Abstract. We here present results from an evaluation of the Radio Occultation Meteorology Satellite Application Facility (ROM SAF) gridded monthly mean climate data record (CDR v1.0),
based on Global Positioning System (GPS) radio occultation (RO) data from the CHAMP (CHAllenging Minisatellite Payload), GRACE (Gravity Recovery and
Climate Experiment), COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate),
and Metop satellite missions.
Systematic differences between RO missions, as well as differences of RO data relative to ERA-Interim
reanalysis data, are quantified. The methods used to generate gridded monthly mean data are described, and
the correction of monthly mean RO climatologies for sampling errors, which is essential for
combining data from RO missions with different sampling characteristics, is evaluated. We find good overall agreement between the ROM SAF gridded monthly mean CDR and the ERA-Interim reanalysis,
particularly in the 8–30 km height interval. Here, the differences largely reflect time-varying biases
in ERA-Interim, suggesting that the RO data record has a better long-term stability than
ERA-Interim. Above 30–40 km altitude, the differences are larger, particularly for the pre-COSMIC era. In the 8–30 km altitude region, the observational data record exhibits a high degree of
internal consistency between the RO satellite missions, allowing us to combine data into
multi-mission records.
For global mean bending angle, the consistency is better than 0.04 %, for refractivity it is better than 0.05 %, and for global mean
dry temperature the consistency is better than 0.15 K in this height interval. At altitudes
up to 40 km, these numbers increase
to 0.08 %, 0.11 %, and 0.50 K, respectively. The numbers can be up to a factor of 2 larger for certain latitude
bands compared to global means. Below about 8 km, the RO mission differences are larger, reducing the possibilities
to generate multi-mission data records.
We also find that the residual sampling errors are about one-third of the original and that they include a component
most likely related to diurnal or semi-diurnal cycles.