Long-term evolution of the calibration constant on a mobile/field campaign water vapour Raman lidar

Abstract. Numerous field campaigns have been carried out to quantify the water vapour content of the atmosphere using vibrational Raman lidar technology. Each of them raises the question of calibration methods, in particular the reliability of this calibration over time. We present a study on the stability of the calibration of the WALI lidar developed at Laboratoire des Sciences du Climat et de l’Environnement in France (LSCE), over a period of 7 years (2016–2022) and across several field campaigns. A calibration method is applied that mainly use radiosondes and, in a few cases, airborne meteorological probes. Complementing the previous approaches, we show that ground-based meteorological measurements can be of great interest for lidar calibration under conditions of vertical stability in the lower troposphere and of good knowledge of the lidar overlap function, with full overlap within the planetary boundary layer. We emphasize that these three calibration approaches remain consistent over time. The observation periods considered here allow us to sample a wide range of water vapour contents in the lower troposphere, from 0.5 g kg^-1 to more than 10 g kg^-1 characteristic of the variabilities expected over the mid-latitudes and even over the Arctic. We observe a variability of more than 10 % in the calibration constant between field experiments conducted with and without laser injection seeding. The root mean square error is between 0.23 and 0.6 g kg^-1, mainly due to the atmospheric variability during the calibration. The bias is small, less than 0.08 g kg^-1. For all the situations studied, the correlation coefficient remains high, above 0.75. The instrumental error is comparable to the 0.4 g kg^-1 recommended by the World Meteorological Organization (WMO). Such a precision requires the use of a significant number of reference profiles and the remaining limitation is due to the uncertainties associated with in situ weather sensors. We note that the use of ground-based measurements does not introduce any more uncertainty in the lidar calibration coefficient than vertical profiles obtained by radiosondes or airborne means. Furthermore, the use of re-analyses can be an interesting option for calibration when there are no operational constraints.