Mesoscale modulation of marine boundary layer water vapor isotopologues during EUREC ⁴ A

Shallow cumulus clouds in trade-wind regions remain an important source of uncertainty in climate projections, with competing interpretations of how mesoscale circulations influence boundary layer moisture. We analyze water vapor isotopologue measurements from the EUREC4A campaign to quantify the mesoscale modulation of marine boundary layer humidity and isotopic composition. Surface δD measurements from R/V Meteor show a strong sensitivity to mesoscale vertical velocity relative to humidity. Regression analyses show that mesoscale ascent partially offsets entrainment-driven isotopic depletion: approximately 0.7 mm s−1 of upward motion is required to counteract the isotopic effect of 1 mm s−1 of entrainment. The strongest correlations between vertical velocity and both δD (r≈0.52) and mixing ratio (r≈0.39) occur within ±200 m of the subcloud layer top. A steady-state flux-form mixed-layer model shows that modest below-cloud rain evaporation can reproduce the observed asymmetric response by adding both vapor mass and isotopic mass with an isotopic composition distinct from the ambient subcloud layer. In the model, an ascent-dependent drizzle source rotates the fitted δD contours relative to humidity, whereas a no-rain control collapses this separation. These results support below-cloud rain evaporation as a physically consistent mechanism through which mesoscale circulations can reorganize moisture pathways without producing commensurate changes in total water vapor.