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.
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| Fig. 1 | Fig. 2 | Fig. 3 |
Fig.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. In the troposphere and lower stratosphere, between 1000 hPa 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, and its more than 5 K colder at about 0.05 hPa. In addition, this colder region spreads into the northern hemisphere near 0.5 hPa, and the southern hemisphere above 0.05 hPa. Differences of more than 2 K are also apparent in both polar regions above about 0.3 hPa.
A similar comparison for the transition from 1988 to 1989 in ERA-Interim (Fig. 2) reveals that in the troposphere and stratosphere (from 1000 hPa to 1 hPa), the differences are, on the whole, of a similar magnitude to those in the 2009/2010 ERA5 transition. However, in the lower to mid mesosphere (from 1 hPa to 0.1 hPa, with the latter being the top of the ERA-Interim domain), the differences in the ERA-Interim transition are generally smaller than in the 2009/2010 ERA5 transition, having magnitudes less than 2 K, except near the equator at about 0.8 hPa where the magnitudes exceed 3 K. The spread in temperature in the ERA5 10 member ensemble (Fig. 3), which gives some indication of the uncertainty, shows that the largest values are located at lower latitudes, in the mid to upper stratosphere, where values exceed 0.8 K. This region merges into a second area of relatively large uncertainty, in the mesosphere of the northern hemisphere, where values exceed 0.7 K in the mid-latitudes of the mid-mesosphere.
