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Atmospheric Model Data Sources

The best analysis is that which allows the IFS models  to subsequently produce forecasts that verify nearest to the actual evolution.  The analysis is not necessarily true to the observations in every respect, though of course the analysis processes (4D-Var and LDAS) try to assimilate them to best effect.  For the purposes of the ensemble, the analysis process also tries to quantify the uncertainty of our estimate of the initial state.  Advanced analysis procedures have to be used to assimilate non-conventional observations. 

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The observations used for the analysis of the atmosphere are available at both synoptic and asynoptic hours and can be divided roughly into direct observations and remote-sensing observations. 

Direct or Ground-based observations 

These consist of observations from:

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• be biased or be reported at incorrect locations or times.  For most observations there are dynamic bias correction mechanisms (e.g. bias correction used on one day may differ from bias correction used the previous day). Incorrect locations are generally flagged out by the monitoring.  Any duplicate observations are filtered out.
• not be representative of the grid resolution or height - perhaps lying in one corner of a grid box or at a different altitude.  Some observations are excluded using certain land/sea mask criteria.  For the other observations, the observation errors include implicitly a representativeness error.
  
• be of non-uniform quality – some radiosondes have good quality, others less so; and absolute calibration can vary with age of the instrument.  For most data types we try to use adapted observation errors.  Radiosondes are also identified by instrumentation type, which facilitates adaptations to the expected data quality.
  

• have insufficient detail - in radiosonde messages, perhaps only significant levels are reported (as in the old style TAC message format) instead of full resolution data (as in the new style BUFR format being gradually introduced around the world).

   

Indirect or Satellite-based observations

These are achieved in two different ways:

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