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Abstract Cloud condensation and hydrometeor evaporation fractionate stable isotopes of water, enriching liquid with heavy isotopes; whereupon updrafts, downdrafts, and rain vertically redistribute water and its isotopes in the lower troposphere. These vertical water fluxes through the marine boundary layer affect low cloud climate feedback and, combined with isotope fractionation, are hypothesized to explain the depletion of tropical precipitation at higher precipitation rates known as the “amount effect.” Here, an efficient and numerically stable quasi‐analytical model simulates the evaporation of raindrops and enrichment of their isotope composition. It is applied to a drop size distribution and subcloud environment representative of Atlantic trade cumulus clouds. Idealized physics experiments artificially zero out selected processes to discern the separate effects on the isotope ratio of raindrops, of exchange with the environment, evaporation, and kinetic molecular diffusion. A parameterization of size‐dependent molecular and eddy diffusion is formulated that enriches raindrops much more strongly (+5‰ for deuterated water [HDO] and +3.5‰ for O) than equilibrium evaporation as they become smaller than 1 mm. The effect on evaporated vapor is also assessed. Rain evaporation enriches subcloud vapor by +12‰ per mm rain (for HDO), explaining observations of enriched vapor in cold pools sourced by evaporatively cooled downdrafts. Drops smaller than 0.5 mm evaporate completely before falling 700 m in typical subtropical marine boundary layer conditions. The early and complete evaporation of these smaller drops in the rain size distribution enriches the vapor produced by rain evaporation.
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Isotopic Equilibration in Convective Downdrafts
Abstract While it is becoming increasingly clear that stable water isotopologues are important tools in the study of atmospheric convection, many questions remain on how different processes affect them. This work is focused on water vapor isotopes in precipitation‐driven downdrafts. Using idealized simulations, the contributions of rain evaporation and liquid‐vapor equilibration processes are analyzed. It is shown that rain evaporation tends to deplete water vapor isotopes throughout the downdraft's descent, whereas equilibration has a neutral or depleting effect in the first half of the downdraft's life cycle and an enriching one in the second half. The total contribution of the two processes is then discussed, and it is argued that equilibration has an overall small effect compared to rain evaporation. Finally, using two sensitivity experiments, it is shown that the role played by equilibration varies significantly in regimes with different concentrations of cloud condensation nuclei.
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- Award ID(s):
- 1945972
- PAR ID:
- 10372526
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 49
- Issue:
- 15
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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