Abstract
Abstract. In conventional atmospheric models, isotope exchange between
liquid, gas, and solid 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 National Center for Atmospheric Research (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 a 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 was 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 ‰. Using initial vertical distribution and lower boundary
conditions of water stable isotopes from satellite data is critical to
obtain successful isotope simulations, without which the δD in water
vapor can be off by about 34 ‰ and 28 ‰, respectively.
Without microphysical fractionation, the δD in water vapor can be off
by about 25 ‰.
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