Hybrid methodology for optimised water vapour mixing ratio profiles from Raman lidar measurements

Abstract. This study presents a hybrid methodology to obtain high temporal resolution calibration constants for water vapour Raman lidar measurements, and posteriorly retrieve high-accuracy water vapour mixing ratio profiles. The hybrid method combines correlative measurements of collocated precipitable water vapour and Numerical Weather Prediction data to reconstruct the profile within the incomplete overlap region. The hybrid methodology is applied to the Raman lidar system, which operated at the EARLINET/ACTRIS station of the University of Granada, Spain, for the period 2009–2022. The system has been continuously updated to meet EARLINET/ACTRIS requirements for aerosol measurements, but the hybrid method has allowed tracking the impact of these changes on calibration constants for water vapour retrievals, and consequently to exploit water vapour mixing ratio profiles that were previously unavailable. The hybrid method was optimised for the Granada station by selecting Global Navigation Satellite System precipitable water vapour data as the most appropriate due to its better agreement with collocated and simultaneous radiosonde data (coefficient of determination of 0.95). Furthermore, the ERA5 reanalysis model was selected as the most appropriate because of its better temporal and spatial resolution and its accuracy when evaluated against radiosonde data. The advantages of the hybrid methodology were evaluated in comparison to traditional calibration methods such as those based on radiosondes or precipitable water vapour data assuming a constant water vapour mixing ratio in the incomplete overlap region. Although all methods generally provided good calibration constants, the hybrid method presented the best assessments under conditions where atmospheric layers were not well-mixed. Comparison with radiosonde data revealed excellent agreement, with a mean bias of −0.1 ± 0.3 g kg−1, a standard deviation of 1.0 ± 0.4 g kg−1 and a coefficient of determination of 0.87 across the entire period and vertical range (0–6 kma.g.l.). The most important result of this study is the ability to continuously evaluate calibration constants in a system that its configuration has been changing over 14 years of operation. This new methodology expanded the dataset from 31 initial cases using collocated radiosondes to more than 2000 values through the hybrid methodology. The posterior application of the hybrid methodology to all Raman lidar measurements enabled the generation of a comprehensive database of water vapour mixing ratio profiles for the entire period 2009–2022. Illustrative cases under different atmospheric conditions are presented to showcase the potential of Raman lidar measurements in monitoring water vapour and to investigate its role in climate dynamics and weather prediction.