| Summary: | The impact of optimal parameters in a non-orographic gravity wave drag parameterization on the middle atmosphere circulation of the Southern hemisphere is examined. Optimal parameters are estimated using a data assimilation technique. The proposed technique aims to reduce the delay in the winter vortex breakdown of the Southern Hemisphere found in general circulation models, which may be associated with a poor representation of gravity wave activity. We introduce two different implementations of the parameter estimation method: an emph{offline} estimation method and a emph{sequential} estimation method. The delay in the zonal-mean zonal-wind transition is largely alleviated by the optimal gravity wave parameters. The sequential method diminishes the model biases during winter vortex evolution, through gravity wave drag alone. On the other hand, the offline method accounts better for the unresolved-resolved wave interactions and the zonal-wind transition. We show that the final warmings in the lower mesosphere are mainly driven by planetary wave breaking. These are affected by changes in the gravity wave drag which are responsible for the stratospheric preconditioning. Parameter estimation during the vortex breakdown is a challenging task that requires the use of sophisticated estimation techniques, because there are strong interactions between unresolved gravity wave drag and planetary waves. Fil: Scheffler, Guillermo Federico. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas Naturales y Agrimensura. Departamento de Matemática; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Pulido, Manuel Arturo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Nordeste. Instituto de Modelado e Innovación Tecnológica. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas Naturales y Agrimensura. Instituto de Modelado e Innovación Tecnológica; Argentina
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