Fixing unit conversion for non-Cgrids

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

Author: erikvansebille

src/parcels/_core/field.py

@@ -330,18 +330,23 @@ def eval(self, time: datetime, z, y, x, particles=None, apply_conversion=True):
         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 apply_conversion and self.U.grid._mesh == "spherical":
+                u /= 1852 * 60.0 * np.cos(np.deg2rad(y))
+                v /= 1852 * 60.0
+
             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)
 
-        if apply_conversion:
-            if self.U.grid._mesh == "spherical":
-                meshJac = 1852 * 60.0 * np.cos(np.deg2rad(y))
-                u = u / meshJac
-                v = v / meshJac
+            if apply_conversion:
+                if self.U.grid._mesh == "spherical":
+                    meshJac = 1852 * 60.0 * np.cos(np.deg2rad(y))
+                    u = u / meshJac
+                    v = v / meshJac
 
         for vel in (u, v, w):
             _update_particle_states_interp_value(particles, vel)