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. Understanding what controls the water vapor isotopic composition ofthe sub-cloud layer (SCL) over tropical oceans (δD0) isa first step towards understanding the water vapor isotopic compositioneverywhere in the troposphere. We propose an analytical model to predictδD0 motivated by the hypothesis that the altitude fromwhich the free tropospheric air originates (zorig) is an importantfactor: when the air mixing into the SCL is lower in altitude, itis generally moister, and thus it depletes the SCL more efficiently.We extend previous simple box models of the SCL by prescribing theshape of δD vertical profiles as a function of humidity profilesand by accounting for rain evaporation and horizontal advection effects.The model relies on the assumption that δD profiles are steeperthan mixing lines, and that the SCL is at steady state, restrictingits applications to timescales longer than daily. In the model, δD0is expressed as a function of zorig, humidity and temperatureprofiles, surface conditions, a parameter describing the steepnessof the δD vertical gradient, and a few parameters describingrain evaporation and horizontal advection effects. We show that δD0does not depend on the intensity of entrainment, in contrast to severalprevious studies that had hoped that δD0 measurementscould help estimate this quantity. Based on an isotope-enabled general circulation model simulation,we show that δD0 variations are mainly controlled by mid-troposphericdepletion and rain evaporation in ascending regions and by sea surfacetemperature and zorig in subsiding regions. In turn, could δD0measurements help estimate zorig and thus discriminate betweendifferent mixing processes? For such isotope-based estimates of zorigto be useful, we would need a precision of a few hundred meters indeep convective regions and smaller than 20 m in stratocumulus regions.To reach this target, we would need daily measurements of δDin the mid-troposphere and accurate measurements of δD0(accuracy down to 0.1 ‰ in the case of stratocumulus clouds,which is currently difficult to obtain). We would also need informationon the horizontal distribution of δD to account for horizontaladvection effects, and full δD profiles to quantify the uncertaintyassociated with the assumed shape for δD profiles. Finally,rain evaporation is an issue in all regimes, even in stratocumulusclouds. Innovative techniques would need to be developed to quantifythis effect from observations.

Controls on the water vapor isotopic composition near the surface of tropical oceans and role of boundary layer mixing processes