DEUTERIUM AND OXYGEN-18 IN PRECIPITATION - ISOTOPIC MODEL, INCLUDING MIXED CLOUD PROCESSES

Modeling the isotopic ratios of precipitation in cold regions meets th e problem of ''switching'' from the vapor-liquid transition to the vap or-ice transition at the onset of snow formation. The one-dimensional model (mixed cloud isotopic model (MCIM)) described in this paper focu ses on the fractionation of water isotopes in mixed clouds, where both liquid droplets and ice crystals can coexist for a given range of tem peratures. This feature is linked to the existence of specific saturat ion conditions within the cloud, allowing droplets to evaporate while the water vapor condensates onto ice crystals. The isotopic compositio n of the different airborne phases and the precipitation is calculated throughout the condensation history of an isolated air mass moving ov er the Antarctic ice sheet. The results of the MCIM are compared to su rface snow data both for the isotopic ratios and the deuterium excesse s. The sensitivity of the model is compared to previous one-dimensiona l models. Our main result is that accounting specifically for the micr ophysics of mixed stratiform clouds (Bergeron-Findesein process) does not invalidate the results of earlier modeling studies.