In order to speed up production, ERA5 is being produced by several parallel experiments, 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 year, which does not become part of the final product, but does overlap with the preceding experiment. This facilitates a comparison of the two experiments to see how well they have converged to a common solution.
Fig.1shows 1 shows the zonal mean temperature (contours) and difference between the final product and the warm up (colours), for the last three months of the overlap year, October - December 2009. From Between 1000 hPa to and 10 hPa the differences are small (less than 0.2 K). From the mid-stratosphere into the mesosphere, the differences are larger, but generally below 2 K. However, in some places the differences are larger. Near the equator, at about 1 hPa the final product is more than 2K warmer than the warm up, whereas above the final product is generally more than 2 K colder, with a maximum greater than 5 K colder at about 0.05 hPa.
