Water Vapor DIAL in Space: Which Performance Should you Expect?

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. And recent studies have shown that the boundary layer moisture and isolated lofted humidity layers play a key role in the initiation of convection.  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 on a low Earth orbit. Thanks to the European spaceborne lidar missions Aeolus/2, EarthCARE, and MERLIN now the major building blocks for such a water vapor DIAL have reached the necessary technological readiness and the last open issue, a high-power laser source at 935 nm, is currently addressed by an ESA project.

A key tool to assess the impact of certain design decisions on the performance is a full end-to-end simulation tool. DLR has developed and kept up to date such a tool over the past years. In our presentation we will show the achievable resolution and precision of a spaceborne H2O-DIAL in dependence of key design parameters like number of wavelengths, laser power, telescope diameter and detector noise for several real-world atmospheric scenes that have been captured with our airborne demonstrator. Special focus will be given to non-standard profile situations where especially passive sounding systems have difficulties due to their limited vertical resolution. This presentation is thought as a starting point for further discussions with potential users of data from a space-borne H2O-DIAL to refine the observational requirements and adjust the lidar-parameters on the system level.