Direct 4D-Var assimilation of all-sky radiances. Part I: Implementation
| Title | Direct 4D-Var assimilation of all-sky radiances. Part I: Implementation
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Technical memorandum
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| Date Published |
04/2010
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| Secondary Title |
ECMWF Technical Memoranda
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| Number |
618
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| Author | |
| Publisher |
ECMWF
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| Abstract | This paper describes a new radiance assimilation scheme for microwave imager observations which unifies the treatment of clear-sky, cloud and precipitation-affected situations, giving an all-sky' approach. The previous approach assimilated radiances in clear skies and retrievals of total column water vapour in clouds and rain. In March 2009, the new approach became operational in the Four-Dimensional Variational Assimilation (4D-Var) system of the European Centre for Medium-Range Weather Forecasts. This approach employs moist physics parameterizations and a full multiple scattering radiative transfer model in the observation operator for all microwave imager observations. Observation operator accuracy, observation error definition and bias correction, basic observational impact, 4D-Var linearity and stability as well as computational cost are described. Because of careful quality control and relatively large observation errors, the all-sky system produces a weaker observational constraint on moisture analysis than the previous system. However, in single-observation experiments where the observations are given the same weight as before, the all-sky system is able to produce 4D-Var analyses that are closer to the observations than the previous approach. Despite the nonlinearity of rain and cloud processes, 4D-Var minimises successfully through the use of an incremental technique. Overall the quality of the 4D-Var minimisation, in terms of number of iterations and conditioning, is unaffected by the new approach. |
| URL | https://www.ecmwf.int/en/elibrary/73599-direct-4d-var-assimilation-all-sky-radiances-part-i-implementation |
| DOI | 10.21957/fhxdy4xou |
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