Spaceborne Water Vapor DIAL: Has the time come now?

Water vapor is the key trace gas component of the air and involved in virtually all relevant atmospheric processes. To know the vertical profile with decent resolution is crucial in all cases. For example, there are several regions of the atmosphere where numerical weather prediction models show biases which are not understood. So, after aerosol/cloud and wind lidars have been very successfully applied within space missions, the natural next step would be the profiling of water vapor by a Differential Absorption Lidar (DIAL) from a satellite in a low Earth orbit. About 20 years ago the ESA EarthExplorer Proposal WALES went through phase A, but was not further selected due to the identified technological risks and the corresponding financial efforts. Thanks to the European spaceborne lidar missions Aeolus/2, EarthCare, and MERLIN now the major building blocks for a such water vapor DIAL have reached the necessary technological readiness to realize such a program within the financial limits of a typical Earth observation mission. We will review the benefits of water vapor profiling by lidar as compared to passive sensors for different applications and then present an updated system design based on the current European space lidar component pool. Finally, results from end-to-end performance simulations will be presented. This presentation is thought as an invitation to the community to think about possible applications of space-borne H2O-lidar data and the corresponding observational requirements.