The AFES-LETKF Experimental Ensemble Reanalysis: ALERA

Abstract

A four-dimensional local ensemble transform Kalman filter (4D-LETKF) is applied to the AFES (AGCM for the Earth Simulator) at a T159/L48 resolution to perform an experimental reanalysis called “ALERA” since May 2005 until October 2006, without using satellite radiances. The medium-term reanalysis confirms for the first time that the 4D-LETKF system is stable for more than a year with a realistic AGCM and real observations. ALERA generates reasonable reanalysis of the nature atmosphere, which is shown by comparing with the NCEP/NCAR reanalysis. ALERA is disadvantageous in the areas where satellite radiances play an essential role such as upper atmosphere and Southern Hemisphere high latitudes, although ensemble spreads capture the large errors. ALERA has large departures from the NCEP/NCAR reanalysis at upper levels above 30 hPa where the departures are larger than what is expected from the ensemble spreads. The ensemble reanalysis provides not only analysis itself but also samples for the analysis error statistics, which offers various potential uses for further studies.