Water vapour isotopes over West Africa as observed from space: which processes control tropospheric H₂O/HDO pair distributions?

Abstract. The West African Monsoon (WAM) represents the main source of rainfall over West Africa and thus has important socio-economic impacts. However, the complex interactions of the large-scale circulation, convective dynamics and microphysical processes pose a substantial challenge to reliably quantify the atmospheric branches of the hydrological cycle in observations and models.

Making use of recent advances in retrieving the isotopic composition of tropospheric water vapour from space, we promote the paired analysis of H₂O and HDO to investigate the moisture pathways and processes associated with the WAM. Data from the state-of-the-art satellite sensors IASI, AIRS and TROPOMI, together with the multi-satellite IMERG precipitation product, serve to characterize the variability of H₂O and HDO (with their ratio product δD) over West Africa from a convective as well as seasonal perspective and with respect to impacts from dynamical and microphysical processes. In particular, we find: (1) Monsoon convection over the Sahel leads to a marked anti-correlation between increasing H₂O and decreasing δD in the free troposphere. This is due to strong dry air intrusions from the Saharan upper troposphere that feed into Sahelian squall line systems, foster rain evaporation and, hence, lead to δD depletion; (2) Over the Guinea Coast, convective precipitation is associated with overall moist and enriched signals without showing significant δD depletion. Here, surface evaporation from the Tropical Atlantic moistens the troposphere, reducing the efficiency of the rain evaporation and the corresponding δD depletion; (3) During the Sahelian monsoon peak, an anti-correlation between increasing precipitation amount and decreasing δD becomes apparent. Thus, this provides observational evidence for the amount effect in tropospheric water vapour, similar to what is known for the isotopic composition in precipitation; (4) When no considerable precipitation occurs, e.g. during the Sahelian winter, the {H₂O, δD} signals point to dry air mass mixing from large-scale circulation.

This study demonstrates that different microphysical and dynamical processes occurring over West Africa leave distinct features in the isotopic composition of water vapour, and, hence, underlines the overall value of using paired H₂O and δD observations from space to study effects of processes that control tropical convection.