Study and mitigation of calibration factor instabilities in a water vapor Raman lidar

Abstract

Abstract. We have investigated calibration variations in the Rameau water vapor Raman lidar. This lidar system was developed by the Institut National de l'Information Géo-graphique et Forestière (IGN) together with the Laboratoire Atmosphères, Milieux, Observations Spatiales (LAT-MOS). It aims at calibrating Global Navigation Satellite System (GNSS) measurements for tropospheric wet delays and sounding the water vapor variability in the lower troposphere. The Rameau system demonstrated good capacity in retrieving water vapor mixing ratio (WVMR) profiles accurately in several campaigns. However, systematic shortterm and long-term variations in the lidar calibration factor pointed to persistent instabilities. A careful testing of each subsystem independently revealed that these instabilities are mainly induced by mode fluctuations in the optic fiber used to couple the telescope to the detection subsystem and by the spatial nonuniformity of the photomultiplier photocathodes. Laboratory tests that replicate and quantify these instability sources are presented. A redesign of the detection subsystem is presented, which, combined with careful alignment procedures, is shown to significantly reduce the instabilities. Outdoor measurements were performed over a period of 5 months to check the stability of the modified lidar system. The calibration changes in the detection subsystem were monitored with lidar profile measurements using a common nitrogen filter in both Raman channels. A short-term stability of 2-3 % and a long-term drift of 2-3 % per month are demonstrated. Compared to the earlier Development of Methodologies for Water Vapour Measurement (DEMEVAP) campaign, this is a 3-fold improvement in the long-term stability of the detection subsystem. The overall water vapor calibration factors were determined and monitored with capacitive humidity sensor measurements and with GPS zenith wet delay (ZWD) data. The changes in the water vapor calibration factors are shown to be fairly consistent with the changes in the nitrogen calibration factors. The nitrogen calibration results can be used to correct the overall calibration factors without the need for additional water vapor measurements to within 1 % per month.

Calibration of Raman lidar water vapor profiles by means of AERONET photometer observations and GDAS meteorological data

“…Polly XT is a multiple-wavelength polarization Raman lidar (Althausen et al, 2009;Engelmann et al, 2016). Three light pulses at wavelengths of 355, 532 and 1064 nm are emitted simultaneously and the elastically backscattered and Raman backscattered light is measured.…”

“…They utilized diffuse sunlight or a standard lamp as external sources to obtain the water vapor calibration constant. The development of all these methods is well discussed by Whiteman et al (2011) and David et al (2017). Leblanc and McDermid (2008) proposed a new hybrid method by combining dependent and independent methods to determine the calibration constant.…”

“…4 we present measurement ex-amples of WVMR and RH profiles and a comparison with accompanying radiosonde observations. (Engelmann et al, 2016). The PWV was measured at daytime by a sun photometer at the CUT-TEPAK site of AERONET.…”

Automated compact mobile Raman lidar for water vapor measurement: instrument description and validation by comparison with radiosonde, GNSS, and high-resolution objective analysis

“…We developed our MRL system to meet these requirements as much as possible within the total material cost of ∼ USD 250 000. The Raman lidar technique is a well-established technique for measuring the water vapor distribution in the troposphere (e.g., Melfi et al, 1969;Whiteman et al, 1992), and the systems have been in operation for decades at stations around the world (Turner et al, 2016;Dinoev et al, 2013;Reichardt et al, 2012;Leblanc et al, 2012). Field-deployable systems have also been developed by several institutes (Whiteman et al, 2012;Chazette et al, 2014;Engelmann et al, 2016).…”

Calibration of Raman lidar water vapor profiles by means of AERONET photometer observations and GDAS meteorological data

“…The development of all these methods are well discussed by Whiteman et al (2011) and David et al (2017). Leblanc and McDermid (2008) proposed a new hybrid method combining dependent and independent methods to determine the calibration constant.…”