Operational ECMWF system September to December 2008.  Averaged over all model layers and entire global atmosphere.  % contribution of different observations to reduction in forecast error.
GPS RO has significant impact (ranked #5 among
all observing systems) in reducing forecast errors, despite the small number of soundings.
Courtesy: Carla Cardinali
and Sean Healy, ECMWF
22 Oct. 2009
Forecast error contribution (%)