Abstract.The

[1] GPS radio occultation measurements from CHAMP, GRACE-A and FORMOSAT-3/COSMIC are used to derive global information on small-scale ionospheric irregularities such as sporadic E layers between January 2002 and December 2007. The investigations are based on the analysis of amplitude variations of the GPS radio occultation signals. The global distribution of ionospheric irregularities shows strong seasonal variations with highest occurrence rates during summer in the middle latitudes. The long-term data set of CHAMP allows for first climatological studies, while the data coverage increases significantly with the combination of CHAMP, GRACE and FORMOSAT-3/ COSMIC measurements. This allows for global maps of sporadic E occurrence rates of very high spatial resolution where the influence of the Earth's magnetic field becomes visible in global sporadic E maps for the first time.

The operational data analysis of the GPS radio occultation experiment aboard the German CHAMP (CHAllenging Minisatellite Payload) satellite mission is described. Continuous Near-Real-Time processing with average time delay of @5 hours between measurement and provision of analysis results is demonstrated. A delay of less than 3 hours is reached for individual events. This is made possible by using an operationally operated ground infrastructure, consisting of a polar downlink station, a globally distributed fiducial GPS ground network, a precise orbit determination facility, an automated occultation processing system and an advanced data center (the Information System and Data Center at GFZ, ISDC). The infrastructure was installed within the CHAMP and the German GPS Atmosphere Sounding Project (GASP). More than 120,000 globally distributed occultation measurements were automatically analysed during 2001 and 2002. A set of @46,000 vertical profiles of refractivity, temperature and water vapor is validated with meteorological analyses from the European Centre for Medium-Range Weather Forecasts (ECMWF) and data from the global radiosonde network. The mean temperature bias in relation to the analyses is less then 0.4 K between 10 and 35 km, the mean deviation of the refractivity is <0.5%. A height dependent standard deviation of @1 K at 10 km and @2 K at 30 km is observed. This result is confirmed by comparing @6,000 CHAMP occultations with corresponding radiosonde measurements. A negative bias of the refractivity in relation to the analyses up to @5% in the Tropics is found in the lower troposphere. It corresponds to mean meridional dry biases of the specific humidity up to @30%. It is shown, that the application of a heuristic retrieval method, based on the Canonical Transform method and the sliding spectral approach, reduces the refractivity bias on average by a factor of @2. The corresponding bias in the specific humidity is reduced by a factor of @3. In mid-latitudes almost no more refractivity bias out of the planetary boundary layer is observed. This is shown by a comparison of

[1] Analysis of atmospheric occultation data from the GPS Meteorology experiment has revealed that the refractivity retrievals in the lower troposphere were systematically smaller than those obtained with numerical weather prediction models. It has been suggested that the bias was due to a combination of atmospheric multipath, critical refraction, and receiver tracking errors. In this paper, we show that a similar bias exists in the CHAMP and SAC-C data and describe the characteristics of the bias based on over 6700 soundings from October 2001. Retrievals obtained using the recently introduced canonical transform method are shown to markedly reduce the refractivity bias; however, a significant bias still remains below 2 km altitude. To better understand the underlying causes of the bias, we perform an end-to-end simulation study that incorporates full-wave signal propagation and realistic receiver tracking effects using an ensemble of atmospheric profiles. We find that atmospheric ducting effects associated with the top of the planetary boundary layer (PBL) at 1-2 km altitude would cause retrieval errors at and below the PBL even in the absence of the receiver errors. Furthermore, current implementation of the receiver tracking algorithm based on an enhanced version of the phase-locked loop could introduce additional errors under the low signal-to-noise ratio conditions that are often encountered in the lower troposphere. The latter problem is expected to be resolved in the near future through the adoption of open-loop tracking and the removal of the navigation modulation from the GPS signal.INDEX TERMS: 3360

A global climatology of multiple tropopauses (MT) is discussed based on Global Positioning System (GPS) radio occultation (RO) data from the German CHAMP (CHAllenging Minisatellite Payload) and the US‐Argentinian SAC‐C (Satelite de Aplicaciones Cientificas‐C) satellite mission for the period May 2001–April 2005. In this study we present first detailed investigations about the geographical and temporal distribution of MT during different seasons. The thickness of the layer between the lowest (first) and highest (last) tropopause has a strong annual cycle. In the vicinity of the subtropical jet (STJ) stream region values vary between 4–5 km during winter and 2–3 km during summer, respectively, whereas higher differences were found on the northern hemisphere. It is shown that the occurrence distribution of MT is in good agreement with the mean climatological location of the STJ (30°–40°) on both hemispheres, in particular during winter time.