Ensemble data assimilation with the NCEP Global Forecast System

Real-data experiments with an ensemble data assimilation system using the NCEP Global Forecast System model were performed and compared with the NCEP Global Data Assimi-lation System (GDAS). All observations in the operational data stream were assimilated for the period January 1 to February 10, 2004, except satellite radiances. Because of computa-tional resource limitations, the comparison was done at lower resolution (triangular truncation at wavenumber 62 with 28 levels) than the GDAS real-time NCEP operational runs (triangular truncation at wavenumber 254 with 64 levels). The ensemble data assimilation system outperformed the reduced-resolution version of the NCEP operational 3D-Var system, with the biggest improvement in data sparse regions. En-semble data assimilation analyses yielded a 24-hour improvement in forecast skill in the South-ern Hemisphere extra-tropics relative to the NCEP 3D-Var system (the 48-hour forecast from the ensemble data assimilation system was as accurate as the 24-hour forecast from the 3D-Var system). Improvements in the data-rich Northern Hemisphere, while still statistically signifi-cant, were more modest. It remains to be seen whether the improvements seen in the Southern Hemisphere will be retained when satellite radiances are assimilated. Three different param-eterizations of background errors unaccounted for in the data assimilation system (including model error) were tested. Adding scaled random differences between adjacent 6-hourly analy-ses from the NCEP-NCAR Reanalysis to each ensemble member (additive inflation) performed slightly better than the other two methods (multiplicative inflation and relaxation-to-prior). 1 1