the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
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.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2024-1613', Anonymous Referee #1, 09 Sep 2024
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RC2: 'Comment on egusphere-2024-1613', Anonymous Referee #2, 23 Oct 2024
This manuscript investigates water vapor isotopes (H2O and HDO) over West Africa, focusing on the West African Monsoon (WAM). Using satellite data from IASI, AIRS, and TROPOMI, the study analyzes moisture pathways and isotopic variations during different monsoon phases. Key findings include an anti-correlation between H2O and δD over the Sahel during monsoon convection, driven by dry air intrusions, while the Guinea Coast shows moist, enriched air without significant δD depletion. The study highlights the value of paired isotopic observations in understanding tropical convection and moisture processes in the region. Overall, this manuscript is well written, and no additional observations or model simulations seem necessary. However, a few minor revisions or updates could help clarify the study and improve the understanding of WAM mechanisms.1. Monsoon retreat phaseThe monsoon consists of rainfall systems moving the convection area north and south. However, this study covers the monsoon onset but lacks analysis of the retreat (withdrawal) phase. There could be asymmetry due to land-sea contrasts and seasonal mean state differences.2. Figures showing contrastThroughout the figures (e.g., regarding non-rain & post-rain periods, Guinea Coast & Sahel), the differences are not clearly visible (except during the monsoon peak month of August) for readers to capture the changes in H2O and δD pairs. The authors may consider adding additional figures highlighting these differences to better illustrate which processes or regions are associated with enrichment or depletion as moisture changes.3. Satellite dataset uncertaintyThe authors mention the previously reported uncertainty in the three satellite datasets, but the reliability of using them specifically for the WAM region is not clearly addressed. Please quantify this uncertainty more explicitly, not only through percentage contours or whisker plots for individual datasets but also by considering the spread between the two datasets (IASI and AIRS). Additionally, the authors could emphasize the benefits of using water isotopes, for instance, by showing more distinct anomalies in δD compared to H2O in those datasets.Citation: https://doi.org/
10.5194/egusphere-2024-1613-RC2
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