Accurate continuous measurements of relative humidity (RH) vertical profiles in the lower troposphere have become a significant scientific challenge. In recent years a synergy of various ground-based remote sensing instruments have been successfully used for RH vertical profiling, which has resulted in the improvement of spatial resolution and, in some cases, of the accuracy of the measurement. Some studies have also suggested the use of high-resolution model simulations as input datasets into RH vertical profiling techniques. In this paper we apply two synergetic methods for RH profiling, including the synergy of lidar with a microwave radiometer and high-resolution atmospheric modeling. The two methods are employed for RH retrieval between 100 and 6000 m with increased spatial resolution, based on datasets from the HygrA-CD (Hygroscopic Aerosols to Cloud Droplets) campaign conducted in Athens, Greece from May to June 2014. RH profiles from synergetic methods are then compared with those retrieved using single instruments or as simulated by high-resolution models. Our proposed technique for RH profiling provides improved statistical agreement with reference to radiosoundings by 27 % when the lidar–radiometer (in comparison with radiometer measurements) approach is used and by 15 % when a lidar model is used (in comparison with WRF-model simulations). Mean uncertainty of RH due to temperature bias in RH profiling was  ∼ 4.34 % for the lidar–radiometer and  ∼ 1.22 % for the lidar–model methods. However, maximum uncertainty in RH retrievals due to temperature bias showed that lidar-model method is more reliable at heights greater than 2000 m. Overall, our results have demonstrated the capability of both combined methods for daytime measurements in heights between 100 and 6000 m when lidar–radiometer or lidar–WRF combined datasets are available.

The authors thank the ITaRS program for generous support. The research leading to this article has received funding from the European Community’s FP7 – PEOPLE 2011 under grant agreement number 289923 – ITARS (Initial Training for Atmospheric Remote Sensing) and has also been supported by the National Core Program – PN 16.40.01.01/2017 and ROSA – STAR project CARESSE. All WRF simulations were performed on the MareNostrum supercomputer hosted by the Barcelona Supercomputing Center (BSC). We acknowledge the project CGL2013- 46736-R, and the Severo Ochoa Programme awarded by the Spanish Government (SEV-2011-00067) and 2014 SGR 522. Additionally, the research leading to these results has received funding from the European Union’s Horizon 2020 Research and Innovation programme under grant agreement no. 602014, project ECARS (East European Centre for Atmospheric Remote Sensing). The Hellenic National Meteorological Service (HNMS) is acknowledged for performing the high-resolution radiosoundings. We acknowledge the team from the University of Cologne for the microwave radiometer calibration and installation the HygrA-CD campaign. The financial support of the ACTRIS Research Infrastructure Project by the European Union’s Horizon 2020 – Research and Innovation Framework Programme, under grant agreement no. 654169 is gratefully acknowledged. The topical editor, Vassiliki Kotroni, thanks two anonymous referees for help in evaluating this paper.

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