Influence of the assimilation scheme on the efficiency of adaptive observations

The influence of assimilation schemes on the efficiency of adaptive observations is investigated, with three-and four-dimensional variational data assimilation (3D-Var and 4D-Var) systems, for 20 cases of cyclogenesis during the Fronts and Atlantic Storm-Track Experiment (FASTEX). During FASTEX, adaptive observations were made in such a way that the forecast of synoptic-scale systems characterized by a weak predictability was improved. This study shows that, on average, adaptive observations significantly influence the forecast for both assimilation schemes. However, with 3D-Var, the inclusion of adaptive observations leads to a blending of improvements and worsenings, resulting in a very weak mean improvement. In contrast, with 4D-Var the forecasts are, on average, significantly improved at the verification time (a mean improvement of about 10% and a maximum improvement of more than 50%) without producing large variability at medium-range. Results confirm that the positive impact of 4D-Var primarily occurs as a result of a change in the initial conditions in the most unstable directions. This positive impact is particularly obvious when adaptive observations are mainly located at the border of the sensitive area. In such cases, the known deficiencies of 3D-Var may be detrimental. This study also demonstrates that the impact of sampling of the sensitive area depends on the assimilation scheme.

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