New screening of cold-air outbreak regions used in 4D-Var all-sky assimilation

TitleNew screening of cold-air outbreak regions used in 4D-Var all-sky assimilation
Publication TypeReport
Date Published02/2015
Series/CollectionEUMETSAT/ECMWF Fellowship Programme Research Reports
Document Number35
AuthorsLonitz, K, Geer, AJ
Event Series/CollectionEUMETSAT/ECMWF Fellowship Programme
Place of publicationShinfield Park, Reading

In this study a new filter for biases in cold-air outbreak (CAO) regions is introduced, which is applied before assimilating microwave radiances using the all-sky approach. In cold-air outbreaks, observed microwave imager radiances are typically warmer than the corresponding model radiances. The low model radiances are believed to be caused by a higher ratio of frozen to liquid cloud water compared to what has been observed. A filter is applied to reject observations from CAO areas as the assimilation of them would degrade the analysis. The current filter also rejects some unbiased microwave imager/sounder radiances, whereas the new filter identifies CAO areas more precisely using predictors of stability and the ratio between liquid and total cloud water. Using the new filter increases the amount of microwave imager data by  1.5% in the winter hemisphere, which corresponds to  0.4% globally. This has a neutral impact on the forecast scores for humidity, temperature and wind, proving that the new filter works well in detecting systematically biased data from CAO regions while rejecting less unbiased data.

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