Deuterium and oxygen 18 in precipitation: Isotopic model, including mixed cloud processes

Abstract

Modeling the isotopic ratios of precipitation in cold regions meets the problem of “switching” from the vapor‐liquid transition to the vapor‐ice transition at the onset of snow formation. The one‐dimensional model (mixed cloud isotopic model (MCIM)) described in this paper focuses on the fractionation of water isotopes in mixed clouds, where both liquid droplets and ice crystals can coexist for a given range of temperatures. This feature is linked to the existence of specific saturation conditions within the cloud, allowing droplets to evaporate while the water vapor condensates onto ice crystals. The isotopic composition of the different airborne phases and the precipitation is calculated throughout the condensation history of an isolated air mass moving over the Antarctic ice sheet. The results of the MCIM are compared to surface snow data both for the isotopic ratios and the deuterium excesses. The sensitivity of the model is compared to previous one‐dimensional models. Our main result is that accounting specifically for the microphysics of mixed stratiform clouds (Bergeron‐Findesein process) does not invalidate the results of earlier modeling studies.