In ECMWF Technical Memo 859 released in January 2020, Simmons and his coauthors
report that "the ERA5 analyses of lower stratospheric temperature exhibit a
pronounced cold bias for the years 2000 to 2006. This is due to specifying
background error covariances for the data assimilation that were inappropriate
prior to availability during 2006 of GNSS radio occultation data in sufficient
numbers to constrain a cold bias of the assimilating ERA5 model. A new set of
analyses, termed ERA5.1, has thus been produced for the period from 2000 to
2006 using the background error covariances that were used to produce the
ERA5 analyses for the years 1979 to 1999. ERA5.1 also includes the more
restrictive ensemble assimilation of SBUV ozone data that was used in production
of ERA5 for 1979 to 1999."
"ERA5.1 provides analyses with better global-mean temperatures in the
stratosphere and uppermost troposphere than provided by ERA5. ERA5.1 stands up
well in comparison with ERA-Interim and other reanalyses in the lower stratosphere,
although there are also lower-stratospheric temperature differences between ERA5
and other reanalyses in the 1980s and 1990s. These are due in part to differences
in radiosonde temperature bias adjustment. The pronounced near-tropopause cold bias
of ERA5 from 2000 to 2006 has implications for the representation of stratospheric
humidity, for which ERA5.1 performs better, though by no means perfectly.
ERA5.1 does not exhibit the spuriously high values of ozone that occur close
to the South Pole in the polar nights of 2003 and 2004 in the ERA5 analyses.
Synoptic evolution in the extratropical stratosphere is seen to be
very similar in two cases involving splitting of the stratospheric polar vortex and
secondary vortex formation by dynamical instability. The ERA5.1 representation of
the QBO agrees slightly better with radiosonde wind data than that of ERA5.
The dataset formed by merging ERA5.1 with ERA5 is generally more homogeneous
over time than ERA5 alone. It nevertheless is problematic for global-mean upper
stratospheric temperature for all but the most recent ten or so years."
"ERA5.1 is very close to ERA5 in the lower and middle troposphere."
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