标题：Nonequilibrium Fractionation During Ice Cloud Formation in iCAM5: Evaluating the Common Parameterization of Supersaturation as a Linear Function of Temperature
期刊名称：Journal of Advances in Modeling Earth Systems
电子版ISSN：1942-2466
出版年度：2019
卷号：11
期号：11
页码：3777-3793
DOI：10.1029/2019MS001764
出版社：John Wiley & Sons, Ltd.
摘要：Supersaturation with respect to ice determines the strength of nonequilibrium fractionation during vapor deposition onto ice or snow and therefore influences the water isotopic composition of vapor and precipitation in cold environments. Historically, most general circulation models formed clouds through saturation adjustment and therefore prevented supersaturation. To match the observed isotopic content, especially the deuterium excess, of snow in polar regions, the saturation ratio with respect to ice ( S i ) was parameterized, usually by assuming a linear dependence of S i on temperature. The Community Atmosphere Model Version 5 (CAM5) no longer applies saturation adjustment for the ice phase and thus allows ice supersaturation. Here, we adapt the isotope‐enabled version of CAM5 to compute nonequilibrium fractionation in ice and mixed‐phase clouds based on S i from the CAM5 microphysics and use it to evaluate the common parameterization of S i . Our results show a wide range of S i predicted by the CAM5 microphysics and reflected in the simulated deuterium excess of Antarctic precipitation; this is overly simplified by the linear parameterization. Nevertheless, a linear function, when properly tuned, can reproduce the average observed relationship between δ D and deuterium excess reasonably well. However, only the model‐predicted S i can capture changes in microphysical conditions under different climate states that are not due to changes in temperature. Furthermore, parametric sensitivity tests show that with the model‐predicted S i , water isotopes are more closely tied to the model microphysics and can therefore constrain uncertain microphysical parameters. Plain Language Abstract The concentration of oxygen and hydrogen isotopes of water depends on meteorological processes such as evaporation from the ocean and cloud formation. Water isotope concentrations measured in ice cores and other natural archives are therefore used to reconstruct Earth's past climate. In Antarctica, isotope concentrations strongly depend on the meteorological conditions during ice and mixed‐phase cloud formation, especially the supersaturation with respect to ice. Isotope‐enabled climate models, which are often used to support interpretations of isotope measurements, commonly prescribe supersaturation with respect to ice as a linear function of temperature. It is unknown how much this simplification affects the representation of water isotope concentrations in the models. Here we use a recently developed isotope‐enabled climate model, which uses a physically based calculation of supersaturation, to evaluate the linear parameterization used in other models. We find that, even though it is less physically realistic, the linear parameterization can represent the average water isotope concentrations reasonably well. We also evaluate how model parameters related to supersaturation affect water isotopes. Our results suggest that water isotopes can potentially be used to improve climate models.
关键词：stable water isotopes;deuterium excess;supersaturation;Antarctica;ice clouds
