Resolved bug in the selection of particle's default z value when it is not provided by the user

src/parcels/_core/particleset.py

@@ -44,7 +44,8 @@ class ParticleSet:
     y :
         List of initial y (latitude) values for particles
     z :
-        Optional list of initial z values for particles. Default is 0m
+        Optional list of initial z values for particles. Default is vertical grid position closest to the surface (z=0)
+        that covers all fields in the fieldset. If none of the fields in the fieldset have a vertical grid, z=0 is used.
     t :
         Optional list of initial t (time) values for particles. Default is fieldset.U.grid.time[0]
     repeatdt : datetime.timedelta or float, optional
@@ -78,11 +79,15 @@ def __init__(
             particle_ids = np.arange(x.size)
 
         if z is None:
-            minz = 0
+            minz = None
             for field in self.fieldset.fields.values():
-                if field.grid.depth is not None:
-                    minz = min(minz, field.grid.depth[0])
-            z = np.ones(x.size) * minz
+                for depth in field.grid.depth:
+                    if minz is None or np.abs(depth) < np.abs(minz):
+                        minz = depth
+            if minz is not None:
+                z = np.ones(x.size) * minz
+            else:
+                z = np.zeros(x.size)
         else:
             z = np.array(z).flatten()
         assert x.size == y.size and x.size == z.size, "x, y, z don't all have the same lengths"

tests/test_particleset.py

@@ -7,11 +7,13 @@
 import xarray as xr
 
 from parcels import (
+    FieldSet,
     Particle,
     ParticleSet,
     ParticleSetWarning,
     Variable,
 )
+from parcels._datasets.structured.generated import simple_UV_dataset
 from tests.common_kernels import DoNothing
 from tests.utils import round_and_hash_float_array
 
@@ -174,3 +176,38 @@ def test_pset_iterator(fieldset):
     for i, particle in enumerate(pset):
         assert particle.particle_id == i
     assert i == npart - 1
+
+
+@pytest.mark.parametrize(
+    "depths",
+    [
+        pytest.param(np.linspace(1, 10, 10), id="all_depths_positive"),
+        pytest.param(np.linspace(-10, -1, 10), id="all_depths_negative"),
+    ],
+)
+def test_pset_default_z_is_in_domain(depths):
+    ds = simple_UV_dataset(dims=(1, len(depths), 10, 10), mesh="flat")
+    ds = ds.assign_coords(depth=depths)
+    fieldset = FieldSet.from_sgrid_conventions(ds, mesh="flat")
+
+    pset = ParticleSet(fieldset, x=[0], y=[0])
+    expected_z = depths[np.argmin(np.abs(depths))]
+    assert np.isclose(pset.z[0], expected_z)
+
+
+@pytest.mark.parametrize(
+    "depths",
+    [
+        pytest.param(np.concatenate([np.linspace(-15, -1, 5), np.linspace(0, 2, 5)]), id="depths_include_zero"),
+        pytest.param(np.concatenate([np.linspace(-9, -3, 3), np.linspace(2, 8, 3)]), id="closest_depth_is_positive"),
+        pytest.param(np.concatenate([np.linspace(-8, -2, 3), np.linspace(3, 9, 3)]), id="closest_depth_is_negative"),
+    ],
+)
+def test_pset_default_z_closest_to_zero(depths):
+    ds = simple_UV_dataset(dims=(1, len(depths), 10, 10), mesh="flat")
+    ds = ds.assign_coords(depth=depths)
+    fieldset = FieldSet.from_sgrid_conventions(ds, mesh="flat")
+
+    pset = ParticleSet(fieldset, x=[0], y=[0])
+    expected_z = depths[np.argmin(np.abs(depths))]
+    assert np.isclose(pset.z[0], expected_z)