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\begin{document}

\title{A Lagrangian perspective on stable water isotopes during the West
African Monsoon}



\author[1]{Christopher Johannes Diekmann}%
\author[1]{Matthias Schneider}%
\author[1]{Peter Knippertz}%
\author[2]{Andries Jan de Vries}%
\author[3]{Stephan Pfahl}%
\author[2]{Franziska Aemisegger}%
\author[2]{Fabienne Dahinden}%
\author[1]{Benjamin Ertl}%
\author[1]{Farahnaz Khosrawi}%
\author[4]{Heini Wernli}%
\author[5]{Peter Braesicke}%
\affil[1]{Karlsruhe Institute of Technology}%
\affil[2]{ETH Zürich}%
\affil[3]{Freie Universität Berlin}%
\affil[4]{ETH Zurich}%
\affil[5]{Karslruhe Institute for Technology}%


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  \date{May 28, 2024}


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\begin{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 \selectlanguage{greek}δ\selectlanguage{english}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, \selectlanguage{greek}δ\selectlanguage{english}D\} distributions. The use of idealized process curves in the
\{H2O, \selectlanguage{greek}δ\selectlanguage{english}D\} phase space allows us to attribute isotopic changes to
contributions from (1) air mass mixing, (2) Rayleigh condensation during
convection, and (3) microphysical processes depleting the vapor beyond
the Rayleigh prediction, i.e., partial rain evaporation in unsaturated
and isotopic equilibration \selectlanguage{greek}δ\selectlanguage{english}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, \selectlanguage{greek}δ\selectlanguage{english}D\} over
Africa and other regions in order to better quantify characteristics of
the hydrological cycle.%
\end{abstract}%



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