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Discussion papers
https://doi.org/10.5194/acp-2018-596
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-596
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 19 Jul 2018

Research article | 19 Jul 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Kinetic mass-transfer calculation of water isotope fractionation due to cloud microphysics in a regional meteorological model

I-Chun Tsai1, Wan-Yu Chen2,3, Jen-Ping Chen2,4, and Mao-Chang Liang5 I-Chun Tsai et al.
  • 1Research Center for Environmental Changes, Academia Sinica, Taipei, Taiwan R.O.C.
  • 2Department of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan, R.O.C.
  • 3Central Weather Bureau, Taipei, Taiwan, R.O.C.
  • 4International Degree Program on Climate Change and Sustainable Development, National Taiwan University, Taipei, Taiwan, R.O.C.
  • 5Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan R. O. C.

Abstract. In conventional atmospheric models, isotope exchange between liquid and gas phases is usually assumed to be in equilibrium, and the highly kinetic phase transformation processes inferred in clouds are yet to be fully investigated. In this study, a two-moment microphysical scheme in the NCAR Weather Research and Forecasting (WRF) model was modified to allow kinetic calculation of isotope fractionation due to various cloud microphysical phase-change processes. A case of moving cold front is selected for quantifying the effect of different factors controlling isotopic composition, including water vapor sources, atmospheric transport, phase transition pathways of water in clouds, and kinetic versus equilibrium mass transfer. A base-run simulation was able to reproduce the ~50‰ decrease in δD that observed during the frontal passage. Sensitivity tests suggest that all the above factors contributed significantly to the variations in isotope composition. The thermal equilibrium assumption commonly used in earlier studies may cause an overestimate of mean vapor-phase δD by 11‰, and the maximum difference can be more than 20‰. Without microphysical fractionation, the δD in water vapor can be off by about 25‰. Also, using initial vertical distribution and lower boundary conditions of water isotopes from satellite data are critical to successful isotope simulations, without which the δD in water vapor can be off by about 34 and 28‰, respectively.

I-Chun Tsai et al.
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I-Chun Tsai et al.
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Short summary
In conventional models, isotope exchange between liquid and gas phases is usually assumed to be in equilibrium, and the highly kinetic phase transformation processes inferred in clouds are yet to be fully investigated. We show that different factors controlling isotopic composition, including water vapor sources, atmospheric transport, phase transition pathways of water in clouds, and kinetic versus equilibrium mass transfer, contributed significantly to the variations in isotope composition.
In conventional models, isotope exchange between liquid and gas phases is usually assumed to be...
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