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 of 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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| Fig. 1 Zonal mean temperature (C) of the final ERA5 HRES product (contours) and zonal mean temperature difference (C) for the final ERA5 HRES product minus the warm up (colours), for October to December 2009. The vertical coordinate is pressure of the model levels. | Fig. 2 Zonal mean temperature (C) of the final ERA-Interim product (contours) and zonal mean temperature difference (C) for the final ERA-Interim product minus the warm up (colours), for October to December 2009. The vertical coordinate is pressure of the model levels. | Fig. 3 Zonal mean temperature (C) of the ERA5 EDA control (contours) and zonal mean ERA5 EDA ensemble spread of temperature (C) (colours), for September to November 2009. The vertical coordinate is pressure of the model levels. |
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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, although ERA-Interim does exhibit transition differences lower down in the atmosphere. 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 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, for September - November 2009, in the ERA5 10 member ensemble (Fig. 3), which gives some indication of the uncertainty, is generally larger than the ERA5 transition differences in the lower latitudes of the troposphere and stratosphere. The largest values of the spread are located in the mid to upper stratosphere, at lower latitudes, where values exceed 0.8 K. This region merges into a second area of relatively large uncertainty, in the northern mid-latitudes of the mid-mesosphere, where values exceed 0.7 K. In general, the spread in the mesosphere is smaller than the ERA5 transition differences.
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| Fig. 4 Zonal mean eastward wind (ms-1) of the final ERA5 HRES product (contours) and zonal mean eastward wind difference (ms-1) for the final ERA5 HRES product minus the warm up (colours), for October to December 2009. The vertical coordinate is pressure of the model levels. | Fig. 5 Zonal mean eastward wind (ms-1) of the final ERA-Interim product (contours) and zonal mean eastward wind difference (ms-1) for the final ERA-Interim product minus the warm up (colours), for October to December 2009. The vertical coordinate is pressure of the model levels. | Fig. 6 Zonal mean eastward wind (ms-1) of the ERA5 EDA control (contours) and zonal mean ERA5 EDA ensemble spread of eastward wind (ms-1) (colours), for September to November 2009. The vertical coordinate is pressure of the model levels. |
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The equatorial differences in eastward wind in ERA-Interim in the lower stratosphere (Fig. 5) are smaller than those in ERA5, though above in the mid to upper stratosphere, the differences are somewhat larger than those in ERA5. In the mesosphere, the differences in ERA-Interim are much smaller than those in ERA5. The spread in the eastward wind in the ERA5 10 member ensemble (Fig. 6) is generally below 2 ms-1 except at low latitudes above about 0.2 hPa, where values can exceed 5 ms-1.
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| Fig. 7 Zonal mean specific humidity (log kg kg-1) of the final ERA5 HRES product (contours) and zonal mean specific humidity difference (%) for the final ERA5 HRES product minus the warm up (colours), for October to December 2009. The vertical coordinate is pressure of the model levels. | Fig. 8 Zonal mean ozone (log kg kg-1) of the final ERA5 HRES product (contours) and zonal mean ozone difference (%) for the final ERA5 HRES product minus the warm up (colours), for October to December 2009. The vertical coordinate is pressure of the model levels. |
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