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Technical Note: Harmonized retrieval of column-integrated atmospheric water vapor from the FTIR network – first examples for long-term records and station trends

by R. Sussmann, T. Borsdorff, M. Rettinger, C. Camy-Peyret, P. Demoulin, P. Duchatelet, E. Mahieu, C. Servais
Atmospheric Chemistry and Physics ()

Abstract

We present a method for harmonized retrieval of integrated water vapor\n(IWV) from existing, long-term, measurement records at the ground-based\nmid-infrared solar FTIR spectrometry stations of the Network for the\nDetection of Atmospheric Composition Change (NDACC). Correlation of IWV\nfrom FTIR with radiosondes shows an ideal slope of 1.00(3). This optimum\nmatching is achieved via tuning one FTIR retrieval parameter, i.e., the\nstrength of a Tikhonov regularization constraining the derivative (with\nrespect to height) of retrieved water profiles given in per cent\ndifference relative to an a priori profile. All other FTIR-sonde\ncorrelation parameters (intercept=0.02(12) mm, bias=0.02(5) mm, standard\ndeviation of coincident IWV differences (stdv)=0.27 mm, R=0.99) are\ncomparable to or better than results for all other ground-based IWV\nsounding techniques given in the literature. An FTIR-FTIR side-by-side\nintercomparison reveals a strong exponential increase in stdv as a\nfunction of increasing temporal mismatch starting at delta t1 min. This\nis due to atmospheric water vapor variability. Based on this result we\nderive an upper limit for the precision of the FTIR IWV retrieval for\nthe smallest delta t(=3.75 min) still giving a statistically sufficient\nsample (32 coincidences), i.e., precision(IWV(FTIR))< 0.05 mm (or 2.2%\nof the mean IWV). The bias of the IWV retrievals from the two different\nFTIR instruments is nearly negligible (0.02(1) mm). The optimized FTIR\nIWV retrieval is set up in the standard NDACC algorithm SFIT 2 without\nchanges to the code. A concept for harmonized transfer of the retrieval\nbetween different stations deals with all relevant control parameters;\nit includes correction for differing spectral point spacings (via\nregularization strength), and final quality selection of the retrievals\n(excluding the highest residuals (measurement minus model), 5% of the\ntotal).\nAs first application examples long-term IWV data sets are retrieved from\nthe FTIR records of the Zugspitze (47.4 degrees N, 11.0 degrees E, 2964\nm a.s.l.) and Jungfraujoch (46.5 degrees N, 8.0 degrees E, 3580 m\na.s.l.) NDACC sites. Station-trend analysis comprises a linear fit after\nsubtracting an intra-annual model (3 Fourier components) and\nconstructing an uncertainty interval {[}95% confidence] via bootstrap\nresampling. For the Zugspitze a significant trend of 0.79 {[}0.65, 0.92]\nmm/decade is found for the time interval {[}1996-2008], whereas for the\nJungfraujoch no significant trend is found. This confirms recent\nfindings that strong variations of IWV trends do occur above land on the\nlocal to regional scale (250 km) in spite of homogeneous surface\ntemperature trends. This paper provides a basis for future exploitation\nof more than a dozen existing, multi-decadal FTIR measurement records\naround the globe for climate studies.

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