Update Nemo tutorial periodic boundaries

Author: erikvansebille

docs/user_guide/examples/tutorial_nemo.ipynb

@@ -155,7 +155,7 @@
     ")\n",
     "\n",
     "pset.execute(\n",
-    "    [parcels.kernels.AdvectionEE],\n",
+    "    parcels.kernels.AdvectionRK2,\n",
     "    runtime=runtime,\n",
     "    dt=np.timedelta64(1, \"D\"),\n",
     "    output_file=pfile,\n",
@@ -203,7 +203,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "# post processing\n",
+    "# handling periodic boundary conditions in post processing\n",
     "df = df.with_columns((pl.col(\"x\") % 360).alias(\"x\"))\n",
     "df = df.with_columns(\n",
     "    pl.when(pl.col(\"x\") <= 180)\n",
@@ -223,8 +223,23 @@
    },
    "outputs": [],
    "source": [
-    "# with a Kernel\n",
-    "def periodicBC(particles, fieldset):  # pragma: no cover\n",
+    "# with a custom Kernel\n",
+    "def AdvectionRK2_periodicBC(particles, fieldset):  # pragma: no cover\n",
+    "    \"\"\"Advection of particles using second-order Runge-Kutta integration,\n",
+    "    keeping particles between -180 and 180 degrees longitude.\n",
+    "    \"\"\"\n",
+    "    (u1, v1) = fieldset.UV[particles]\n",
+    "    x1 = particles.x + u1 * 0.5 * particles.dt\n",
+    "    y1 = particles.y + v1 * 0.5 * particles.dt\n",
+    "    x1 = np.where(x1 > 180, x1 - 360, x1)  # keep particles within [-180, 180]\n",
+    "\n",
+    "    (u2, v2) = fieldset.UV[\n",
+    "        particles.t + 0.5 * particles.dt, particles.z, y1, x1, particles\n",
+    "    ]\n",
+    "    particles.dx += u2 * particles.dt\n",
+    "    particles.dy += v2 * particles.dt\n",
+    "\n",
+    "    # update particles.dx to stay within [-180, 180]\n",
     "    particles.dx = np.where(\n",
     "        particles.x + particles.dx > 180, particles.dx - 360, particles.dx\n",
     "    )\n",
@@ -236,7 +251,7 @@
     ")\n",
     "\n",
     "pset.execute(\n",
-    "    [parcels.kernels.AdvectionEE, periodicBC],\n",
+    "    AdvectionRK2_periodicBC,\n",
     "    runtime=runtime,\n",
     "    dt=np.timedelta64(1, \"D\"),\n",
     "    output_file=pfile,\n",