddev, default 1.)
  • Nearest Neighbour
Emulate Interpolation Nearest Neighbour by picking first point (note that, when Nearest Method is Sample, a random nearest point is picked)
  • No
no distance weighting, average input values (irrespective of distance)

On multiple input points, weights are normalised linearly to unity. Associated options supporting Distance Weighting (described above):

Non Linear

QQQ. Possible values are:

Simulated Missing Value

Only available if Non Linear is Simulated Missing Value. QQQ (description)

Simulated Missing Value Epsilon

Only available if Non Linear is Simulated Missing Value. QQQ (description)

Land-sea mask parameters

Distance Weighting With LSM

Only available if Interpolation  is Nearest LSM. Possible values are On and Off.

LSM Weight Adjustment

Only available if Distance Weighting With Lsm is not Off. QQQ.

LSM

LSM Selection Input/Output

Specifies whether the input/output LSM file will come from LSM Named Input/Output (named, default) or LSM File Input/Output (file).

LSM Named Input/Output

Select one of the predefined names from the following list:

LSM File Input/Output

Provide the path to an input/output LSM GRIB file.

LSM Interpolation Input/Output

If input/output is not the same geometry (grid) as provided input/output LSM (respectivelly), interpolate with this method to a temporary LSM with required geometry.

LSM Value Threshold Input/Output

For GRIB-based LSM (this excludes '1km' and '10min'), the threshold for condition (value ≥ threshold) to distinguish land (true) from sea (false).

GRIB Output

Accuracy

Specifies the output GRIB bitsPerValue. If left empty, this will take the value from the input fields. This option can also be used to simply change the number of bits per value in a Fieldset if no other processing options are given. Note that if Packing  is set to ieee, then the only valid values for this parameter are 32 and 64.

Packing

Specifies the output GRIB packingType. Possible values are (depending on build-time configuration):

Edition

Specifies the output GRIB edition (or format). Note that format conversion is not supported.

Possible values are: