Abstract
Abstract. Radio occultation (RO) sensing is used to probe the earth's atmosphere in order to obtain information about its physical properties. With a main interest in the parameters of the neutral atmosphere, there is the need to perform a correction of the ionospheric contribution to the bending angle. Since this correction is an approximation to first order, there exists an ionospheric residual, which can be expected to be larger when the ionization is high (day versus night, high versus low solar activity). The ionospheric residual systematically affects the accuracy of the atmospheric parameters at low altitudes, at high altitudes (above 25–30 km) it even is an important error source. In climate applications this could lead to a time dependent bias which induces wrong trends in atmospheric parameters at high altitudes. The first goal of our work was to study and characterize this systematic residual error. In a second step we developed a simple correction method, based purely on observational data, to reduce this residual for large ensembles of RO profiles. In order to tackle this problem, we analyzed the bending angle bias of CHAMP and COSMIC RO data from 2001–2011. We could observe that the nighttime bending angle bias stays constant over the whole period of 11 yr, while the daytime bias increases from low to high solar activity. As a result, the difference between nighttime and daytime bias increases from about −0.05 μrad to −0.4 μrad. This behavior paves the way to correct the solar cycle dependent bias of daytime RO profiles. In order to test the newly developed correction method we performed a simulation study, which allowed to separate the influence of the ionosphere and the neutral atmosphere. Also in the simulated data we observed a similar increase in the bias in times from low to high solar activity. In this simulation we performed the climatological ionospheric correction of the bending angle data, by using the bending angle bias characteristics of a solar cycle as a correction factor. After the climatological ionospheric correction the bias of the simulated data improved significantly, not only in the bending angle but also in the retrieved temperature profiles.
Highlights
Atically affects the accuracy of the atmospheric parameters at low altitudes, at high altitudes it even is an important error source
The radio occultation (RO) method (Melbourne et al, 1994; Klituerstoinsskatieelltitael.l,im19b9s7o;uHHndayjijndgerttoeacll.oh,ng2iq0yu0e2a.) Mnisdeaansuarcetmiveentssataerleperformed when a globaEl paorstithionSinygsstyestmem (GPS) satellite transmits an electromagnetiSc csiigenanl,cwehsich is recorded at a low earth orbit (LEO) satellite
In order to detect the residual ionospheric error, we investigated the bending angle bias over a time period from 2001 to 2011, using CHAMP (CHAllenging Minisatellite Payload) and Formosat-3/COSMIC (Constellation Observing System for Meteorology, Ionosphere and Climate) RO data, comparing we thank G. Kirchengast (WEGC) Occultation Processing System version 5.4 (OPSv5.4) (Steiner et al, 2009; Pirscher, 2010) and UCAR (University Corporation for Atmospheric Research) data processing (Kuo et al, 2004; Ho et al, 2009)
Summary
In order to detect the residual ionospheric error, we investigated the bending angle bias (see Sect. 2.3) over a time period from 2001 to 2011, using CHAMP (CHAllenging Minisatellite Payload) and Formosat-3/COSMIC (Constellation Observing System for Meteorology, Ionosphere and Climate) RO data, comparing WEGC Occultation Processing System version 5.4 (OPSv5.4) (Steiner et al, 2009; Pirscher, 2010) and UCAR (University Corporation for Atmospheric Research) data processing (Kuo et al, 2004; Ho et al, 2009). 2.3) over a time period from 2001 to 2011, using CHAMP (CHAllenging Minisatellite Payload) and Formosat-3/COSMIC (Constellation Observing System for Meteorology, Ionosphere and Climate) RO data, comparing WEGC Occultation Processing System version 5.4 (OPSv5.4) (Steiner et al, 2009; Pirscher, 2010) and UCAR (University Corporation for Atmospheric Research) data processing (Kuo et al, 2004; Ho et al, 2009). CHAMP data have been available from May 2001 to September 2008 and COSMIC data were used from August 2006 to September 2011. The UCAR/CDAAC (COSMIC Data Analysis and Archive Center) retrieval (version 2010.2640) starts with raw GPS amplitude and phase measurements as well as raw GPS and LEO orbit tracking data. The WEGC OPSv5.4 retrieval starts with excess phase profiles and precise orbit information, provided by UCAR/CDAAC
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