A Simple Bias Correction Algorithm for use in Data Assimiliation

This report examines the e ect of model bias within a data assimilation sys tem For this study we use the Goddard Earth Observing System GEOS Data Assimilation System DAS which employs the Incremental Analysis Updating IAU procedure In such a procedure the after analysis minus rst guess in crements are used as external forcing within a general circulation model GCM to provide corrective adjustments for data assimilation It is shown that for a GCM which contains a climatological model bias the climatology of the as similation will be biased from the climatology of the after analysis A modi ed IAU procedure is presented which e ectively removes the bias from the assim ilation The procedure may also be modi ed for incorporation into traditional intermittent data assimilation approaches