We present an intercomparison of three years of measurements of<br />integrated water vapor (IWV) performed by the mid-infrared solar FTIR<br />(Fourier Transform Infra-Red) instrument on the summit of Mt. Zugspitze<br />(2964 ma.s.l.) and by the nearby near-infrared differential absorption<br />lidar (DIAL) at the Schneefernerhaus research station (2675 ma.s.l.).<br />The solar FTIR was shown to be one of the most accurate and precise IWV<br />sounders in recent work (Sussmann et al., 2009) and is taken as the<br />reference here. By calculating the FTIR-DIAL correlation (22 min<br />coincidence interval, 15 min integration time) we derive an almost ideal<br />slope of 0.996 (10), a correlation coefficient of R = 0.99, an IWV<br />intercept of -0.039 (42) mm (-1.2% of the mean), and a bias of -0.052<br />(26) mm (-1.6% of the mean) from the scatter plot. By selecting a<br />subset of coincidences with an optimum temporal and spatial matching<br />between DIAL and FTIR, we obtain a conservative estimate of the<br />precision of the DIAL in measuring IWV which is better than 0.1mm (3.2%<br />of the mean). We found that for a temporal coincidence interval of 22<br />min the difference in IWV measured by these two systems is dominated by<br />the volume mismatch (horizontal distance: 680 m). The outcome from this<br />paper is twofold: (1) the IWV soundings by FTIR and DIAL agree very well<br />in spite of the differing wavelength regions with different<br />spectroscopic line parameters and retrieval algorithms used. (2) In<br />order to derive an estimate of the precision of state-of-the-art IWV<br />sounders from intercomparison experiments, it is necessary to use a<br />temporal matching on time scales shorter than 10 min and a spatial<br />matching on the 100-m scale.
Vogelmann, H., Sussmann, R., Trickl, T., & Borsdorff, T. (2011). Intercomparison of atmospheric water vapor soundings from the differential absorption lidar (DIAL) and the solar FTIR system on Mt. Zugspitze. Atmospheric Measurement Techniques, 4(5), 835–841. https://doi.org/10.5194/amt-4-835-2011