|Title||The direct assimilation of principal components of IASI spectra in the ECMWF 4D-Var|
|Year of Publication||2012|
|Authors||Matricardi, M, McNally, A|
|Secondary Title||Technical Memorandum|
The European Centre for Medium-Range Weather Forecasts (ECMWF) 4D-Var data assimilation system has been modified to allow the direct assimilation of Principal Component (PC) scores derived from spectra measured by the Infrared Atmospheric Sounding Interferometer (IASI). Testing of a prototype system where 165 IASI radiances are replaced by just 20 PC scores shows significant computational savings with no detectable loss of skill in the resulting analyses or forecasts. Indeed in some respects the assimilation of PC scores leads to marginal improvements over the traditional radiance based assimilation.