Differential absorption lidar for monitoring water vapour isotope HDO in the lower troposphere – WaVIL

The overarching objective of the proposal is to develop a compact, transportable differential absorption lidar for measuring concentration of water vapor isotope HDO, at high spatio-temporal resolution in the lower troposphere with an unprecedented accuracy. The Water Vapor Isotope Lidar is a unique, innovative remote sensing instrument that will for the first time ever allow monitoring water vapor and HDO isotopic abundance profiles, to advance knowledge on the water cycle at scales relevant for meteorological and climate studies.

The proposed Water Vapor Isotope Lidar WaVIL will enable measuring water vapor mixing ratio, HDO mixing ratio and the relative abundance of HDO (designated in the following by dD) from the Earth surface, while pointing to the zenith. The system will be designed to allow operation in various climatic regions, i.e. from the Tropics to the Polar Regions. We are aiming at performing dD measurements with an absolute error less than 6‰ in the tropical and mid-latitude PBLs and less than 12‰ in the polar PBL. The vertical and temporal resolution will be on the order of 100 m and 10 minutes. The measurement of dD in the free troposphere (and below 3-4 km) will be made with an absolute error less than 20‰ with the same vertical and temporal resolution. The expected precisions are on the order of or better than the columnar measurements obtained with other remote sensing techniques BUT with a much greater resolution on the vertical. Furthermore, the WaVIL system will make water vapor mixing ratio measurements with a relative statistical error less than 1% below 3 km (less than 5‰ in the PBL). At mid-latitudes, this translates into an error less than 0.1 g kg-1 (less than 160 ppmv). The stringent precision of 1% imposed for H2O measurements in the lower troposphere will pave the way toward the detection of other climate-relevant greenhouse gases (CH4, CO2).

WaVIL accurate and high-resolution isotope observations will enhance our comprehension of the water vapor budget (segregation of natural and anthropogenic sources) as well as the life cycle of shallow low level clouds in a variety of climatic region (Tropics to the Poles). This will be done by enabling an unambiguous determination of the origin of the air masses associated with each of the processes contributing to the maintenance or the demise of clouds during their life cycle. Long time-series of accurate highly-vertically-resolved profiles of dD measured by the WaVIL lidar will be of paramount importance to address this challenge in synergy with models. They will contribute to improve the explicit and/or sub-grid representation of these processes in mesoscale, regional and climate models.

Furthermore, observations made with the WaVIL lidar system will help advance the interpretation of current and future remote sensing measurements of isotopes from space, as well as reduce the biases and uncertainties associated with these products. Reliable space-borne retrievals of isotope abundance are of sheer value to analyze the impact of the changing climate on the water cycle.

The novel combination of high resolution H20/HDO observations obtained by lidar over a significant depth of the troposphere will contribute to advance knowledge on processes such as evaporation at the surface, evapotranspiration by ecosystems, vertical and lateral mixing in the atmosphere, condensation during cloud formation and water phase changes, processes that are currently very difficult to observe in the troposphere by means of in situ instruments.