Isotopic Evidence for Ice Growth by Riming in Precipitation

Abstract. In this study, we investigate the impact of riming on the relative composition of oxygen and hydrogen isotopes or d-excess in precipitation. Riming, where supercooled liquid droplets freeze directly on ice particles, is an important process of precipitation formation in mixed-phase clouds but has long been assumed to occur without isotopic fractionation. We used an independent indicator of riming, the terminal fall velocity of snow particles, and correlated it with the d-excess of snow or rain precipitation (ranging from –23 to +45 ‰) in polar (Arctic, Antarctic), mid-latitude and tropical regions. Our results show an inverse correlation of d-excess with terminal fall velocity, which increases with riming, indicating that lower d-excess reflects variable extents of riming during precipitation formation. The lower d-excess of rimed ice results from a partial loss of the accreted liquid by evaporation, and possibly splashing and shedding, before freezing is complete. This contrasts with a higher d-excess of ice that grows by the vapor deposition process. We conclude that low d-excess from riming can explain the spatial variations of Greenland and Antarctica surface snow that were previously attributed to changes in source moisture origin. Our results also help to explain the wide range of d-excess observed in daily precipitation compared to a much narrower range in surface snow or ice cores. Further, spatial or temporal differences in d-excess would allow for the estimation of variations in the rimed mass fraction, which, in turn, can be used as observational constraints for improving microphysics schemes in climate models.