A set of global observing experiments has been run by denying classes of observations from the ECMWF 4D-Var assimilation and forecast system. This gives indications on the efficiency of the use of observations in this particular system. The tested observing sytems were mainly satellite data but also included TEMP/PILOT, aircraft, DRIBU.

Some interesting conclusions can be drawn about the ECMWF 4D- Var system. Each tested observing system has a notable positive impact on the medium-range forecast performance. The conclusions are not all the same at short range. The TOVS radiances have a large impact on all areas and ranges. Drifting buoys have only impact in the Southern Hemisphere. In the Northern Hemi Hemisphere, the relative impact of various data types varies regionally.