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Observation of the atmospheric climate and the detection of changes require high quality data. Radio Occultation (RO), based on signals from Global Positioning System (GPS) satellites, provides a new climate record with high quality and vertical resolution in the upper troposphere and lower stratosphere (UTLS). RO data are considered a climate benchmark data type since they are based on timing with precise atomic clocks, tied to the international definition of the second. Long-term stability and the consistency of RO data stemming from different satellites (without need for inter-calibration) make RO well suited for climate applications. First RO data exist from the GPS/Met proof-of-concept mission intermittently in the years 1995 to 1997. Continuous RO observations are available from the CHAMP satellite from September 2001 to September 2008, complemented and continued by SAC-C, GRACE, Formosat-3/COSMIC, and MetOp/GRAS data.
We discuss the utility of RO-based climatologies for observing the atmosphere and for providing climate change indicators, i.e., parameters and regions reacting most sensitive to climate change. Results reveal that RO accessible parameters cover the whole UTLS as useful climate indicators, being most robust in the tropics. Refractivity is most sensitive in the LS, pressure near the tropopause, and temperature in the UT and LS.
We present results of a climate change detection study based on RO data revealing significant climate trends relative to natural variability in the UTLS region within 9?25 km (300?30 hPa). While an emerging warming trend in the tropical UT is obscured by El Niñ�o variability, a significant cooling trend is revealed in the LS in February 1997 to 2009. A consistent trend signal is also detected in refractivity. The results are in agreement with trends in radiosonde records though those trends are not significant themselves due to less stable error characteristics. Climate model trends basically agree as well but they show less warming/cooling contrast across the tropical tropopause. Beside comparison to conventional upper air satellite data, intercomparison of RO data from different processing centers confirms the consistency of RO data products fulfilling the needs of climate monitoring.
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