The dynamics of ice clouds in the TTL as inferred from the isotopic composition of water vapor.

Abstract:

We show that the profile of the isotopic composition of water vapor can be used to quantify the contribution of deep convection to water vapor in the UT/LS region. The contribution of UT/LS cirrus to radiation balance means that the control of moisture to the TTL is of particular importance. The various processes affecting UT/LS water include convective sources of water (injection of convective ice and subsaturated convective air), dehydration via in situ cirrus formation and sedimentation, and moistening from mixing with extratropical air, but none have been quantified with certainty. We show that these processes can be combined into a one-dimensional advection-dilution model for the isotopic ratio of TTL water vapor, with sources and sinks due to convective moistening and in situ cirrus formation, most of whose parameters can be described from observations and reanalysis. This model can then be fit to an averaged tropical profile of water vapor isotopic ratio in the TTL. The result allows us to back out the convective contribution to UT/LS water vapor. That water vapor contributes directly to in-situ cirrus formation because any injection of water vapor into the subsaturated UT/LS must subsequently be removed by in-situ dehydration. Using isotopic profiles from the ACE-FTS solar-occultation instrument and assuming timescales for evaporation and removal by sedimentation, we demonstrate that convective injection of water vapor significantly increases the production of in-situ cirrus over expectations from large-scale uplift alone. We also show that the turnaround point of UT/LS isotopic profiles – the altitude where isotopic composition begins to increase with height – is readily explained as reflecting the bulk isotopic composition in overshooting convection.