TropopauseEstimate

This obsfunction creates a first-guess estimate of the tropopause pressure that is based on latitude with some adjustment for day-of-year. An optional parameter can convert the final answer from pressure to height using convert_p2z: true. The code in this method is crude and purely designed for estimating the tropopause when lacking a model-derived estimate that may otherwise arrive via GeoVaLs.

To begin, the code assumes an equatorial belt of 15 degrees north and south of the equator then applyies a linear transition toward the poles starting from 130 hPa and lowering to 370 hPa. To account for the seasons, the so-called equator is shifted to be about one month delayed from actual solar solstice to mimic that July (January) is typically hotter and has a corresponding higher tropopause than June (December) in the northern (southern) hemisphere.

Options

convert_p2z: true will use an ultra simple approximation of ICAO standard atmosphere from pressure to height because the code is making a tropopause estimate only.

tropo_equator is used to specify the pressure of the tropopause in the equatorial belt.

tropo_pole is used to specify the pressure of the tropopause at the poles.

Example 1

The most useful example of this obsfunction is to reject satellite-derived atmospheric motion vectors (satwinds) data when their vertical level information implies they exist well above the tropopause since clouds (which are tracked to provide a motion vector) are not likely to occur in the clear air of the stratosphere. This is handled by a Difference Check filter in which the MetaData/pressure is more than some threshold lower (higher altitude) than the supposed tropopause.

- filter: Difference Check
  filter variables:
  - name: windEastward
  - name: windNorthward
  reference: ObsFunction/TropopauseEstimate
  options:
    - tropo_equator: 13000         # 130 hPa
    - tropo_pole: 37000            # 370 hPa
  value: MetaData/pressure
  minvalue: -5000                  # 50 hPa above tropopause level, negative p-diff

Example 2

Another possible usage for this obsfunction is to inflate the observational error of water vapor (specificHumidity) or satellite radiance data when above the tropopause where clouds are nearly impossible to form.