Adding unit conversions for precipitation amount and surface snow amount

.zenodo.json

@@ -244,6 +244,10 @@
       "affiliation": "BSC, Spain",
       "name": "Ghosh, Supriyo",
       "orcid": "0000-0003-0581-9907"
+    },
+    {
+      "affiliation": "DLR, Germany",
+      "name": "Caspers, Laura"
     }
   ],
   "description": "ESMValCore: A community tool for pre-processing data from Earth system models in CMIP and running analysis scripts.",

CITATION.cff

@@ -248,6 +248,10 @@ authors:
     family-names: Ghosh
     given-names: Supriyo
     orcid: "https://orcid.org/0000-0003-0581-9907"
+  -
+    affiliation: "DLR, Germany"
+    family-names: Caspers
+    given-names: Laura
 
 cff-version: 1.2.0
 date-released: 2026-07-15

doc/recipe/preprocessor.rst

@@ -2589,6 +2589,10 @@ For example, this enables conversions between precipitation fluxes measured in
 versa).
 Currently, the following special conversions are supported:
 
+* ``precipitation_amount`` (``kg m-2``) --
+  ``lwe_thickness_of_precipitation_amount`` (``mm``)
+* ``surface_snow_amount`` (``kg m-2``) --
+  ``lwe_thickness_of_snowfall_amount`` (``mm``)
 * ``precipitation_flux`` (``kg m-2 s-1``) --
   ``lwe_precipitation_rate`` (``mm day-1``)
 * ``water_evaporation_flux`` (``kg m-2 s-1``) --

esmvalcore/iris_helpers.py

@@ -401,6 +401,14 @@ def has_unstructured_grid(cube: Cube) -> bool:
 # special case need to be "physically identical", e.g., 1 kg m-2 s-1 "equals" 1
 # mm s-1 for precipitation
 _SPECIAL_UNIT_CONVERSIONS: list[list[tuple[str | None, str]]] = [
+    [
+        ("precipitation_amount", "kg m-2"),
+        ("lwe_thickness_of_precipitation_amount", "mm"),
+    ],
+    [
+        ("surface_snow_amount", "kg m-2"),
+        ("lwe_thickness_of_snowfall_amount", "mm"),
+    ],
     [
         ("precipitation_flux", "kg m-2 s-1"),
         ("lwe_precipitation_rate", "mm s-1"),

esmvalcore/preprocessor/_units.py

@@ -38,6 +38,11 @@ def convert_units(cube: Cube, units: str | Unit) -> Cube:
 
     Currently, the following special conversions are supported:
 
+
+    * ``precipitation_amount`` (``kg m-2``) --
+      ``lwe_thickness_of_precipitation_amount`` (``mm``)
+    * ``surface_snow_amount`` (``kg m-2``) --
+      ``lwe_thickness_of_snowfall_amount`` (``mm``)
     * ``precipitation_flux`` (``kg m-2 s-1``) --
       ``lwe_precipitation_rate`` (``mm day-1``)
     * ``water_evaporation_flux`` (``kg m-2 s-1``) --

tests/unit/preprocessor/_units/test_convert_units.py

@@ -85,6 +85,36 @@ def test_convert_ozone_content_du_to_m(self):
             [[0.0, 1e-2], [2e-2, 3e-2]],
         )
 
+    def test_convert_precipitation_amount(self):
+        """Test special conversion of precipitation_amount."""
+        self.arr.standard_name = "precipitation_amount"
+        self.arr.units = "kg m-2"
+        result = convert_units(self.arr, "mm")
+        self.assertEqual(
+            result.standard_name,
+            "lwe_thickness_of_precipitation_amount",
+        )
+        self.assertEqual(result.units, "mm")
+        np.testing.assert_allclose(
+            result.data,
+            [[0.0, 1.0], [2.0, 3.0]],
+        )
+
+    def surface_snow_amount(self):
+        """Test special conversion of surface_snow_amount."""
+        self.arr.standard_name = "surface_snow_amount"
+        self.arr.units = "kg m-2"
+        result = convert_units(self.arr, "mm")
+        self.assertEqual(
+            result.standard_name,
+            "lwe_thickness_of_snowfall_amount",
+        )
+        self.assertEqual(result.units, "mm")
+        np.testing.assert_allclose(
+            result.data,
+            [[0.0, 1.0], [2.0, 3.0]],
+        )
+
     def test_convert_water_evaporation_flux(self):
         """Test special conversion of water_evaporation_flux."""
         self.arr.standard_name = "water_evaporation_flux"