A Calibration of the MeteoSwiss RAman Lidar for Meteorological Observations (RALMO)Water Vapour Mixing Ratio Measurements using a Radiosonde Trajectory Method

With only 50% downtime from 2007-2016, the RALMO lidar in Payerne, Switzerland, has one of the largest continuous lidar data sets available. These measurements will be used to produce an extensive lidar water vapour climatology using the Optimal Estimation Method introduced by Sica and Haefele (2016). We will compare our improved technique for external calibration using radiosonde trajectories with the standard external methods, and present the evolution of the lidar constant from 2007 to 2016.

Application of the lamp mapping technique for overlap function for Raman lidar systems

Traditionally, the lidar water vapor mixing ratio (WVMR) is corrected for overlap using data from another instrument, such as a radiosonde. Here we introduce a new experimental method to determine the overlap function using the lamp mapping technique (LMT), which relies on the lidar optics and detection system. The LMT discussed here involves a standard halogen lamp being scanned over the aperture of a Raman lidar telescope in synchronization with the lidar detection system [Appl. Opt.50, 4622 (2011)APOPAI0003-693510.1364/AO.50.004622, Appl. Opt.53, 8538 (2014)APOPAI0003-693510.1364/AO.53.008535]. In this paper, we show results for a LMT-determined overlap function for individual channels, as well as a WVMR overlap function. We found that the LMT-determined WVMR overlap functions deviate within 5% of the traditional radiosonde-determined overlap.

Assessing the temperature dependence of narrow-band Raman water vapor lidar measurements: a practical approach

Narrow-band detection of the Raman water vapor spectrum using the lidar technique introduces a concern over the temperature dependence of the Raman spectrum. Various groups have addressed this issue either by trying to minimize the temperature dependence to the point where it can be ignored or by correcting for whatever degree of temperature dependence exists. The traditional technique for performing either of these entails accurately measuring both the laser output wavelength and the water vapor spectral passband with combined uncertainty of approximately 0.01 nm. However, uncertainty in interference filter center wavelengths and laser output wavelengths can be this large or larger. These combined uncertainties translate into uncertainties in the magnitude of the temperature dependence of the Raman lidar water vapor measurement of 3% or more. We present here an alternate approach for accurately determining the temperature dependence of the Raman lidar water vapor measurement. This alternate approach entails acquiring sequential atmospheric profiles using the lidar while scanning the channel passband across portions of the Raman water vapor Q-branch. This scanning is accomplished either by tilt-tuning an interference filter or by scanning the output of a spectrometer. Through this process a peak in the transmitted intensity can be discerned in a manner that defines the spectral location of the channel passband with respect to the laser output wavelength to much higher accuracy than that achieved with standard laboratory techniques. Given the peak of the water vapor signal intensity curve, determined using the techniques described here, and an approximate knowledge of atmospheric temperature, the temperature dependence of a given Raman lidar profile can be determined with accuracy of 0.5% or better. A Mathematica notebook that demonstrates the calculations used here is available from the lead author.

Comments on "Accuracy of Raman lidar water vapor calibration and its applicability to long-term measurements"

In a recent publication, Leblanc and McDermid [Appl. Opt., 47, 5592 (2008)]APOPAI0003-693510.1364/AO.47.005592 proposed a hybrid calibration technique for Raman water vapor lidar involving a tungsten lamp and radiosondes. Measurements made with the lidar telescope viewing the calibration lamp were used to stabilize the lidar calibration determined by comparison with radiosonde. The technique provided a significantly more stable calibration constant than radiosondes used alone. The technique involves the use of a calibration lamp in a fixed position in front of the lidar receiver aperture. We examine this configuration and find that such a configuration likely does not properly sample the full lidar system optical efficiency. While the technique is a useful addition to the use of radiosondes alone for lidar calibration, it is important to understand the scenarios under which it will not provide an accurate quantification of system optical efficiency changes. We offer examples of these scenarios. Scanning of the full telescope aperture with the calibration lamp can circumvent most of these limitations. Based on the work done to date, it seems likely that the use of multiple calibration lamps in different fixed positions in front of the telescope may provide sufficient redundancy for long-term calibration needs. Further full-aperture scanning experiments, performed over an extended period of time, are needed to determine a "best practice" for the use of multiple calibration lamps in the hybrid technique.