Further clean-up of tutorial

Author: erikvansebille

docs/user_guide/examples/tutorial_unitconverters.ipynb

@@ -23,7 +23,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "Let's first import the relevant modules, and generate a simple dataset on a 2D spherical mesh, with `U`, `V` and `temperature` data arrays, with the velocities 1 m/s and the temperature 20C.\n"
+    "Let's first import the relevant modules, and generate a simple dataset on a 2D spherical mesh, with `U`, `V` and `temperature` data arrays, with the velocities 1 m s<sup>-1</sup> and the temperature 20°C.\n"
    ]
   },
   {
@@ -67,7 +67,7 @@
     "When using a `FieldSet` method for a specific dataset, such as `from_copernicusmarine()`, the grid information is known and parsed by Parcels, so we do not have to add the `mesh` argument.\n",
     "```\n",
     "\n",
-    "Plotting the `U` field indeed shows a uniform 1 m/s eastward flow.\n"
+    "Plotting the `U` field indeed shows a uniform 1 m s<sup>-1</sup> eastward flow.\n"
    ]
   },
   {
@@ -107,7 +107,7 @@
     "However, printing the velocites directly shows something perhaps surprising. Here, we use the square-bracket field-interpolation notation to print the field value at (5W, 40N, 0m depth) at time 0. \n",
     "\n",
     "```{note}\n",
-    "Sampling a velocity in Parcels is done by calling the `fieldset.UV` VectorField; see the [Field Sampling tutorial](https://docs.oceanparcels.org/en/latest/examples/tutorial_sampling.html#Sampling-velocity-fields) for more information._\n",
+    "Sampling a velocity in Parcels is done by calling the `fieldset.UV` VectorField; see also the section \"Sampling U and V separately\" below.\n",
     "```\n"
    ]
   },
@@ -133,17 +133,17 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "While the temperature field indeed is 20C, as we defined, these printed velocities are much smaller.\n",
+    "While the temperature field indeed is 20°C, as we defined, these printed velocities are much smaller.\n",
     "\n",
-    "This is because Parcels converts under the hood from m/s to degrees/s.\n"
+    "This is because Parcels converts under the hood from m s<sup>-1</sup> to degrees s<sup>-1</sup>."
    ]
   },
   {
    "attachments": {},
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "Indeed, if we multiply the value of the U field with 1852 \\* 60 \\* cos(lat) (the number of meters in 1 degree of longitude), we get the expected 1 m/s.\n"
+    "Indeed, if we multiply the value of the U field with 1852 \\* 60 \\* cos(lat) (the number of meters in 1 degree of longitude), we get the expected 1 m s<sup>-1</sup>.\n"
    ]
   },
   {
@@ -196,6 +196,30 @@
     ")"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Don't sample U and V separately"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "Sampling `U` and `V` separately will _not_ convert to degrees s<sup>-1</sup>, so these velocities cannot be used directly for advection on spherical coordinates. This is one of the main reasons to always use the `UV` VectorField for velocity sampling in Parcels.\n"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "for fld in [fieldset.U, fieldset.V]:\n",
+    "    print(f\"{fld.name}: {fld.eval(time, z, lat, lon)}\")"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},