Estimation of the Cressman term for a barotropic model through optimization with use of the adjoint model

This paper presents an estimate of the Cressman term that appears in the divergent barotropic model as a corrector of planetary-wave retrogression. In contrast with earlier studies, the term varies as a function of geographical location. The estimation is performed using the adjoint model. The results deviate from those earlier derived theoretically and applied in routine forecasting. The Cressman term can now be viewed as a corrector of the systematic error or as a baroclinity parameter. The proposed form of the Cressman term can also be interpreted as a forcing parameter, maintaining the troughs and ridges of the main circulation by affecting the free long waves. Parameter estimation using the adjoint model has shown its potential in these experiments. Not only are the parameter values determined but new interpretations and approaches have been found.