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

Risi, Camille; Galewsky, Joseph; Reverdin, Gilles; Brient, Florent

Understanding what controls the water vapor isotopic composition of the sub-cloud layer (SCL) over tropical oceans (inline-formulaδD0) is a first step towards understanding the water vapor isotopic composition everywhere in the troposphere. We propose an analytical model to predict inline-formulaδD0 motivated by the hypothesis that the altitude from which the free tropospheric air originates (inline-formulazorig) is an important factor: when the air mixing into the SCL is lower in altitude, it is generally moister, and thus it depletes the SCL more efficiently. We extend previous simple box models of the SCL by prescribing the shape of inline-formulaδD vertical profiles as a function of humidity profiles and by accounting for rain evaporation and horizontal advection effects. The model relies on the assumption that inline-formulaδD profiles are steeper than mixing lines, and that the SCL is at steady state, restricting its applications to timescales longer than daily. In the model, inline-formulaδD0 is expressed as a function of inline-formulazorig, humidity and temperature profiles, surface conditions, a parameter describing the steepness of the inline-formulaδD vertical gradient, and a few parameters describing rain evaporation and horizontal advection effects. We show that inline-formulaδD0 does not depend on the intensity of entrainment, in contrast to several previous studies that had hoped that inline-formulaδD0 measurements could help estimate this quantity.

Based on an isotope-enabled general circulation model simulation, we show that inline-formulaδD0 variations are mainly controlled by mid-tropospheric depletion and rain evaporation in ascending regions and by sea surface temperature and inline-formulazorig in subsiding regions. In turn, could inline-formulaδD0 measurements help estimate inline-formulazorig and thus discriminate between different mixing processes? For such isotope-based estimates of inline-formulazorig to be useful, we would need a precision of a few hundred meters in deep convective regions and smaller than 20 m in stratocumulus regions. To reach this target, we would need daily measurements of inline-formulaδD in the mid-troposphere and accurate measurements of inline-formulaδD0 (accuracy down to 0.1 ‰ in the case of stratocumulus clouds, which is currently difficult to obtain). We would also need information on the horizontal distribution of inline-formulaδD to account for horizontal advection effects, and full inline-formulaδD profiles to quantify the uncertainty associated with the assumed shape for inline-formulaδD profiles. Finally, rain evaporation is an issue in all regimes, even in stratocumulus clouds. Innovative techniques would need to be developed to quantify this effect from observations.



Risi, Camille / Galewsky, Joseph / Reverdin, Gilles / et al: Controls on the water vapor isotopic composition near the surface of tropical oceans and role of boundary layer mixing processes. 2019. Copernicus Publications.


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