Abstract
AbstractWe 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 O and its isotopologue HDO (expressed as 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 {O, } distributions. The use of idealized process curves in the {O, } 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 {O, } over Africa and other regions in order to better quantify characteristics of the hydrological cycle.
Highlights
The meteorology and hydrology of West Africa is dominated by the complex WestAfrican Monsoon (WAM) system (Fink et al, 2017)
Several studies have emphasized the potential of the paired analysis of H2 O and δD, as this allows for evaluating effects of different moisture processes on tropospheric water vapor, such as air mass mixing (Noone et al, 2011; González et al, 2016; Lacour et al, 2017), condensation (Noone, 2012; Schneider et al, 2016), rain evaporation (Worden et al, 2007; Field et al, 2010), and deep convection (Bolot et al, 2013; Lacour et al, 2018)
The aim of our Lagrangian process attribution procedure is to provide a framework for interpreting the isotopic composition of tropospheric moisture in a chosen target region by means of individual moisture pathways
Summary
African Monsoon (WAM) system (Fink et al, 2017). The onset of the WAM is characterized by a shift of maximum rainfall from the Guinea Coast to the Sahel (Sultan & Janicot, 2003; Fitzpatrick et al, 2015), where the rainfall is crucial for the livelihoods of the local population in terms of water resources. Several studies have emphasized the potential of the paired analysis of H2 O and δD, as this allows for evaluating effects of different moisture processes on tropospheric water vapor, such as air mass mixing (Noone et al, 2011; González et al, 2016; Lacour et al., 2017), condensation (Noone, 2012; Schneider et al, 2016), rain evaporation (Worden et al, 2007; Field et al, 2010), and deep convection (Bolot et al, 2013; Lacour et al, 2018) In this context, Noone (2012) derived a theoretical framework for characterizing the vari-. We introduce the model data and trajectory tool that we use to calculate the backward trajectories for the Sahelian mid-troposphere during the WAM season 2016, and explain the process-attribution strategy
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