Intercomparison of arctic XH 2 O observations from three ground-based Fourier transform infrared networks and application for satellite validation

Abstract. In this paper, we compare column-averaged dry-air mole fractions of water\nvapor ( XH2O ) retrievals from the COllaborative Carbon Column\nObserving Network (COCCON) with retrievals from two co-located high-resolution\nFourier transform infrared (FTIR) spectrometers as references at two boreal\nsites, Kiruna, Sweden, and Sodankyla, Finland, from 6 March 2017 to\n20 September 2019. In the framework of the Network for the Detection\nof Atmospheric Composition Change (NDACC), an FTIR spectrometer is operated at\nKiruna. The H2O product derived from these observations has been\ngenerated with the MUlti-platform remote Sensing of Isotopologues\nfor investigating the Cycle of Atmospheric water (MUSICA) processor. In\nSodankyla, a Total Carbon Column Observing Network (TCCON) spectrometer\nis operated, and the official XH2O data as provided by TCCON are used\nfor this study. The datasets are in good overall agreement, with COCCON data\nshowing a wet bias of ( 49.20±58.61 ) ppm (( 3.33±3.37 ) %,\n R2=0.9992 ) compared with MUSICA NDACC and ( 56.32±45.63 ) ppm\n(( 3.44±1.77 ) %, R2=0.9997 ) compared with TCCON. Furthermore, the a priori H2O volume mixing ratio (VMR) profiles (MAP) used as a priori information in the TCCON retrievals (also adopted for COCCON\nretrievals) are evaluated with respect to radiosonde (Vaisala RS41) profiles\nat Sodankyla. The MAP and radiosonde profiles show similar shapes and a\ngood linear correlation of integrated XH2O , indicating that MAP is a\nreasonable approximation of the true atmospheric state and an appropriate\nchoice for the scaling retrieval methods as applied by COCCON and TCCON.\nCOCCON shows a reduced dry bias ( −14.96  %) in comparison with TCCON ( −19.08  %) with respect to radiosonde XH2O . Finally, we investigate the quality of satellite data at high latitudes. For\nthis purpose, the COCCON XH2O is compared with retrievals from the\nInfrared Atmospheric Sounding Interferometer (IASI) generated with the\nMUSICA processor (MUSICA IASI) and with retrievals from the TROPOspheric\nMonitoring Instrument (TROPOMI). Both paired datasets generally show good\nagreement and similar correlations at the two sites. COCCON measures 4.64 % less XH2O at Kiruna and 3.36 % less at Sodankyla with respect to MUSICA IASI, whereas COCCON measures 9.71 % more XH2O at Kiruna and 7.75 % more at Sodankyla compared with TROPOMI. Our study supports the assumption that COCCON also delivers a\nwell-characterized XH2O data product. This emphasizes that this approach might complement the TCCON network with respect to satellite validation efforts. This is the first published study where COCCON XH2O has been compared with MUSICA NDACC and TCCON retrievals and has been used for MUSICA IASI and TROPOMI validation.