MOisture Transport pathways and Isotopologues in water Vapour (MOTIV)

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The coupling between atmospheric water and circulation via radiative effects, latent heat release and cloud feedback mechanisms is a main limitation in our current understanding of the climate system. A comparison of different models or model setups with different resolutions and parameterisations can give valuable insight into the underlying problems, but for critical model tests observational data are required. In this context tropospheric water vapour isotopologues can make a unique contribution. Their ratios provide information about the source conditions of atmospheric water and the involved transformation processes in the atmosphere and in clouds. There has recently been large progress in modelling and observing these ratios, such that a combined analysis is now feasible at high spatial and temporal resolution and on a global scale. The aim of this project is to establish tropospheric water vapour isotopologues as an observational tool for testing the model representation of atmospheric moisture pathways, thereby contributing to the aforementioned key challenges in climate research. To achieve statistical robustness, we will generate an unprecedented amount of free tropospheric {H2O, deltaD}-pairs (deltaD is the standardised ratio between HD16O and H216O). For the first time a validated observational dataset will be available that covers large areas, long time periods and include morning and evening measurements. At the same time, a high-resolution meteorological model with isotopologue representation will be used for analyzing moisture sources and pathways and their associated isotopologue signals. This combined observational-modeling approach will provide unique opportunities for model evaluation and for advancing the understanding of the involved processes. The potential of the isotopologues will be demonstrated in three different climatologically interesting regions. In Europe our approach will provide valuable insight into the key relationship between moisture sources and isotopologue signals in highly variable weather conditions. Over the subtropical North Atlantic the isotopologues will be used for tracing mixing between the marine boundary layer and the free troposphere, whose discrepant treatment in models is thought to be one important reason for a large cloud feedback uncertainty in climate models. Over West Africa the isotopologues serve to evaluate the model representation of the West African monsoon, particularly the associated horizontal moisture transport, terrestrial moisture recycling and the diurnal variations related to vertical mixing. Of particular interest will be the role of organised convection in influencing the monsoon circulation and the associated water pathways. Together these results will help to identify and better understand deficits in existing weather and climate models and provide a new framework to guide future model improvement.