The Impacts of Representing the Correlation of Errors in Radar Data Assimilation. Part II: Model Output as Background Estimates

Abstract

Abstract In data assimilation, analyses are generally obtained by combining a “background,” taken from a previously initiated model forecast, with observations from different instruments. For optimal analyses, the error covariance of all information sources must be properly represented. In the case of radar data assimilation, such representation is of particular importance since measurements are often available at spatial resolutions comparable to that of the model grid. Unfortunately, misrepresenting the covariance of radar errors is unavoidable as their true structure is unknown. This two-part study investigates the impacts of misrepresenting the covariance of errors when dense observations, such as radar data, are available. Experiments are performed in an idealized context. In Part I, analyses were obtained by using artificially simulated background and observation estimates. For the second part presented here, background estimates from a convection-resolving model were used. As before, analyses were generated with the same input data but with different misrepresentation of errors. The impacts of these misrepresentations can be quantified by comparing the two sets of analyses. It was found that the correlation of both the background and observation errors had to be represented to improve the quality of analyses. Of course, the concept of “errors” depends on how the “truth” is considered. When the truth was considered as an unknown constant, as opposed to an unknown random variable, background errors were found to be biased. Correcting these biases was found to significantly improve the quality of analyses.