From 0c4fa4549179a69a55b1ed423733478e6f71da56 Mon Sep 17 00:00:00 2001
From: Maureen Cohen <maureenjcohen@gmail.com>
Date: Tue, 7 Jul 2026 14:08:54 +0100
Subject: [PATCH 01/16] Added SphericalMesh object to new mesh.py module.
 Contains only planetary radius at this time.

---
 src/parcels/_core/mesh.py | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)
 create mode 100644 src/parcels/_core/mesh.py

diff --git a/src/parcels/_core/mesh.py b/src/parcels/_core/mesh.py
new file mode 100644
index 000000000..7a48bb3d8
--- /dev/null
+++ b/src/parcels/_core/mesh.py
@@ -0,0 +1,24 @@
+import numpy as np
+
+class SphericalMesh:
+    """ Spherical mesh object with configurable planetary radius. 
+    
+        Pass to FieldSet object as ``mesh=SphericalMesh(radius=...)``. 
+        radius is in meters; None reverts degree to meter conversion 
+        to 1852 * 60 ."""
+    
+    def __init__(self, radius: float | None = None):
+        self.radius = radius
+
+    @property
+    def deg2m(self) -> float: 
+        """ Meters per degree of arc."""
+        if self.radius is None:
+            return 1852 * 60.0
+        else:
+            return self.radius * np.pi / 180.0
+        
+    def __repr__(self) -> str:
+        return f"SphericalMesh(radius={self.radius})"
+    
+    
\ No newline at end of file

From 3ddff6051bf5cc2bc605c09a699cf8631cbc4788 Mon Sep 17 00:00:00 2001
From: Maureen Cohen <maureenjcohen@gmail.com>
Date: Tue, 7 Jul 2026 14:10:41 +0100
Subject: [PATCH 02/16] Add SphericalMesh object to imports in __init__.py.

---
 src/parcels/__init__.py | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/src/parcels/__init__.py b/src/parcels/__init__.py
index 78227247b..81c901ca9 100644
--- a/src/parcels/__init__.py
+++ b/src/parcels/__init__.py
@@ -22,6 +22,7 @@
 from parcels._core.basegrid import BaseGrid
 from parcels._core.uxgrid import UxGrid
 from parcels._core.xgrid import XGrid
+from parcels._core.mesh import SphericalMesh
 
 from parcels._core.statuscodes import (
     AllParcelsErrorCodes,
@@ -54,6 +55,7 @@
     "BaseGrid",
     "UxGrid",
     "XGrid",
+    "SphericalMesh",
     # Status codes and errors
     "AllParcelsErrorCodes",
     "FieldInterpolationError",

From ffa6d858ccaebfd086c8fb5f253a9e2e64597326 Mon Sep 17 00:00:00 2001
From: Maureen Cohen <maureenjcohen@gmail.com>
Date: Tue, 7 Jul 2026 14:28:24 +0100
Subject: [PATCH 03/16] Added SphericalMesh object unpacking to Xgrid.

---
 src/parcels/_core/xgrid.py | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/src/parcels/_core/xgrid.py b/src/parcels/_core/xgrid.py
index 91a48af5d..95bc97fd1 100644
--- a/src/parcels/_core/xgrid.py
+++ b/src/parcels/_core/xgrid.py
@@ -14,6 +14,7 @@
 from parcels._core.index_search import _search_1d_array, _search_indices_curvilinear_2d
 from parcels._sgrid.accessor import _get_dim_to_axis_mapping
 from parcels._sgrid.core import SGRID_PADDING_TO_XGCM_POSITION
+from parcels._core.mesh import SphericalMesh
 
 _FIELD_DATA_ORDERING: Sequence[ptyping.XgcmAxisDirection] = "TZYX"
 _XGRID_AXES_ORDERING: Sequence[ptyping.XgridAxis] = "ZYX"
@@ -169,7 +170,12 @@ def __init__(self, model_data: xr.Dataset, mesh):
         self._ds = model_data
         grid = XgcmLikeGrid(self.sgrid_metadata, model_data)
         self.xgcm_grid = grid
-        self._mesh = mesh
+        if isinstance(mesh, SphericalMesh):
+            self._mesh = "spherical"
+            self._radius = mesh.radius
+        else:
+            self._mesh = mesh
+            self._radius = mesh.radius
         self._spatialhash = None
         ds = model_data
 
@@ -248,6 +254,14 @@ def _datetimes(self):
     @property
     def time(self):
         return self._datetimes.astype(np.float64) / 1e9
+    
+    @property
+    def deg2m(self) -> float:
+        """ Metres per degree of arc for this grid's mesh. """
+        if self._radius is None:
+            return 1852 * 60.0
+        else:
+            return self._radius * np.pi / 180.0
 
     @cached_property
     def xdim(self) -> int:

From f566fe10c3221f83ca9171ea34f2c03a266ecf22 Mon Sep 17 00:00:00 2001
From: Maureen Cohen <maureenjcohen@gmail.com>
Date: Tue, 7 Jul 2026 14:33:13 +0100
Subject: [PATCH 04/16] Switched self._radius = mesh.radius to None in second
 branch of if/else block.

---
 src/parcels/_core/xgrid.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/parcels/_core/xgrid.py b/src/parcels/_core/xgrid.py
index 95bc97fd1..d1ce68666 100644
--- a/src/parcels/_core/xgrid.py
+++ b/src/parcels/_core/xgrid.py
@@ -175,7 +175,7 @@ def __init__(self, model_data: xr.Dataset, mesh):
             self._radius = mesh.radius
         else:
             self._mesh = mesh
-            self._radius = mesh.radius
+            self._radius = None
         self._spatialhash = None
         ds = model_data
 

From 69fa293e16139b4f1dc62e2a6c8d5d040f247203 Mon Sep 17 00:00:00 2001
From: Maureen Cohen <maureenjcohen@gmail.com>
Date: Tue, 7 Jul 2026 16:11:30 +0100
Subject: [PATCH 05/16] Applied SphericalMesh/radius imports to Uxgrid as well.

---
 src/parcels/_core/uxgrid.py | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/src/parcels/_core/uxgrid.py b/src/parcels/_core/uxgrid.py
index 39201c2e8..970738bd5 100644
--- a/src/parcels/_core/uxgrid.py
+++ b/src/parcels/_core/uxgrid.py
@@ -7,6 +7,7 @@
 
 from parcels._core.basegrid import BaseGrid
 from parcels._core.index_search import GRID_SEARCH_ERROR, _search_1d_array, uxgrid_point_in_cell
+from parcels._core.mesh import SphericalMesh
 from parcels._typing import assert_valid_mesh
 
 _UXGRID_AXES = Literal["Z", "FACE"]
@@ -41,7 +42,12 @@ def __init__(self, grid: ux.grid.Grid, z: ux.UxDataArray, mesh) -> None:
         if z.ndim != 1:
             raise ValueError("z must be a 1D array of vertical coordinates")
         self.z = z
-        self._mesh = mesh
+        if isinstance(mesh, SphericalMesh):
+            self._mesh = "spherical"
+            self._radius = mesh.radius
+        else:
+            self._mesh = mesh
+            self._radius = None
         self._spatialhash = None
 
         assert_valid_mesh(mesh)
@@ -72,6 +78,14 @@ def get_axis_dim(self, axis: _UXGRID_AXES) -> int:
             return len(self.z.values)
         elif axis == "FACE":
             return self.uxgrid.n_face
+        
+    @property
+    def deg2m(self) -> float:
+        """ Metres per degree of arc for this grid's mesh. """
+        if self._radius is None:
+            return 1852 * 60.0
+        else:
+            return self._radius * np.pi / 180.0
 
     def search(self, z, y, x, ei=None, tol=1e-6):
         """

From 6ec7382c27ed99fffa4a3e95b94c2edc2ddc4156 Mon Sep 17 00:00:00 2001
From: Maureen Cohen <maureenjcohen@gmail.com>
Date: Tue, 7 Jul 2026 16:14:19 +0100
Subject: [PATCH 06/16] Adjust assert_valid_mesh in typing to allow
 SphericalMesh to pass.

---
 src/parcels/_typing.py | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/src/parcels/_typing.py b/src/parcels/_typing.py
index e8993cb05..57f84bb60 100644
--- a/src/parcels/_typing.py
+++ b/src/parcels/_typing.py
@@ -73,4 +73,7 @@ def _validate_against_pure_literal(value, typing_literal):
 
 # Assertion functions to clean user input
 def assert_valid_mesh(value: Any):
+    from parcels._core.mesh import SphericalMesh
+    if isinstance(value, SphericalMesh)
+        return
     _validate_against_pure_literal(value, Mesh)

From b10e32c32ebcaca7e6c9149a325ed191a1d1bf13 Mon Sep 17 00:00:00 2001
From: Maureen Cohen <maureenjcohen@gmail.com>
Date: Tue, 7 Jul 2026 16:16:11 +0100
Subject: [PATCH 07/16] Bugfix: missed colon in if isinstance(value,
 SphericalMesh) statement.

---
 src/parcels/_typing.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/parcels/_typing.py b/src/parcels/_typing.py
index 57f84bb60..9c32aa111 100644
--- a/src/parcels/_typing.py
+++ b/src/parcels/_typing.py
@@ -74,6 +74,6 @@ def _validate_against_pure_literal(value, typing_literal):
 # Assertion functions to clean user input
 def assert_valid_mesh(value: Any):
     from parcels._core.mesh import SphericalMesh
-    if isinstance(value, SphericalMesh)
+    if isinstance(value, SphericalMesh):
         return
     _validate_against_pure_literal(value, Mesh)

From 493ef8fd5974a46e8a537829e3b8420d34ee47f7 Mon Sep 17 00:00:00 2001
From: Maureen Cohen <maureenjcohen@gmail.com>
Date: Tue, 7 Jul 2026 16:19:54 +0100
Subject: [PATCH 08/16] Moved SphericalMesh import to top rather than within
 function.

---
 src/parcels/_typing.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/parcels/_typing.py b/src/parcels/_typing.py
index 9c32aa111..779fcdffb 100644
--- a/src/parcels/_typing.py
+++ b/src/parcels/_typing.py
@@ -10,6 +10,7 @@
 from collections.abc import Callable, Mapping
 from datetime import datetime
 from typing import TYPE_CHECKING, Any, Literal, get_args
+from parcels._core.mesh import SphericalMesh
 
 import numpy as np
 from cftime import datetime as cftime_datetime
@@ -73,7 +74,6 @@ def _validate_against_pure_literal(value, typing_literal):
 
 # Assertion functions to clean user input
 def assert_valid_mesh(value: Any):
-    from parcels._core.mesh import SphericalMesh
     if isinstance(value, SphericalMesh):
         return
     _validate_against_pure_literal(value, Mesh)

From 08fec2c4d72f4b36d0b3d3ab960f5d1477c99ad7 Mon Sep 17 00:00:00 2001
From: Maureen Cohen <maureenjcohen@gmail.com>
Date: Tue, 7 Jul 2026 16:52:12 +0100
Subject: [PATCH 09/16] Replaced hard-coded instances of 1852 * 60 with calls
 to deg2m attribute.

---
 src/parcels/_core/kernel.py                   |  2 +-
 src/parcels/_core/utils/interpolation.py      | 12 +++----
 src/parcels/interpolators/_uxinterpolators.py |  4 +--
 src/parcels/interpolators/_xinterpolators.py  | 16 ++++-----
 src/parcels/kernels/_advection.py             | 10 +++---
 src/parcels/kernels/_advectiondiffusion.py    | 36 +++++++++----------
 6 files changed, 40 insertions(+), 40 deletions(-)

diff --git a/src/parcels/_core/kernel.py b/src/parcels/_core/kernel.py
index f50c2876e..6a526461f 100644
--- a/src/parcels/_core/kernel.py
+++ b/src/parcels/_core/kernel.py
@@ -143,7 +143,7 @@ def check_fieldsets_in_kernels(self, kernel):  # TODO v4: this can go into anoth
                     self.fieldset.add_context("RK45_tol", 10)
                 if self.fieldset.U.grid._mesh == "spherical":
                     self.fieldset.RK45_tol /= (
-                        1852 * 60
+                        self.fieldset.U.grid.deg2m
                     )  # TODO does not account for zonal variation in meter -> degree conversion
                 if not hasattr(self.fieldset, "RK45_min_dt"):
                     warnings.warn(
diff --git a/src/parcels/_core/utils/interpolation.py b/src/parcels/_core/utils/interpolation.py
index a48c0ff21..f8c477500 100644
--- a/src/parcels/_core/utils/interpolation.py
+++ b/src/parcels/_core/utils/interpolation.py
@@ -61,12 +61,12 @@ def dphidxsi3D_lin(zeta: float, eta: float, xsi: float) -> tuple[list[float], li
 
 
 def dxdxsi3D_lin(
-    hexa_z: list[float], hexa_y: list[float], hexa_x: list[float], zeta: float, eta: float, xsi: float, mesh: Mesh
+    hexa_z: list[float], hexa_y: list[float], hexa_x: list[float], zeta: float, eta: float, xsi: float, mesh: Mesh,
+    deg2m: float = 1852 * 60.0
 ) -> tuple[float, float, float, float, float, float, float, float, float]:
     dphidxsi, dphideta, dphidzet = dphidxsi3D_lin(zeta, eta, xsi)
 
     if mesh == 'spherical':
-        deg2m = 1852 * 60.
         rad = np.pi / 180.
         lat = (1-xsi) * (1-eta) * hexa_y[0] + \
                  xsi  * (1-eta) * hexa_y[1] + \
@@ -92,9 +92,10 @@ def dxdxsi3D_lin(
 
 
 def jacobian3D_lin(
-    hexa_z: list[float], hexa_y: list[float], hexa_x: list[float], zeta: float, eta: float, xsi: float, mesh: Mesh
+    hexa_z: list[float], hexa_y: list[float], hexa_x: list[float], zeta: float, eta: float, xsi: float, mesh: Mesh,
+    deg2m: float = 1852 * 60.0
 ) -> float:
-    dxdxsi, dxdeta, dxdzet, dydxsi, dydeta, dydzet, dzdxsi, dzdeta, dzdzet = dxdxsi3D_lin(hexa_z, hexa_y, hexa_x, zeta, eta, xsi, mesh)
+    dxdxsi, dxdeta, dxdzet, dydxsi, dydeta, dydzet, dzdxsi, dzdeta, dzdzet = dxdxsi3D_lin(hexa_z, hexa_y, hexa_x, zeta, eta, xsi, mesh, deg2m)
 
     jac = (
         dxdxsi * (dydeta * dzdzet - dzdeta * dydzet)
@@ -174,10 +175,9 @@ def interpolate(phi: Callable[[float], list[float]], f: list[float], xsi: float)
     return np.dot(phi(xsi), f)
 
 
-def _geodetic_distance(lat1: float, lat2: float, lon1: float, lon2: float, mesh: Mesh, lat: float) -> float:
+def _geodetic_distance(lat1: float, lat2: float, lon1: float, lon2: float, mesh: Mesh, lat: float, deg2m: float = 1852 * 60.0) -> float:
     if mesh == "spherical":
         rad = np.pi / 180.0
-        deg2m = 1852 * 60.0
         return np.sqrt(((lon2 - lon1) * deg2m * np.cos(rad * lat)) ** 2 + ((lat2 - lat1) * deg2m) ** 2)
     else:
         return np.sqrt((lon2 - lon1) ** 2 + (lat2 - lat1) ** 2)
diff --git a/src/parcels/interpolators/_uxinterpolators.py b/src/parcels/interpolators/_uxinterpolators.py
index 80e804475..0b04b1de6 100644
--- a/src/parcels/interpolators/_uxinterpolators.py
+++ b/src/parcels/interpolators/_uxinterpolators.py
@@ -171,8 +171,8 @@ def interp(
         u = vectorfield.U.interp_method.interp(particle_positions, grid_positions, vectorfield.U)
         v = vectorfield.V.interp_method.interp(particle_positions, grid_positions, vectorfield.V)
         if vectorfield.grid._mesh == "spherical":
-            u /= 1852 * 60 * np.cos(np.deg2rad(particle_positions["y"]))
-            v /= 1852 * 60
+            u /= vectorfield.grid.deg2m * np.cos(np.deg2rad(particle_positions["y"]))
+            v /= vectorfield.grid.deg2m
 
         if "3D" in vectorfield.vector_type:
             w = vectorfield.W.interp_method.interp(particle_positions, grid_positions, vectorfield.W)
diff --git a/src/parcels/interpolators/_xinterpolators.py b/src/parcels/interpolators/_xinterpolators.py
index d20313ffb..d851e42c5 100644
--- a/src/parcels/interpolators/_xinterpolators.py
+++ b/src/parcels/interpolators/_xinterpolators.py
@@ -149,8 +149,8 @@ def interp(
         u = _xlinear.interp(particle_positions, grid_positions, vectorfield.U)
         v = _xlinear.interp(particle_positions, grid_positions, vectorfield.V)
         if vectorfield.grid._mesh == "spherical":
-            u /= 1852 * 60 * np.cos(np.deg2rad(particle_positions["y"]))
-            v /= 1852 * 60
+            u /= vectorfield.grid.deg2m * np.cos(np.deg2rad(particle_positions["y"]))
+            v /= vectorfield.grid.deg2m
 
         if vectorfield.W:
             w = _xlinear.interp(particle_positions, grid_positions, vectorfield.W)
@@ -201,16 +201,16 @@ def interp(
             px[1:] = np.where(px[1:] - px[0] > 180, px[1:] - 360, px[1:])
             px[1:] = np.where(-px[1:] + px[0] > 180, px[1:] + 360, px[1:])
         c1 = i_u._geodetic_distance(
-            py[0], py[1], px[0], px[1], grid._mesh, np.einsum("ij,ji->i", i_u.phi2D_lin(0.0, xsi), py)
+            py[0], py[1], px[0], px[1], grid._mesh, np.einsum("ij,ji->i", i_u.phi2D_lin(0.0, xsi), py), grid.deg2m
         )
         c2 = i_u._geodetic_distance(
-            py[1], py[2], px[1], px[2], grid._mesh, np.einsum("ij,ji->i", i_u.phi2D_lin(eta, 1.0), py)
+            py[1], py[2], px[1], px[2], grid._mesh, np.einsum("ij,ji->i", i_u.phi2D_lin(eta, 1.0), py), grid.deg2m
         )
         c3 = i_u._geodetic_distance(
-            py[2], py[3], px[2], px[3], grid._mesh, np.einsum("ij,ji->i", i_u.phi2D_lin(1.0, xsi), py)
+            py[2], py[3], px[2], px[3], grid._mesh, np.einsum("ij,ji->i", i_u.phi2D_lin(1.0, xsi), py), grid.deg2m
         )
         c4 = i_u._geodetic_distance(
-            py[3], py[0], px[3], px[0], grid._mesh, np.einsum("ij,ji->i", i_u.phi2D_lin(eta, 0.0), py)
+            py[3], py[0], px[3], px[0], grid._mesh, np.einsum("ij,ji->i", i_u.phi2D_lin(eta, 0.0), py), grid.deg2m
         )
 
         def _create_selection_dict(dims, zdir=False):
@@ -283,7 +283,7 @@ def _compute_corner_data(data, selection_dict) -> np.ndarray:
         Vvel = (1 - eta) * V0 + eta * V1
 
         if grid._mesh == "spherical":
-            jac = i_u._compute_jacobian_determinant(py, px, eta, xsi) * 1852 * 60.0
+            jac = i_u._compute_jacobian_determinant(py, px, eta, xsi) * grid.deg2m
         else:
             jac = i_u._compute_jacobian_determinant(py, px, eta, xsi)
 
@@ -304,7 +304,7 @@ def _compute_corner_data(data, selection_dict) -> np.ndarray:
             v = v.compute()
 
         if grid._mesh == "spherical":
-            conversion = 1852 * 60.0 * np.cos(np.deg2rad(particle_positions["y"]))
+            conversion = grid.deg2m * np.cos(np.deg2rad(particle_positions["y"]))
             u /= conversion
             v /= conversion
 
diff --git a/src/parcels/kernels/_advection.py b/src/parcels/kernels/_advection.py
index 10e7ee676..bcf133361 100644
--- a/src/parcels/kernels/_advection.py
+++ b/src/parcels/kernels/_advection.py
@@ -243,12 +243,12 @@ def AdvectionAnalytical(particles, fieldset):  # pragma: no cover
     else:
         dz = 1.0
 
-    c1 = i_u._geodetic_distance(py[0], py[1], px[0], px[1], grid.mesh, np.dot(i_u.phi2D_lin(0.0, xsi), py))
-    c2 = i_u._geodetic_distance(py[1], py[2], px[1], px[2], grid.mesh, np.dot(i_u.phi2D_lin(eta, 1.0), py))
-    c3 = i_u._geodetic_distance(py[2], py[3], px[2], px[3], grid.mesh, np.dot(i_u.phi2D_lin(1.0, xsi), py))
-    c4 = i_u._geodetic_distance(py[3], py[0], px[3], px[0], grid.mesh, np.dot(i_u.phi2D_lin(eta, 0.0), py))
+    c1 = i_u._geodetic_distance(py[0], py[1], px[0], px[1], grid.mesh, np.dot(i_u.phi2D_lin(0.0, xsi), py), grid.deg2m)
+    c2 = i_u._geodetic_distance(py[1], py[2], px[1], px[2], grid.mesh, np.dot(i_u.phi2D_lin(eta, 1.0), py), grid.deg2m)
+    c3 = i_u._geodetic_distance(py[2], py[3], px[2], px[3], grid.mesh, np.dot(i_u.phi2D_lin(1.0, xsi), py), grid.deg2m)
+    c4 = i_u._geodetic_distance(py[3], py[0], px[3], px[0], grid.mesh, np.dot(i_u.phi2D_lin(eta, 0.0), py), grid.deg2m)
     rad = np.pi / 180.0
-    deg2m = 1852 * 60.0
+    deg2m = grid.deg2m
     meshJac = (deg2m * deg2m * math.cos(rad * particles.y)) if grid.mesh == "spherical" else 1
     dxdy = i_u._compute_jacobian_determinant(py, px, eta, xsi) * meshJac
 
diff --git a/src/parcels/kernels/_advectiondiffusion.py b/src/parcels/kernels/_advectiondiffusion.py
index 3593e2538..7b7a96276 100644
--- a/src/parcels/kernels/_advectiondiffusion.py
+++ b/src/parcels/kernels/_advectiondiffusion.py
@@ -8,14 +8,14 @@
 __all__ = ["AdvectionDiffusionEM", "AdvectionDiffusionM1", "DiffusionUniformKh"]
 
 
-def meters_to_degrees_zonal(deg, lat):  # pragma: no cover
+def meters_to_degrees_zonal(deg, lat, deg2m):  # pragma: no cover
     """Convert square meters to square degrees longitude at a given latitude."""
-    return deg / pow(1852 * 60.0 * np.cos(lat * np.pi / 180), 2)
+    return deg / pow(deg2m * np.cos(lat * np.pi / 180), 2)
 
 
-def meters_to_degrees_meridional(deg):  # pragma: no cover
+def meters_to_degrees_meridional(deg, deg2m):  # pragma: no cover
     """Convert square meters to square degrees latitude."""
-    return deg / pow(1852 * 60.0, 2)
+    return deg / pow(deg2m, 2)
 
 
 def AdvectionDiffusionM1(particles, fieldset):  # pragma: no cover
@@ -40,26 +40,26 @@ def AdvectionDiffusionM1(particles, fieldset):  # pragma: no cover
     Kxp1 = fieldset.Kh_zonal[particles.t, particles.z, particles.y, particles.x + fieldset.dres, particles]
     Kxm1 = fieldset.Kh_zonal[particles.t, particles.z, particles.y, particles.x - fieldset.dres, particles]
     if fieldset.Kh_zonal.grid._mesh == "spherical":
-        Kxp1 = meters_to_degrees_zonal(Kxp1, particles.y)
-        Kxm1 = meters_to_degrees_zonal(Kxm1, particles.y)
+        Kxp1 = meters_to_degrees_zonal(Kxp1, particles.y, fieldset.Kh_zonal.grid.deg2m)
+        Kxm1 = meters_to_degrees_zonal(Kxm1, particles.y, fieldset.Kh_zonal.grid.deg2m)
     dKdx = (Kxp1 - Kxm1) / (2 * fieldset.dres)
 
     u, v = fieldset.UV[particles.t, particles.z, particles.y, particles.x, particles]
     kh_zonal = fieldset.Kh_zonal[particles.t, particles.z, particles.y, particles.x, particles]
     if fieldset.Kh_zonal.grid._mesh == "spherical":
-        kh_zonal = meters_to_degrees_zonal(kh_zonal, particles.y)
+        kh_zonal = meters_to_degrees_zonal(kh_zonal, particles.y, fieldset.Kh_zonal.grid.deg2m)
     bx = np.sqrt(2 * kh_zonal)
 
     Kyp1 = fieldset.Kh_meridional[particles.t, particles.z, particles.y + fieldset.dres, particles.x, particles]
     Kym1 = fieldset.Kh_meridional[particles.t, particles.z, particles.y - fieldset.dres, particles.x, particles]
     if fieldset.Kh_meridional.grid._mesh == "spherical":
-        Kyp1 = meters_to_degrees_meridional(Kyp1)
-        Kym1 = meters_to_degrees_meridional(Kym1)
+        Kyp1 = meters_to_degrees_meridional(Kyp1, fieldset.Kh_meridional.grid.deg2m)
+        Kym1 = meters_to_degrees_meridional(Kym1, fieldset.Kh_meridional.grid.deg2m)
     dKdy = (Kyp1 - Kym1) / (2 * fieldset.dres)
 
     kh_meridional = fieldset.Kh_meridional[particles.t, particles.z, particles.y, particles.x, particles]
     if fieldset.Kh_meridional.grid._mesh == "spherical":
-        kh_meridional = meters_to_degrees_meridional(kh_meridional)
+        kh_meridional = meters_to_degrees_meridional(kh_meridional, fieldset.Kh_meridional.grid.deg2m)
     by = np.sqrt(2 * kh_meridional)
 
     # Particle positions are updated only after evaluating all terms.
@@ -89,27 +89,27 @@ def AdvectionDiffusionEM(particles, fieldset):  # pragma: no cover
     Kxp1 = fieldset.Kh_zonal[particles.t, particles.z, particles.y, particles.x + fieldset.dres, particles]
     Kxm1 = fieldset.Kh_zonal[particles.t, particles.z, particles.y, particles.x - fieldset.dres, particles]
     if fieldset.Kh_zonal.grid._mesh == "spherical":
-        Kxp1 = meters_to_degrees_zonal(Kxp1, particles.y)
-        Kxm1 = meters_to_degrees_zonal(Kxm1, particles.y)
+        Kxp1 = meters_to_degrees_zonal(Kxp1, particles.y, fieldset.Kh_zonal.grid.deg2m)
+        Kxm1 = meters_to_degrees_zonal(Kxm1, particles.y, fieldset.Kh_zonal.grid.deg2m)
     dKdx = (Kxp1 - Kxm1) / (2 * fieldset.dres)
     ax = u + dKdx
 
     kh_zonal = fieldset.Kh_zonal[particles.t, particles.z, particles.y, particles.x, particles]
     if fieldset.Kh_zonal.grid._mesh == "spherical":
-        kh_zonal = meters_to_degrees_zonal(kh_zonal, particles.y)
+        kh_zonal = meters_to_degrees_zonal(kh_zonal, particles.y, fieldset.Kh_zonal.grid.deg2m)
     bx = np.sqrt(2 * kh_zonal)
 
     Kyp1 = fieldset.Kh_meridional[particles.t, particles.z, particles.y + fieldset.dres, particles.x, particles]
     Kym1 = fieldset.Kh_meridional[particles.t, particles.z, particles.y - fieldset.dres, particles.x, particles]
     if fieldset.Kh_meridional.grid._mesh == "spherical":
-        Kyp1 = meters_to_degrees_meridional(Kyp1)
-        Kym1 = meters_to_degrees_meridional(Kym1)
+        Kyp1 = meters_to_degrees_meridional(Kyp1, fieldset.Kh_meridional.grid.deg2m)
+        Kym1 = meters_to_degrees_meridional(Kym1, fieldset.Kh_meridional.grid.deg2m)
     dKdy = (Kyp1 - Kym1) / (2 * fieldset.dres)
     ay = v + dKdy
 
     kh_meridional = fieldset.Kh_meridional[particles.t, particles.z, particles.y, particles.x, particles]
     if fieldset.Kh_meridional.grid._mesh == "spherical":
-        kh_meridional = meters_to_degrees_meridional(kh_meridional)
+        kh_meridional = meters_to_degrees_meridional(kh_meridional, fieldset.Kh_meridional.grid.deg2m)
     by = np.sqrt(2 * kh_meridional)
 
     # Particle positions are updated only after evaluating all terms.
@@ -143,8 +143,8 @@ def DiffusionUniformKh(particles, fieldset):  # pragma: no cover
     kh_meridional = fieldset.Kh_meridional[particles]
 
     if fieldset.Kh_zonal.grid._mesh == "spherical":
-        kh_zonal = meters_to_degrees_zonal(kh_zonal, particles.y)
-        kh_meridional = meters_to_degrees_meridional(kh_meridional)
+        kh_zonal = meters_to_degrees_zonal(kh_zonal, particles.y, fieldset.Kh_zonal.grid.deg2m)
+        kh_meridional = meters_to_degrees_meridional(kh_meridional, fieldset.Kh_meridional.grid.deg2m)
 
     bx = np.sqrt(2 * kh_zonal)
     by = np.sqrt(2 * kh_meridional)

From a2779136791019b0a5a6e9221d43a1f980b7e458 Mon Sep 17 00:00:00 2001
From: Maureen Cohen <maureenjcohen@gmail.com>
Date: Tue, 7 Jul 2026 17:37:52 +0100
Subject: [PATCH 10/16] Added three tests: test that deg2m calculation works,
 test calculation for different input args (string vs SphericalMesh object,
 radius given or not), short advection test with radii None, Mars, Venus,
 Earth.

---
 tests/test_mesh.py | 44 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 44 insertions(+)
 create mode 100644 tests/test_mesh.py

diff --git a/tests/test_mesh.py b/tests/test_mesh.py
new file mode 100644
index 000000000..df4b24412
--- /dev/null
+++ b/tests/test_mesh.py
@@ -0,0 +1,44 @@
+import numpy as np
+import pytest
+
+from parcels import FieldSet, ParticleSet, SphericalMesh
+from parcels.kernels import AdvectionRK4
+from parcels._datasets.structured.generated import simple_UV_dataset
+
+EARTH_DEG2M = 1852 * 60.0
+
+def test_spherical_mesh_deg2m():
+    assert SphericalMesh().radius is None
+    assert SphericalMesh().deg2m == EARTH_DEG2M
+    r = 3389500.0                   # Mars radius
+    assert SphericalMesh(radius=r).deg2m == pytest.approx(r * np.pi / 180)
+
+@pytest.mark.parametrize("mesh, exp_radius, exp_deg2m",
+                          [
+                              ("spherical", None, EARTH_DEG2M),
+                              (SphericalMesh(), None, EARTH_DEG2M),
+                              (SphericalMesh(radius=3389500.0), 3389500.0, 3389500.0 * np.pi / 180),
+                          ],
+                          )
+def test_xgrid_radius_and_deg2m(mesh, exp_radius, exp_deg2m):
+    grid = FieldSet.from_sgrid_conventions(simple_UV_dataset(), mesh=mesh).U.grid
+    assert grid._mesh == "spherical"
+    assert grid._radius == exp_radius
+    assert grid.deg2m == pytest.approx(exp_deg2m)
+
+@pytest.mark.parametrize("radius", [None, 3389500.0, 6051800.0, 6371000.0]) # Mars, Venus, Earth
+def test_advection_uses_custom_radius(radius, npart=10):
+    ds = simple_UV_dataset()
+    ds["U"].data[:] = 1.0
+    fieldset = FieldSet.from_sgrid_conventions(ds, mesh=SphericalMesh(radius=radius))
+
+    runtime = 7200
+    startlat = np.linspace(0, 80, npart)
+    startlon = 20.0 + np.zeros(npart)
+    pset = ParticleSet(fieldset, x=startlon, y=startlat)
+    pset.execute(AdvectionRK4, runtime=runtime, dt=np.timedelta64(15, "m"))
+
+    deg2m = EARTH_DEG2M if radius is None else radius * np.pi / 180
+    expected_dlon = runtime / (deg2m * np.cos(np.deg2rad(pset.y)))
+    np.testing.assert_allclose(pset.x - startlon, expected_dlon, atol=1e-5)
+    np.testing.assert_allclose(pset.y, startlat, atol=1e-5)

From da32a50635169fe041c18ffc442150c8b15f82ab Mon Sep 17 00:00:00 2001
From: Maureen Cohen <maureenjcohen@gmail.com>
Date: Tue, 7 Jul 2026 17:53:21 +0100
Subject: [PATCH 11/16] Added a check to SphericalMesh init to catch bad values
 of radius: nonon-numeric, negative, or 0. Added matching tests to test_mesh.
 Co-authored by: Claude <noreply@anthropic.com>).

---
 src/parcels/_core/mesh.py |  5 ++++-
 tests/test_mesh.py        | 10 ++++++++++
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/src/parcels/_core/mesh.py b/src/parcels/_core/mesh.py
index 7a48bb3d8..334674613 100644
--- a/src/parcels/_core/mesh.py
+++ b/src/parcels/_core/mesh.py
@@ -8,6 +8,10 @@ class SphericalMesh:
         to 1852 * 60 ."""
     
     def __init__(self, radius: float | None = None):
+        if radius is not None and not isinstance(radius, (int, float, np.number)):
+            raise TypeError(f"radius must be a number of None, got {type(radius).__name__}")
+        if radius is not None and radius <= 0:
+            raise ValueError(f"radius must be positive, got {radius}")
         self.radius = radius
 
     @property
@@ -21,4 +25,3 @@ def deg2m(self) -> float:
     def __repr__(self) -> str:
         return f"SphericalMesh(radius={self.radius})"
     
-    
\ No newline at end of file
diff --git a/tests/test_mesh.py b/tests/test_mesh.py
index df4b24412..62baa490b 100644
--- a/tests/test_mesh.py
+++ b/tests/test_mesh.py
@@ -42,3 +42,13 @@ def test_advection_uses_custom_radius(radius, npart=10):
     expected_dlon = runtime / (deg2m * np.cos(np.deg2rad(pset.y)))
     np.testing.assert_allclose(pset.x - startlon, expected_dlon, atol=1e-5)
     np.testing.assert_allclose(pset.y, startlat, atol=1e-5)
+
+@pytest.mark.parametrize("bad_radius", ["6371000", [6371000], (1, 2), {}])
+def test_spherical_mesh_rejects_non_numeric_radius(bad_radius):
+    with pytest.raises(TypeError):
+        SphericalMesh(radius=bad_radius)
+
+@pytest.mark.parametrize("bad_radius", [0, -1.0, -6371000])
+def test_spherical_mesh_rejects_nonpos_radius(bad_radius):
+    with pytest.raises(ValueError):
+        SphericalMesh(radius=bad_radius)
\ No newline at end of file

From 72be313963f4e369d37272923db1c2048515411e Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Thu, 9 Jul 2026 16:11:41 +0000
Subject: [PATCH 12/16] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 src/parcels/_core/mesh.py                | 21 +++++++++---------
 src/parcels/_core/utils/interpolation.py |  4 +++-
 src/parcels/_core/uxgrid.py              |  4 ++--
 src/parcels/_core/xgrid.py               |  6 ++---
 src/parcels/_typing.py                   |  3 ++-
 tests/test_mesh.py                       | 28 ++++++++++++++----------
 6 files changed, 38 insertions(+), 28 deletions(-)

diff --git a/src/parcels/_core/mesh.py b/src/parcels/_core/mesh.py
index 334674613..b9ca044af 100644
--- a/src/parcels/_core/mesh.py
+++ b/src/parcels/_core/mesh.py
@@ -1,12 +1,14 @@
 import numpy as np
 
+
 class SphericalMesh:
-    """ Spherical mesh object with configurable planetary radius. 
-    
-        Pass to FieldSet object as ``mesh=SphericalMesh(radius=...)``. 
-        radius is in meters; None reverts degree to meter conversion 
-        to 1852 * 60 ."""
-    
+    """Spherical mesh object with configurable planetary radius.
+
+    Pass to FieldSet object as ``mesh=SphericalMesh(radius=...)``.
+    radius is in meters; None reverts degree to meter conversion
+    to 1852 * 60 .
+    """
+
     def __init__(self, radius: float | None = None):
         if radius is not None and not isinstance(radius, (int, float, np.number)):
             raise TypeError(f"radius must be a number of None, got {type(radius).__name__}")
@@ -15,13 +17,12 @@ def __init__(self, radius: float | None = None):
         self.radius = radius
 
     @property
-    def deg2m(self) -> float: 
-        """ Meters per degree of arc."""
+    def deg2m(self) -> float:
+        """Meters per degree of arc."""
         if self.radius is None:
             return 1852 * 60.0
         else:
             return self.radius * np.pi / 180.0
-        
+
     def __repr__(self) -> str:
         return f"SphericalMesh(radius={self.radius})"
-    
diff --git a/src/parcels/_core/utils/interpolation.py b/src/parcels/_core/utils/interpolation.py
index f8c477500..214ba1933 100644
--- a/src/parcels/_core/utils/interpolation.py
+++ b/src/parcels/_core/utils/interpolation.py
@@ -175,7 +175,9 @@ def interpolate(phi: Callable[[float], list[float]], f: list[float], xsi: float)
     return np.dot(phi(xsi), f)
 
 
-def _geodetic_distance(lat1: float, lat2: float, lon1: float, lon2: float, mesh: Mesh, lat: float, deg2m: float = 1852 * 60.0) -> float:
+def _geodetic_distance(
+    lat1: float, lat2: float, lon1: float, lon2: float, mesh: Mesh, lat: float, deg2m: float = 1852 * 60.0
+) -> float:
     if mesh == "spherical":
         rad = np.pi / 180.0
         return np.sqrt(((lon2 - lon1) * deg2m * np.cos(rad * lat)) ** 2 + ((lat2 - lat1) * deg2m) ** 2)
diff --git a/src/parcels/_core/uxgrid.py b/src/parcels/_core/uxgrid.py
index 970738bd5..8cfb41ec5 100644
--- a/src/parcels/_core/uxgrid.py
+++ b/src/parcels/_core/uxgrid.py
@@ -78,10 +78,10 @@ def get_axis_dim(self, axis: _UXGRID_AXES) -> int:
             return len(self.z.values)
         elif axis == "FACE":
             return self.uxgrid.n_face
-        
+
     @property
     def deg2m(self) -> float:
-        """ Metres per degree of arc for this grid's mesh. """
+        """Metres per degree of arc for this grid's mesh."""
         if self._radius is None:
             return 1852 * 60.0
         else:
diff --git a/src/parcels/_core/xgrid.py b/src/parcels/_core/xgrid.py
index d1ce68666..dbd0a3c0e 100644
--- a/src/parcels/_core/xgrid.py
+++ b/src/parcels/_core/xgrid.py
@@ -12,9 +12,9 @@
 import parcels._typing as ptyping
 from parcels._core.basegrid import BaseGrid
 from parcels._core.index_search import _search_1d_array, _search_indices_curvilinear_2d
+from parcels._core.mesh import SphericalMesh
 from parcels._sgrid.accessor import _get_dim_to_axis_mapping
 from parcels._sgrid.core import SGRID_PADDING_TO_XGCM_POSITION
-from parcels._core.mesh import SphericalMesh
 
 _FIELD_DATA_ORDERING: Sequence[ptyping.XgcmAxisDirection] = "TZYX"
 _XGRID_AXES_ORDERING: Sequence[ptyping.XgridAxis] = "ZYX"
@@ -254,10 +254,10 @@ def _datetimes(self):
     @property
     def time(self):
         return self._datetimes.astype(np.float64) / 1e9
-    
+
     @property
     def deg2m(self) -> float:
-        """ Metres per degree of arc for this grid's mesh. """
+        """Metres per degree of arc for this grid's mesh."""
         if self._radius is None:
             return 1852 * 60.0
         else:
diff --git a/src/parcels/_typing.py b/src/parcels/_typing.py
index 779fcdffb..61fc28160 100644
--- a/src/parcels/_typing.py
+++ b/src/parcels/_typing.py
@@ -10,11 +10,12 @@
 from collections.abc import Callable, Mapping
 from datetime import datetime
 from typing import TYPE_CHECKING, Any, Literal, get_args
-from parcels._core.mesh import SphericalMesh
 
 import numpy as np
 from cftime import datetime as cftime_datetime
 
+from parcels._core.mesh import SphericalMesh
+
 if TYPE_CHECKING:
     import xgcm
 
diff --git a/tests/test_mesh.py b/tests/test_mesh.py
index 62baa490b..296b0291a 100644
--- a/tests/test_mesh.py
+++ b/tests/test_mesh.py
@@ -2,31 +2,35 @@
 import pytest
 
 from parcels import FieldSet, ParticleSet, SphericalMesh
-from parcels.kernels import AdvectionRK4
 from parcels._datasets.structured.generated import simple_UV_dataset
+from parcels.kernels import AdvectionRK4
 
 EARTH_DEG2M = 1852 * 60.0
 
+
 def test_spherical_mesh_deg2m():
     assert SphericalMesh().radius is None
     assert SphericalMesh().deg2m == EARTH_DEG2M
-    r = 3389500.0                   # Mars radius
+    r = 3389500.0  # Mars radius
     assert SphericalMesh(radius=r).deg2m == pytest.approx(r * np.pi / 180)
 
-@pytest.mark.parametrize("mesh, exp_radius, exp_deg2m",
-                          [
-                              ("spherical", None, EARTH_DEG2M),
-                              (SphericalMesh(), None, EARTH_DEG2M),
-                              (SphericalMesh(radius=3389500.0), 3389500.0, 3389500.0 * np.pi / 180),
-                          ],
-                          )
+
+@pytest.mark.parametrize(
+    "mesh, exp_radius, exp_deg2m",
+    [
+        ("spherical", None, EARTH_DEG2M),
+        (SphericalMesh(), None, EARTH_DEG2M),
+        (SphericalMesh(radius=3389500.0), 3389500.0, 3389500.0 * np.pi / 180),
+    ],
+)
 def test_xgrid_radius_and_deg2m(mesh, exp_radius, exp_deg2m):
     grid = FieldSet.from_sgrid_conventions(simple_UV_dataset(), mesh=mesh).U.grid
     assert grid._mesh == "spherical"
     assert grid._radius == exp_radius
     assert grid.deg2m == pytest.approx(exp_deg2m)
 
-@pytest.mark.parametrize("radius", [None, 3389500.0, 6051800.0, 6371000.0]) # Mars, Venus, Earth
+
+@pytest.mark.parametrize("radius", [None, 3389500.0, 6051800.0, 6371000.0])  # Mars, Venus, Earth
 def test_advection_uses_custom_radius(radius, npart=10):
     ds = simple_UV_dataset()
     ds["U"].data[:] = 1.0
@@ -43,12 +47,14 @@ def test_advection_uses_custom_radius(radius, npart=10):
     np.testing.assert_allclose(pset.x - startlon, expected_dlon, atol=1e-5)
     np.testing.assert_allclose(pset.y, startlat, atol=1e-5)
 
+
 @pytest.mark.parametrize("bad_radius", ["6371000", [6371000], (1, 2), {}])
 def test_spherical_mesh_rejects_non_numeric_radius(bad_radius):
     with pytest.raises(TypeError):
         SphericalMesh(radius=bad_radius)
 
+
 @pytest.mark.parametrize("bad_radius", [0, -1.0, -6371000])
 def test_spherical_mesh_rejects_nonpos_radius(bad_radius):
     with pytest.raises(ValueError):
-        SphericalMesh(radius=bad_radius)
\ No newline at end of file
+        SphericalMesh(radius=bad_radius)

From 0c3c1f047713e099634f27bd836fc5fa86d4a561 Mon Sep 17 00:00:00 2001
From: Maureen Cohen <53348730+maureenjcohen@users.noreply.github.com>
Date: Fri, 10 Jul 2026 10:30:15 +0100
Subject: [PATCH 13/16] Update src/parcels/_core/mesh.py

Co-authored-by: Erik van Sebille <e.vansebille@uu.nl>
---
 src/parcels/_core/mesh.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/parcels/_core/mesh.py b/src/parcels/_core/mesh.py
index b9ca044af..28f160b21 100644
--- a/src/parcels/_core/mesh.py
+++ b/src/parcels/_core/mesh.py
@@ -11,7 +11,7 @@ class SphericalMesh:
 
     def __init__(self, radius: float | None = None):
         if radius is not None and not isinstance(radius, (int, float, np.number)):
-            raise TypeError(f"radius must be a number of None, got {type(radius).__name__}")
+            raise TypeError(f"radius must be a number or None, got {type(radius).__name__}")
         if radius is not None and radius <= 0:
             raise ValueError(f"radius must be positive, got {radius}")
         self.radius = radius

From ddac215d621fedf93257a2258bd4f446e4392fa2 Mon Sep 17 00:00:00 2001
From: Maureen Cohen <53348730+maureenjcohen@users.noreply.github.com>
Date: Fri, 10 Jul 2026 10:30:40 +0100
Subject: [PATCH 14/16] Update src/parcels/_core/mesh.py

Co-authored-by: Erik van Sebille <e.vansebille@uu.nl>
---
 src/parcels/_core/mesh.py | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/src/parcels/_core/mesh.py b/src/parcels/_core/mesh.py
index 28f160b21..fc9f10147 100644
--- a/src/parcels/_core/mesh.py
+++ b/src/parcels/_core/mesh.py
@@ -5,8 +5,9 @@ class SphericalMesh:
     """Spherical mesh object with configurable planetary radius.
 
     Pass to FieldSet object as ``mesh=SphericalMesh(radius=...)``.
-    radius is in meters; None reverts degree to meter conversion
-    to 1852 * 60 .
+    radius is in meters; None reverts to default for Earth, where 
+    arcdegree to meter conversion is defined as 1852 * 60
+    (1852 meters per arcminute * 60 arcminutes per arcdegree).
     """
 
     def __init__(self, radius: float | None = None):

From d28fa4c814a36e67b2f8db14118935e0cd6e1784 Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Fri, 10 Jul 2026 09:30:49 +0000
Subject: [PATCH 15/16] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 src/parcels/_core/mesh.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/parcels/_core/mesh.py b/src/parcels/_core/mesh.py
index fc9f10147..4bbd96aad 100644
--- a/src/parcels/_core/mesh.py
+++ b/src/parcels/_core/mesh.py
@@ -5,7 +5,7 @@ class SphericalMesh:
     """Spherical mesh object with configurable planetary radius.
 
     Pass to FieldSet object as ``mesh=SphericalMesh(radius=...)``.
-    radius is in meters; None reverts to default for Earth, where 
+    radius is in meters; None reverts to default for Earth, where
     arcdegree to meter conversion is defined as 1852 * 60
     (1852 meters per arcminute * 60 arcminutes per arcdegree).
     """

From ce76ee3a3ad0358d5e1808b5e35aae7b84a252d5 Mon Sep 17 00:00:00 2001
From: Maureen Cohen <53348730+maureenjcohen@users.noreply.github.com>
Date: Fri, 10 Jul 2026 10:31:11 +0100
Subject: [PATCH 16/16] Update src/parcels/_core/uxgrid.py

Co-authored-by: Erik van Sebille <e.vansebille@uu.nl>
---
 src/parcels/_core/uxgrid.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/parcels/_core/uxgrid.py b/src/parcels/_core/uxgrid.py
index 8cfb41ec5..8cd86508e 100644
--- a/src/parcels/_core/uxgrid.py
+++ b/src/parcels/_core/uxgrid.py
@@ -81,7 +81,7 @@ def get_axis_dim(self, axis: _UXGRID_AXES) -> int:
 
     @property
     def deg2m(self) -> float:
-        """Metres per degree of arc for this grid's mesh."""
+        """Metres per arcdegree for this grid's mesh."""
         if self._radius is None:
             return 1852 * 60.0
         else: