A Lagrangian Perspective on Stable Water Isotopes During the West African Monsoon

Abstract

We present a Lagrangian framework for identifying mechanisms that control the isotopic composition of mid-tropospheric water vapor in the Sahel region during the West African Monsoon 2016. In this region mixing between contrasting air masses, strong convective activity, as well as surface and rain evaporation lead to high variability in the distribution of stable water isotopologues. Using backward trajectories based on high-resolution isotope-enabled model data, we obtain information not only about the source regions of Sahelian air masses, but also about the evolution of H2O and its isotopologue HDO (expressed as delta D) along the pathways of individual air parcels. We sort the full trajectory ensemble into groups with similar transport pathways and hydro-meteorological properties, such as precipitation and relative humidity, and investigate the evolution of the corresponding paired {H2O, delta D} distributions. The use of idealized process curves in the {H2O, delta D} phase space allows us to attribute isotopic changes to contributions from (a) air mass mixing, (b) Rayleigh condensation during convection, and (c) microphysical processes depleting the vapor beyond the Rayleigh prediction, i.e., partial rain evaporation in unsaturated and isotopic equilibration in saturated conditions. Different combinations of these processes along the trajectory ensembles are found to determine the final isotopic composition in the Sahelian troposphere during the monsoon. The presented Lagrangian framework is a powerful tool for interpreting tropospheric water vapor distributions. In the future, it will be applied to satellite observations of {H2O, delta D} over Africa and other regions in order to better quantify characteristics of the hydrological cycle.