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A Spectral Nudging Technique for Dynamical Downscaling Purposes

Hans von StorchInstitute of Hydrophysics, GKSS Research Centre, Geesthacht, Germany

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Heike LangenbergInstitute of Hydrophysics, GKSS Research Centre, Geesthacht, Germany

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Frauke FeserInstitute of Hydrophysics, GKSS Research Centre, Geesthacht, Germany

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Abstract

The “spectral nudging” method imposes time-variable large-scale atmospheric states on a regional atmospheric model. It is based on the idea that regional-scale climate statistics are conditioned by the interplay between continental-scale atmospheric conditions and such regional features as marginal seas and mountain ranges. Following this “downscaling” idea, the regional model is forced to satisfy not only boundary conditions, possibly in a boundary sponge region, but also large-scale flow conditions inside the integration area.

In the present paper the performance of spectral nudging in an extended climate simulation is examined. Its success in keeping the simulated state close to the driving state at larger scales, while generating smaller-scale features is demonstrated, and it is also shown that the standard boundary forcing technique in current use allows the regional model to develop internal states conflicting with the large-scale state. It is concluded that spectral nudging may be seen as a suboptimal and indirect data assimilation technique.

Corresponding author address: Dr. Hans von Storch, Institute of Hydrophysics, GKSS Research Centre, Max-Planck-Straße, D-21502 Geesthacht, Germany.

Email: lehmannn@gkss.de

Abstract

The “spectral nudging” method imposes time-variable large-scale atmospheric states on a regional atmospheric model. It is based on the idea that regional-scale climate statistics are conditioned by the interplay between continental-scale atmospheric conditions and such regional features as marginal seas and mountain ranges. Following this “downscaling” idea, the regional model is forced to satisfy not only boundary conditions, possibly in a boundary sponge region, but also large-scale flow conditions inside the integration area.

In the present paper the performance of spectral nudging in an extended climate simulation is examined. Its success in keeping the simulated state close to the driving state at larger scales, while generating smaller-scale features is demonstrated, and it is also shown that the standard boundary forcing technique in current use allows the regional model to develop internal states conflicting with the large-scale state. It is concluded that spectral nudging may be seen as a suboptimal and indirect data assimilation technique.

Corresponding author address: Dr. Hans von Storch, Institute of Hydrophysics, GKSS Research Centre, Max-Planck-Straße, D-21502 Geesthacht, Germany.

Email: lehmannn@gkss.de

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