ModelHeightAdjustedSpecificHumidity

This ObsFunction adjusts an observed specific humidity value to preserve relative humidity when temperature has been height-adjusted from station elevation to model height.

The algorithm:

1. Compute RH at station height: \(RH = q_{obs} / q_{sat}(T_{obs}, P_{station})\)

2. Compute adjusted q at model surface: \(q_{adj} = RH \cdot q_{sat}(T_{adj}, P_{model\_surface})\)

where \(T_{adj}\) is the height-adjusted temperature from a prior Variable Assignment (e.g., using ModelHeightAdjustedAirTemperature).

If station pressure is missing for a location, model surface pressure is used as fallback for both saturation calculations. Since \(q_{sat} \propto e_{sat}(T)/P\), when the same pressure is used in both steps the effect largely cancels, making the adjustment primarily driven by the temperature difference.

The saturation vapor pressure uses the Sonntag formulation, and the saturated specific humidity uses the Gill/UKMO formulation: \(q_{sat} = \varepsilon \cdot e_{sat} / (P - (1-\varepsilon) \cdot e_{sat})\).

Parameters

All parameters are optional and have sensible defaults for typical surface obs processing.

• observed specific humidity variable: [optional, default ObsValue/specificHumidityAt2M] The observed specific humidity variable to adjust.

• observed temperature variable: [optional, default ObsValue/airTemperatureAt2M] The original observed temperature at station height, used to compute RH from \(q_{obs}\).

• adjusted temperature variable: [optional, default DerivedObsValue/airTemperatureAt2M] The height-adjusted temperature (from a prior Variable Assignment).

• station pressure variable: [optional, default ObsValue/stationPressure] Observed station pressure for computing \(q_{sat}\) at station height. Falls back to model surface pressure if missing.

• model surface pressure variable: [optional, default GeoVaLs/air_pressure_at_surface] Model surface pressure for computing \(q_{sat}\) at model surface height.

Example yaml

This ObsFunction is designed to be used in a chain after temperature adjustment:

obs filters:
# Step 1: Height-adjust temperature
- filter: Variable Assignment
  assignments:
  - name: DerivedObsValue/airTemperatureAt2M
    function:
      name: ObsFunction/ModelHeightAdjustedAirTemperature
      options:
        elevation:
          name: MetaData/stationElevation
        model height:
          from: terrain
        temperature lapse rate in K/km:
          scheme: constant
          value: 6.5

# Step 2: Height-adjust specific humidity preserving RH
- filter: Variable Assignment
  assignments:
  - name: DerivedObsValue/specificHumidityAt2M
    function:
      name: ObsFunction/ModelHeightAdjustedSpecificHumidity