Calculation
Standard and significant levels for encoding in BUFR and TEMP messages are calculated in this order:
- Surface thermodynamic and winds
- Missing temperature
- Highest thermodynamic
- 110mb thermodynamic
- Inversions
- Station base pressure
- 110mb winds
- Highest wind
- Extrapolated altitudes for mandatory levels not covered by sounding
- GDL thermodynamic significant levels
- GDL winds significant levels
- Tropopause
- Missing RH
- Standard levels
- Max winds
- 62626 group levels
Significant levels
WMO guidelines mandate that “the reported significant levels alone shall make it possible to reconstruct the air temperature and humidity profiles [and wind speed and direction profiles] within the limits of the criteria specified”.
Significant levels can be further divided into mandatory significant levels (which will be present in every report) and additional levels (which fill in enough additional data to reconstruct the shape of the vertical profile).
Mandatory thermodynamic significant levels are:
- Surface level (or termination level for descending soundings that do not reach surface)
- Highest level (can be aircraft reference level for descending soundings)
- 110mb level (data somewhere between 110 and 100 mb)
- Bases and tops of inversions, only when base of layer is below the higher of the first tropopause or 300mb, and:
- Layer is >20mb thick
- Change in temperature is >2.5C
- Change in RH is >20%
Mandatory winds significant levels are:
- Surface level
- Highest level
- 110mb level
Additional significant levels are then filled in so that the linearly interpolated temperature and relative humidity from the series of levels will be within 1ºC and 15% RH of the original profile. For winds, the linearly interpolated profiles should be within 10º and 5m/s. See GDL Significant Levels.
Source
WMO-No. 306 Manual on Codes
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