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.
Galewsky et al. (Tue,) studied this question.
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