The importance of GNSS Radio Occultation data in the ERA5 global reanalysis
Presenter:
Hans Hersbach
Hans Hersbach
ECMWF
Invited talk
At the European Centre for Medium-Range Weather Forecasts (ECMWF), reanalysis is a key contribution to the Copernicus Climate Change Service (C3S) that is implemented at ECMWF on behalf of the European Commission. As such ERA5 is the latest full-observing-system reanalysis, providing global estimates of the atmosphere, ocean waves and land-surface quantities. Currently, data are available from 1979 onwards, while an extension back to 1950 is being produced.
The role of reanalyses in climate monitoring applications is now widely recognized. By optimally combining observations and models, reanalyses indeed provide consistent \"maps without gaps\" of ECVs and strive to ensure integrity and coherence in the representation of the main Earth system cycles (e.g. water, energy).
The ERA5 four-dimensional variational assimilation method (4D-Var) includes a method for estimating observation biases (VarBC) that respects the heterogeneity within the observing system. This on one hand provides ERA5 products that are less sensitive to inhomogeneities and biases in its input data. On the other hand the resulting bias estimates can also provide more homogeneous long-term observational records. The method of VarBC does rely on 'anchor' observations that can be ingested as sufficiently bias free.
In the stratosphere and upper troposphere, the GNSS Radio Occultation data provide such an important source of unbiased information. Coverage started with data from CHAMP (2001-2008) followed by much larger numbers from COSMIC in 2006, SAC-C, TerraSAR-X, TanDEM-X, GRACE A, FY-3C and from GRAS on METOP-A, METOP-B and (recently) METOP-C.
This presentation will highlight the importance of GNSS-RO data in ERA5, and the consequence of its non-availability on the ERA5 mean state and uncertainty estimate prior to its availability.
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