WindDirAngleDiff

This obsfunction computes the wind direction angle difference between the observation and model. Since the wind component variables are used in JEDI, a translation to wind direction is made within the code. The purpose of this function is to reject observed wind data when the observation differs from the model background (first guess) by a threshold of angular difference in wind direction, typically done with the Bounds Check filter. Since light wind conditions (nearly calm) could be considered OK, there is an optional parameter minimum_uv that can be set (default is 0.5 m/s) below which the check will not be applied.

Options

test_hofx is an option to compare the results against a reference (such as GsiHofX) when executed with test_OpsOperator.x application.

minimum_uv is used to ignore large wind direction discrepancies (between model and obs) under light wind conditions. The default value is 0.5 m/s.

Example

For application in satellite-derived wind quality control, a difference of wind direction greater than 50 degrees is often used to reject the wind information. The following yaml illustrates its usage together with the Bounds Check filter. The test_hofx parameter is shown below (set to GsiHofX), however, when running UFO HofX application, this option should be removed since the incomming model background field would be used instead of a pre-existing value of hofx.

- filter: Bounds Check
  filter variables:
  - name: eastward_wind
  - name: northward_wind
  test variables:
  - name: WindDirAngleDiff@ObsFunction
    options:
      minimum_uv: 1.0        # Wind components less than this value are ignored in the wind angle difference check
      #test_hofx: GsiHofX     # Only if testing against pre-existing hofx data; otherwise reference is hofx

  maxvalue: 50