Final Technical Note of 'Impact assessment of commercial GNSS-RO data'

The addition of Spire and COSMIC-2 data is a large increase in the GNSS-RO data being assimi-
lated at ECMWF and Met Office NWP systems. Both institutions have investigated the effect of Spire
and COSMIC-2 data running Observing System Experiments in their state-of-the-art NWP systems.
They found that the addition of COSMIC-2 and Spire observations is beneficial, with adding Spire
on top of COSMIC-2 showing further improvements for temperature, humidity and wind. In conclu-
sion, Met Office and ECMWF would assimilate the Spire data operationally if it becomes available.
Additionally, ECMWF have compared the impact of how the Spire data is processed, by compar-
ing bending angles provided directly by Spire with Spire bending angles produced at EUMETSAT.
Furthermore, this extensive GNSS-RO dataset also provides the opportunity to investigate how the
impact of GNSS-RO data scales with observation number. For this reason, we have performed a set
Ensembles of Data Assimilation (EDA) experiments. We have found that increasing the number of
GNSS-RO data reduces the ensemble spread, especially at 100 hPa and higher and in the Tropics.
In this region, we also found a linear relationship between EDA spread and variance in first guess
departures of radiosonde temperature observations. For the Northern and Southern Hemisphere the
decrease in EDA spread was less strong when more GNSS-RO data has been added, which might be
linked inter alia to the heterogeneously distributed GNSS-RO data in the outer-tropics.