Long-term evolution of the calibration constant on a mobile 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 (Water vapour and Aerosol Lidar, now renamed Weather and Aerosol LIdar) lidar developed at Laboratoire des Sciences du Climat et de l’Environnement (LSCE) in France, over a period of 7 years (2016–2022) and across several field campaigns. A calibration method is applied mainly using radiosondes and, in a few cases, airborne meteorological probes. Complementing the previous approaches, we show that ground-based meteorological measurements can be of great value for lidar calibration under conditions of vertical stability in the lower troposphere and provide good knowledge of the lidar overlap function, with full overlap within the planetary boundary layer. Using statistical criteria, we emphasize that these three calibration approaches should 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. From comparisons between lidar and in situ measurements (radiosondes and/or flights), 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 between the lidar and in situ reference measurements 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 reanalyses can be an interesting option for calibration if the lidar profiles are not used in the models themselves, e.g. by means of assimilation.