Return tuple even when there is no time interval

This commit also fixes the unstructured stommel gyre example dataset to
be consistent in the variable placement with FESOM2. Specifically, the
pressure, U, and V fields are face registered and the vertical
velocities are vertex registered.

Author: fluidnumericsJoe

parcels/_datasets/unstructured/generic.py

@@ -7,7 +7,7 @@
 __all__ = ["Nx", "datasets"]
 
 T = 13
-Nx = 20
+Nx = 10
 vmax = 1.0
 delta = 0.1
 TIME = xr.date_range("2000", "2001", T)
@@ -44,12 +44,12 @@ def _stommel_gyre_delaunay():
     uxgrid.attrs["Conventions"] = "UGRID-1.0"
 
     # Define arrays U (zonal), V (meridional) and P (sea surface height)
-    U = np.zeros((1, nz1, lat.size), dtype=np.float64)
-    V = np.zeros((1, nz1, lat.size), dtype=np.float64)
+    U = np.zeros((1, nz1, uxgrid.n_face), dtype=np.float64)
+    V = np.zeros((1, nz1, uxgrid.n_face), dtype=np.float64)
     W = np.zeros((1, nz, lat.size), dtype=np.float64)
-    P = np.zeros((1, nz1, lat.size), dtype=np.float64)
+    P = np.zeros((1, nz1, uxgrid.n_face), dtype=np.float64)
 
-    for i, (x, y) in enumerate(zip(lon_flat, lat_flat, strict=False)):
+    for i, (x, y) in enumerate(zip(uxgrid.face_lon, uxgrid.face_lat, strict=False)):
         xi = x / 60.0
         yi = y / 60.0
 
@@ -61,10 +61,10 @@ def _stommel_gyre_delaunay():
         data=U,
         name="U",
         uxgrid=uxgrid,
-        dims=["time", "nz1", "n_node"],
+        dims=["time", "nz1", "n_face"],
         coords=dict(
             time=(["time"], [TIME[0]]),
-            nz1=(["nz1"], [0]),
+            nz1=(["nz1"], zc),
         ),
         attrs=dict(
             description="zonal velocity", units="m/s", location="node", mesh="delaunay", Conventions="UGRID-1.0"
@@ -74,7 +74,7 @@ def _stommel_gyre_delaunay():
         data=V,
         name="V",
         uxgrid=uxgrid,
-        dims=["time", "nz1", "n_node"],
+        dims=["time", "nz1", "n_face"],
         coords=dict(
             time=(["time"], [TIME[0]]),
             nz1=(["nz1"], zc),
@@ -100,7 +100,7 @@ def _stommel_gyre_delaunay():
         data=P,
         name="p",
         uxgrid=uxgrid,
-        dims=["time", "nz1", "n_node"],
+        dims=["time", "nz1", "n_face"],
         coords=dict(
             time=(["time"], [TIME[0]]),
             nz1=(["nz1"], zc),

parcels/_index_search.py

@@ -38,7 +38,7 @@ def _search_time_index(field: Field, time: datetime):
     if the sampled value is outside the time value range.
     """
     if field.time_interval is None:
-        return 0
+        return 0, 0
 
     if time not in field.time_interval:
         _raise_time_extrapolation_error(time, field=None)