A long-term data set for monitoring atmospheric water vapour using lidar techniques requires the calibration of Raman lidar water vapour profiles that vary randomly around some mean value (often addressed as a calibration constant that depends only on the instrument setup) and does not involve step jumps of unknown magnitude. Such step jumps in calibration increase the time required to detect atmospheric trends, which is already typically measured in decades [Weatherhead et. al., 1998; Boers and Meijgaard, 2009]. For this reason, it is important to carefully examine any calibration technique developed for ensuring stable and long-term calibrations. Absolute and relative, but also hybrid calibration methods have been developed. More recently, reference calibration lamps, which are traceable to NMIs standards, have proven to be robust for absolute calibration of water vapour Raman lidar to reduce systematic uncertainties and may represent a common reference for all the available systems.

Another challenge for Raman lidars to ensure the collection of water vapour long-term measurements for climate applications is to improve their daytime observing capability. Raman lidars have been shown to provide high resolution water vapour measurements in several experiments, but these measurements are typically restricted to night-time only, as Raman scattering is a weak physical process and the high solar background radiation during the day tends to mask these signals. During daytime, a few water vapour Raman lidars have already proven to be able to measure water vapour up to 3-4 km above ground level. Only DIAL systems can do better, but they do worse in the UT/LS at night compared to Raman lidar. Most of the water vapour Raman lidar systems are not operated during daytime and this generates a discontinuity in the water vapour monitoring in the troposphere in a climatological sense. The use of commercial systems, Raman lidar or DIAL, designed to operate on a continuous basis, can mitigate the gap but with moderate to high costs, though their performance needs to be carefully assessed in advance. Further technological improvements of lidar techniques for measuring water vapour are also expected but over the mid and long term.

In addition, the improvement of synergy of water vapour Raman lidar with other measurement techniques represents an alternative solution upon which to invest. For example, the ACTRIS-2 and HD(CP) projects are working on this aspect to provide users with a synergetic lidar-