In order to speed up production, ERA5 is being produced by several parallel experiments, each covering a different period, which are then appended together to create the final product. The disadvantage of this approach, is that there can be discontinuities in the final product at the transition points between the different experiments. Here, we document the transition at the end of 2009 and the beginning of 2010. Each experiment begins with a warm up yearof a few months, which does not become part of the final product, but does overlap with the experiment for the preceding period. This overlap facilitates a comparison of the two experiments to see how well they have converged to a common solution.
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