Intercomparison of atmospheric water vapor soundings from the differential absorption lidar (DIAL) and the solar FTIR system on Mt. Zugspitze

摘要

We present an intercomparison of three years of measurements of integrated watervapor (IWV) performed by the mid-infrared solar FTIR (Fourier TransformInfra-Red) instrument on the summit ofMt. Zugspitze (2964 m a.s.l.) and by the nearby near-infrared differential absorptionlidar (DIAL) at the Schneefernerhaus research station (2675 m a.s.l.). Thesolar FTIR was shown to be one of the most accurate and precise IWVsounders in recent work (Sussmann et al., 2009) and is taken as the referencehere. By calculating the FTIR-DIAL correlation (22 min coincidence interval,15 min integration time) we derive an almost ideal slope of 0.996 (10), acorrelation coefficient of = 0.99, an IWV intercept of −0.039 (42) mm (−1.2 % ofthe mean), and a bias of −0.052 (26) mm (−1.6 % of the mean) from the scatter plot.By selecting a subset of coincidences with an optimum temporal and spatial matchingbetween DIAL and FTIR, we obtain a conservative estimate of the precision of theDIAL in measuring IWV which is better than 0.1 mm (3.2 % of the mean). We foundthat for a temporal coincidence interval of 22 min the difference in IWVmeasured by these two systems is dominated by the volume mismatch (horizontaldistance: 680 m). The outcome from this paper is twofold: (1) the IWV soundingsby FTIR and DIAL agree very well in spite of the differing wavelength regionswith different spectroscopic line parameters and retrieval algorithms used. (2) In order to derive an estimate of the precision of state-of-the-art IWV soundersfrom intercomparison experiments, it is necessary to use a temporal matching ontime scales shorter than 10 min and a spatial matching on the 100-m scale.