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Observations from the GPS radio occultation (GPSRO) satellite technique  and from the newly established GCOS Reference Upper Air Network (GRUAN)  are both candidates to serve as reference observations in the Global  Climate Observing System (GCOS). Such reference observations are key to  decrease existing uncertainties in upper-air climate research. There are  now more than 12 years of data available from GPSRO, with the  recognized properties high accuracy, global coverage, high vertical  resolution, and long-term stability. These properties make GPSRO  a suitable choice for comparison studies with other upper-air  observational systems. The GRUAN network consists of reference radiosonde  ground stations (16 at present), which adhere to the GCOS climate  monitoring principles. In this study, we intercompare GPSRO temperature  and humidity profiles and Vaisala RS90/92 data from the "standard"  global radiosonde network over the whole 2002 to 2013 time  frame. Additionally, we include the first years of GRUAN data (using  Vaisala RS92), available since 2009. GPSRO profiles which occur within  3 h and 300 km of radiosonde launches are used. Overall  very good agreement is found between all three data sets with temperature  differences usually less than 0.2 K. In the stratosphere above  30 hPa, temperature differences are larger but still within  0.5 K. Day/night comparisons with GRUAN data reveal small  deviations likely related to a warm bias of the radiosonde data at high  altitudes, but also residual errors from the GPSRO retrieval process  might play a role. Vaisala RS90/92 specific humidity exhibits a dry bias  of up to 40% in the upper troposphere, with a smaller bias at  lower altitudes within 15%. GRUAN shows a marked improvement in  the bias characteristics, with less than 5% difference to GPSRO,  up to 300 hPa. GPSRO dry temperature and physical temperature are  validated using radiosonde data as reference. We find that GPSRO provides  valuable long-term stable reference observations with well-defined error  characteristics for climate applications and for anchoring other  upper-air measurements

Climate intercomparison of GPS radio occultation, RS90/92 radiosondes and GRUAN from 2002 to 2013