Assessing the Effects of Data Selection with DAO's Physical-space Statistical Analysis System

A global, physical-space statistical analysis system is used to assess the effects of data selection in optimal interpolation (OI). Both systems use identical error covariance statistics and first guess, and are presented with the same observational data. Spectral decomposition of analysis increments in terms of spherical harmonics reveals that OI increments have an excessive amount of energy in higher wavenumbers and an unrealistically large ratio of divergence to vorticity. At least in part, dynamical imbalances in the OI analyzed state can be attributed to approximate method of solution of the analysis equation, and are not entirely due to the crude geostrophic constraint built into the covariance model. 1 Introduction Practical implementation of statistical interpolation schemes (sometimes referred to as optimal interpolation or OI) requires many simplifying assumptions for computational feasibility. Among these approximations is data selection, an ad-hoc procedure to reduce obs..