Adding XLinear_Velocity interpolator #2461

erikvansebille · 2026-01-09T15:26:08Z

This PR adds an explicit XLinear_Velocity interpolator, effectively moving the code from inside field.py. This improves transparency what is being done in interpolation.

Parcels/src/parcels/_core/field.py

Lines 307 to 315 in eeedce8

    
           if self._vector_interp_method is None: 
        
               u = self.U._interp_method(particle_positions, grid_positions, self.U) 
        
               v = self.V._interp_method(particle_positions, grid_positions, self.V) 
        
               if "3D" in self.vector_type: 
        
                   w = self.W._interp_method(particle_positions, grid_positions, self.W) 
        
               else: 
        
                   w = 0.0 
        
           else: 
        
               (u, v, w) = self._vector_interp_method(particle_positions, grid_positions, self)

First of all, do you agree this would be a good change?

A few more more things to do

base the choice between Grid_Velcoity and XLinear_Velocity would be based on grid metadata, in FieldSet creation. @VeckoTheGecko, could you help?
Make similar functions for UxGrids. @fluidnumerics-joe, could you pick that up?
Remove the code linked above from field.py (so that VectorFields can't have _vector_interp_method=None

Following the discussion around zonal and meridional spherical conversion in #2455

To confirm unit conversion works

Temporarily, as #2461 will move this unit conversion to the vector-interpolators

src/parcels/_core/field.py

fluidnumerics-joe · 2026-01-13T13:57:18Z

Making sure I understand.. This PR would remove the fallback to component-wise interpolation and require a vector field interpolator to be defined ?

erikvansebille · 2026-01-13T14:02:09Z

Making sure I understand.. This PR would remove the fallback to component-wise interpolation and require a vector field interpolator to be defined ?

No, it would make it more explicit. I moved the code we already had into its own Ux_Velocity interpolator:

Parcels/src/parcels/interpolators.py

Lines 714 to 730 in f1ce1e8

    
           def Ux_Velocity( 
        
               particle_positions: dict[str, float | np.ndarray], 
        
               grid_positions: dict[_UXGRID_AXES, dict[str, int | float | np.ndarray]], 
        
               vectorfield: VectorField, 
        
           ): 
        
               """Interpolation kernel for Vectorfields of velocity on a UxGrid.""" 
        
               u = vectorfield.U._interp_method(particle_positions, grid_positions, vectorfield.U) 
        
               v = vectorfield.V._interp_method(particle_positions, grid_positions, vectorfield.V) 
        
               if vectorfield.grid._mesh == "spherical": 
        
                   u /= 1852 * 60 * np.cos(np.deg2rad(particle_positions["lat"])) 
        
                   v /= 1852 * 60 
        
               if "3D" in vectorfield.vector_type: 
        
                   w = vectorfield.W._interp_method(particle_positions, grid_positions, vectorfield.W) 
        
               else: 
        
                   w = 0.0 
        
               return u, v, w

The advantage is that users can now more easily see how velocities are interpolated (since Interplators are much more public-facing), and that they can also more easily make adaptations (such as e.g. changing from spherical conversion to ellipsoid conversion in lines 723/724)

OK?

VeckoTheGecko

Just did a full review - looks good! One question out of my own curiousity

tests/test_advection.py

fluidnumerics-joe · 2026-01-13T14:31:39Z

src/parcels/_core/field.py

        vector_interp_method: Callable | None = None,
    ):
-        _assert_str_and_python_varname(name)
+        if vector_interp_method is None:


Wouldn't this require that users provide a vector interpolation method here and not allow fall back onto component wise interpolation ?

Yep, but the vector_interp_method can fall back on the component wise interpolation. That's exactly what's done in

Parcels/src/parcels/interpolators.py

Lines 714 to 730 in f1ce1e8

def Ux_Velocity(

particle_positions: dict[str, float | np.ndarray],

grid_positions: dict[_UXGRID_AXES, dict[str, int | float | np.ndarray]],

vectorfield: VectorField,

):

"""Interpolation kernel for Vectorfields of velocity on a UxGrid."""

u = vectorfield.U._interp_method(particle_positions, grid_positions, vectorfield.U)

v = vectorfield.V._interp_method(particle_positions, grid_positions, vectorfield.V)

if vectorfield.grid._mesh == "spherical":

u /= 1852 * 60 * np.cos(np.deg2rad(particle_positions["lat"]))

v /= 1852 * 60

if "3D" in vectorfield.vector_type:

w = vectorfield.W._interp_method(particle_positions, grid_positions, vectorfield.W)

else:

w = 0.0

return u, v, w

Adding XLinear_Velocity interpolator

1e17a46

github-project-automation bot added this to Parcels development Jan 9, 2026

github-project-automation bot moved this to Backlog in Parcels development Jan 9, 2026

erikvansebille added 2 commits January 11, 2026 20:40

Making unit tests of zonal and meridional flow stricter

49a0a82

Following the discussion around zonal and meridional spherical conversion in #2455

Also testing horizontal advection in 3D for spherical mesh

34570eb

To confirm unit conversion works

erikvansebille mentioned this pull request Jan 12, 2026

Implementing convert.nemo_to_sgrid() #2460

Open

erikvansebille added 4 commits January 12, 2026 18:24

Merge branch 'v4-dev' into add_XLinearVelocity

d95d5a1

Adding XLinear_Velocity as default vector_interp_method

b6079cb

forcing vector_interp_method to not be None

ac21aa2

Adding Ux_Velocity interpolator

842b543

erikvansebille added a commit that referenced this pull request Jan 13, 2026

Fixing unit conversion for non-Cgrids

76db3a2

Temporarily, as #2461 will move this unit conversion to the vector-interpolators

erikvansebille and others added 6 commits January 13, 2026 10:26

Merge branch 'v4-dev' into add_XLinearVelocity

2789c71

Updating apply_conversion descriptions

4c50a6a

Converting velocities inside Interpolators

53b847e

Adding vector_interp_method to all tests

cd4dcee

Adding vector_interp_method to explanation_kernelloop

4248b3f

Merge branch 'v4-dev' into add_XLinearVelocity

f1ce1e8

erikvansebille commented Jan 13, 2026

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erikvansebille marked this pull request as ready for review January 13, 2026 13:57

erikvansebille requested a review from VeckoTheGecko January 13, 2026 13:57

Moving ValueError when no vector_interp_method to top of initi

30e1245

VeckoTheGecko approved these changes Jan 13, 2026

View reviewed changes

tests/test_advection.py Show resolved Hide resolved

erikvansebille merged commit ab2cba3 into v4-dev Jan 13, 2026
11 checks passed

erikvansebille deleted the add_XLinearVelocity branch January 13, 2026 14:18

github-project-automation bot moved this from Backlog to Done in Parcels development Jan 13, 2026

fluidnumerics-joe reviewed Jan 13, 2026

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Adding XLinear_Velocity interpolator #2461

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	if self._vector_interp_method is None:
	u = self.U._interp_method(particle_positions, grid_positions, self.U)
	v = self.V._interp_method(particle_positions, grid_positions, self.V)
	if "3D" in self.vector_type:
	w = self.W._interp_method(particle_positions, grid_positions, self.W)
	else:
	w = 0.0
	else:
	(u, v, w) = self._vector_interp_method(particle_positions, grid_positions, self)

	def Ux_Velocity(
	particle_positions: dict[str, float \| np.ndarray],
	grid_positions: dict[_UXGRID_AXES, dict[str, int \| float \| np.ndarray]],
	vectorfield: VectorField,
	):
	"""Interpolation kernel for Vectorfields of velocity on a UxGrid."""
	u = vectorfield.U._interp_method(particle_positions, grid_positions, vectorfield.U)
	v = vectorfield.V._interp_method(particle_positions, grid_positions, vectorfield.V)
	if vectorfield.grid._mesh == "spherical":
	u /= 1852 * 60 * np.cos(np.deg2rad(particle_positions["lat"]))
	v /= 1852 * 60

	if "3D" in vectorfield.vector_type:
	w = vectorfield.W._interp_method(particle_positions, grid_positions, vectorfield.W)
	else:
	w = 0.0
	return u, v, w

Adding XLinear_Velocity interpolator #2461

Adding XLinear_Velocity interpolator #2461

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fluidnumerics-joe commented Jan 13, 2026 •

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