Mesoscale modulation of marine boundary layer water vapor isotopologues during EUREC4A

Abstract. Shallow cumulus clouds in trade-wind regions remain a major source of uncertainty in climate projections, with conflicting hypotheses about how mesoscale circulations affect boundary layer moisture. We analyze water vapor isotopologue measurements from the EUREC⁴A campaign to quantify mesoscale modulation of marine boundary layer humidity and composition. Surface δD measurements from R/V Meteor show remarkably strong sensitivity to mesoscale vertical motions, responding 7.5 times more strongly than humidity when normalized by observed standard deviations. Mesoscale upward motion counteracts entrainment-driven isotopic depletion with an efficiency of 1.19, meaning 1 mm s^-1 of vertical velocity more than cancels 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 (SCL) top. A flux-form mixed-layer model reproduces these asymmetric responses, providing mechanistic understanding of how mesoscale circulations fundamentally modulate boundary layer moisture processes.