EPPT-2590: Fire Severity Index: Add a FireSeverityIndex class to IMPROVER

improver/api/__init__.py

@@ -62,6 +62,7 @@
     "FieldTexture": "improver.utilities.textural",
     "FillRadarHoles": "improver.nowcasting.utilities",
     "FineFuelMoistureContent": "improver.fire_weather.fine_fuel_moisture_content",
+    "FireSeverityIndex": "improver.fire_weather.fire_severity_index",
     "FreezingRain": "improver.precipitation.freezing_rain",
     "FrictionVelocity": "improver.wind_calculations.wind_downscaling",
     "GenerateClearskySolarRadiation": "improver.generate_ancillaries.generate_derived_solar_fields",

improver/fire_weather/__init__.py

@@ -30,7 +30,7 @@ class FireWeatherIndexBase(BasePlugin):
     The Canadian Forest Fire Weather Index System requires specific units
     for all calculations. These are fixed and cannot be overridden:
 
-    - Temperature: degrees Celsius (degC)
+    - Temperature: degrees Celsius (Celsius)
     - Precipitation: millimeters (mm)
     - Relative humidity: dimensionless fraction (1)
     - Wind speed: kilometers per hour (km/h)
@@ -50,7 +50,7 @@ class FireWeatherIndexBase(BasePlugin):
     # Fixed unit conversions for all cube types used in fire weather calculations
     # These units are required by the Canadian FWI System and cannot be changed
     _REQUIRED_UNITS: dict[str, str] = {
-        "temperature": "degC",
+        "temperature": "Celsius",
         "precipitation": "mm",
         "relative_humidity": "1",
         "wind_speed": "km/h",
@@ -85,6 +85,7 @@ class FireWeatherIndexBase(BasePlugin):
         "input_ffmc": (0.0, 101.0),  # Valid FFMC range
         "input_dmc": (0.0, None),  # DMC is non-negative
         "input_dc": (0.0, None),  # DC is non-negative
+        "canadian_forest_fire_weather_index": (0.0, 100.0),  # FWI valid range
     }
 
     # Valid output ranges for warning checks (output_name: (min, max))

improver/fire_weather/fire_severity_index.py

@@ -0,0 +1,50 @@
+# (C) Crown Copyright, Met Office. All rights reserved.
+#
+# This file is part of 'IMPROVER' and is released under the BSD 3-Clause license.
+# See LICENSE in the root of the repository for full licensing details.
+"""Plugin to calculate the Fire Severity Index (Daily Severity Rating)."""
+
+import numpy as np
+from iris.cube import Cube
+
+from improver.fire_weather import FireWeatherIndexBase
+
+
+class FireSeverityIndex(FireWeatherIndexBase):
+    """
+    Plugin to calculate the Fire Severity Index, also known as the
+    Daily Severity Rating (DSR).
+
+    The DSR provides a numerical rating of the difficulty of controlling fires.
+    It is derived from the Fire Weather Index (FWI) and represents the daily
+    fire load and the expected effort required for fire suppression.
+
+    This process is adapted directly from:
+        Equations and FORTRAN Program for the
+        Canadian Forest Fire Weather Index System
+        (C.E. Van Wagner and T.L. Pickett, 1985).
+        Page 8, Equation 31.
+
+    Expected input units:
+        - Fire Weather Index (FWI): dimensionless
+    """
+
+    INPUT_CUBE_NAMES = ["canadian_forest_fire_weather_index"]
+    OUTPUT_CUBE_NAME = "fire_severity_index"
+
+    canadian_forest_fire_weather_index: Cube
+
+    def _calculate(self) -> np.ndarray:
+        """Calculates the Daily Severity Rating (DSR) from FWI.
+
+        From Van Wagner and Pickett (1985), Page 8: Equation 31.
+
+        Returns:
+            np.ndarray: The calculated DSR values.
+        """
+        fwi_data = self.canadian_forest_fire_weather_index.data
+
+        # Equation 31: DSR = 0.0272 * FWI^1.77
+        dsr = 0.0272 * fwi_data**1.77
+
+        return dsr

improver_tests/fire_weather/__init__.py

@@ -73,7 +73,7 @@ def make_input_cubes(
     Example:
         >>> cubes = make_input_cubes(
         ...     [
-        ...         ("air_temperature", 20.0, "degC", False),
+        ...         ("air_temperature", 20.0, "Celsius", False),
         ...         ("lwe_thickness_of_precipitation_amount", 1.0, "mm", True),
         ...     ]
         ... )