High-Temporal-Resolution Variations in Snowfall Water Isotopes Driven by Cloud and Precipitation Processes

Stable water isotopes in precipitation serve as a useful tool for understanding past and present atmospheric water cycles, including evaporation, mixing, transport, and precipitation. However, observational studies focusing on changes in stable water isotopes during cloud and precipitation processes remain limited. Here, we measured the stable water isotopic compositions of snowfall and snow crystal shapes at 1-2 h intervals during a single cyclone-associated snowfall event to clarify changes in isotopic composition during cloud and precipitation processes. Early in the observation period, high abundances of dendrites and sector/plate crystals corresponded to elevated snowfall δ 18 O values and deuterium excess (d-excess), whereas later shifts toward bullet and combination crystal types coincided with lower values. These results demonstrate that the isotopic composition of snowfall reflects the temperature during snow crystal growth. The observed fluctuations in snowfall d-excess showed a strong positive correlation with δ 18 O. We suggest that these fluctuations were primarily driven by ambient temperature during crystal formation, as explained by Rayleigh distillation. Furthermore, riming during snow crystal growth shifted the isotopic ratios toward higher values. This study demonstrated that the high-temporal-resolution isotopic fluctuations observed during a single snowfall event reflected cloud and precipitation processes. Detailed analyses of stable water isotopes in conjunction with individual snow crystal shapes may help elucidate in-cloud microphysical mechanisms, improve the parameterization of cloud and precipitation processes in isotope-enabled atmospheric models, and enhance the interpretation of ice-core proxy records.