Add index first_autumn_frost (closes #89)

climix/etc/metadata.yml

@@ -275,3 +275,43 @@ indices:
       long_name: Simple precipitation intensity index
       definition: PRCPTOT / Nwetdays
       comment: Mean daily total precipitation during days having at least 1 mm
+
+  faf:
+    reference: B4EST
+    period:
+      allowed:
+        annual:
+        monthly:
+        seasonal: JASOND
+      default: seasonal
+    output:
+      var_name: faf
+      standard_name:
+      proposed_standard_name: first_occurrence_of_air_temperature_below_threshold
+      long_name: First Autumn Frost (day-of-year during Jul-Dec when Tmin < 0 degC)
+      units: days
+      cell_methods:
+        - time: minimum within days
+        - time: minimum over days
+    input:
+      var_name: tasmin
+      standard_name: air_temperature
+      cell_methods:
+        - time: minimum
+    index_function:
+      name: first_occurrence
+      parameters:
+        threshold:
+          kind: quantity
+          long_name: Threshold value for daily maximum air temperature
+          standard_name: air_temperature
+          data: 0
+          units: degree_Celsius
+        condition:
+          kind: operator
+          operator: '<'
+    ET:
+      short_name:
+      long_name:
+      definition:
+      comment: