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. 

The ECMWF Land Data Assimilation System (LDAS) provides the land-surface analysis including the screen-level parameters analyses, the snow depth analysis, the soil moisture analysis, the soil temperature and snow temperature analysis.   

When the analysis makes large changes to the background state there are two main possibilities:

It is important to inspect closely these areas to help assess shortcomings of evolution in previous forecasts or the effect of the latest observations on the current forecast.

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:

Surface pressure reports are used over land and sea.  For wind speed and direction, observations from ships and buoys are used, but not those from land stations or even from islands or coastal stations.  Some screen level humidity data is used in the analysis, but screen temperature data is not used. However for the separate analysis of soil moisture screen temperature and dew point observations over land are actively used; in so doing observed temperatures are adjusted to the 2m level in the model by taking account of the vertical distance between the observations and grid point altitude first.  Cloud cover is not assimilated.

A scheme to account for the drift of radiosondes and consequent location of the observations during the ascent of the device was introduced in cycle 45r1 released in June 2018. Prior to this observed values at all altitudes were assigned to the location of the radiosonde station.

Several aspects need to be considered before and during the assimilation process.  Observations may:

Indirect or Satellite-based observations

These are achieved in two different ways:

Radiance assimilation takes the viewing geometry into account, by evaluating the radiative transfer along slantwise paths instead of assuming a nadir (overhead) viewpoint at all times.  This has been introduced recently for some satellite data (e.g. clear sky radiances) but not yet for everything.

Satellite data is important because:

 However, several aspects need to be considered.  Satellite data:

 is an indirect measurement and requires accurate observation operators to translate model quantities into observed ones.

Geostationary and Polar Orbiter satellite data have different strengths.

Satellite data is vital for an effective analysis and the use satellite observations is increasing rapidly.


Fig24.A: Pie chart showing the proportion of data types used by the IFC assimilation.  ATMS predominate. Ground-based observations constitute a relatively small proportion.




Users need to be aware of potential problems with the forecast due to deficiencies in coverage of data or conflict of observations with background fields.  Users should inspect: