Impact of ERS-1/ERS-2 scatterometer tandem operations on the ECMWF 3D-Var assimilation system

Didier Le Meur, ECMWF
Shinfield Park
Reading, Berkshire RG2 9AX, England
Tel (44) 1189 499 476/ Fax (44) 1189 869 450
E-mail stl@ecmwf.int


ERS scatterometer data have been assimilated operationally in the
ECMWF NWP model since January 1996.  The method followed relies on
the use of a three-dimensional variational analysis scheme
(3D-Var), in which the model state is updated by minimizing a cost
function measuring both its distance from its background estimate
and from the observations available. The scatterometer data are
processed as pairs of ambiguous wind vectors, leaving the
minimization process to select the right direction through the
application of a two-minima observational cost term.

The 3D-Var analysis, providing the most appropriate solution to
the ambiguity problem, has allowed improvements in the
ECMWF analysed and first guess surface winds through the use of
the scatterometer winds.  Clear improvements
have also been observed in the quality of the short range
forecasts, especially in the Southern Hemisphere where conventional
observations are sparse.  However, the impact on the medium-range
forecasts is neutral on average.

The data coverage provided by the single, 500 km wide swath of the
ERS scatterometers is probably one major limitation of these data.
Only one scatterometer has been used operationally
starting with the ERS-1 instrument and switching to
ERS-2 only once it was fully commissioned in April 1996
shortly before ERS-1 was switched off in June 1996.

The tandem operations of ERS-1 and ERS-2 allowed valid wind
data to be retrieved in real time from both scatterometers for a
few weeks. This period was chosen as an opportunity to study
the impact of the increased data coverage obtained by using both of
them together.

The presentation will describe four parallel assimilation
experiments over the first week of April 1996 within the
3D-Var system, using successively no scatterometer data, ERS-1 or
ERS-2 data only, and both ERS-1 and ERS-2 data.  The different
analysis and first guess fields obtained from these experiments
will be compared with each other, and evaluated with respect to the
data excluded from the analysis. Comparisons will also be made
between the associated forecasts.

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry