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Presenter: Ron Gelaro

Seminar Title: Assessing the impact of observations in the NASA GEOS-5 atmospheric data assimilation system

With the adjoint of a data assimilation system (forecast model plus analysis scheme), the impact of any or all assimilated observations on measures of forecast or analysis skill can be estimated accurately and efficiently. The approach is especially well suited for assessing the impact of hyper-spectral satellite instruments on numerical weather forecasts because it easily allows aggregation of results in terms of individual data types, channels or locations, all computed simultaneously based on a single pass of the adjoint system.

The NASA Global Modeling and Assimilation Office (GMAO) has developed the adjoint of the GEOS-5 atmospheric data assimilation system, consisting of the GEOS-5 finite volume atmospheric model and Gridpoint Statistical Interpolation (GSI) analysis scheme developed at the National Centers for Environmental Prediction (NCEP). In this study, the impacts of various observing systems, including the Atmospheric Infrared Sounder (AIRS), are examined during July 2005 and January 2006. It is found that both conventional and satellite observations contribute significantly to the reduction of forecast errors, with asymmetries in the magnitudes of their impacts depending on the season and hemisphere. Map views of these impacts reveal possible deficiencies in the usage of some observation types. The adjoint-based impact calculations are compared with results from standard observing system experiments (OSEs). The two approaches are shown to provide unique, but complementary, information. The adjoint method also reveals explicit redundancies and dependencies between observing system impacts as observations are added or removed. Understanding these dependencies poses a major challenge for optimizing the use of the current observational network and defining requirements for future observing systems.

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