Isotopic Analysis for Tracing Vertical Growth Trajectories of Hailstones

The growth trajectory of hailstones within clouds has remained elusive due to the inability to trace them directly, impeding the comprehension of their underlying growth mechanisms. This study investigated hailstone vertical growth trajectories by detecting the stable isotope signatures (²H and ¹⁸O compositions) of different shells in 27 hailstones from 9 hailstorms, which allowed us to capture the ambient temperature during hailstone growth. The vertical growth trajectories were obtained by comparing the isotopic compositions of water condensate in clouds, derived from the Adiabatic Model, with those measured in hailstones. Although hailstone growth was primarily observed in the −10°C to −30°C temperature layer, the embryo formation height and subsequent growth trajectories significantly varied among hailstones. Embryos formed over a wide range of temperatures (−8.7°C to −33.4°C); four originated at temperatures above −15°C and 16 originated at temperatures below −20°C, suggesting ice nuclei composed of bioproteins and mineral dust, respectively. Among the 27 measured hailstones, 3 exhibited minimal vertical movement, 16 exhibited a monotonic rise or fall, and the remaining 8 exhibited alternating up-down trajectories; only one experienced “recycling” during up-down drifting. Trajectory analysis revealed that similar-sized hailstones from a single storm tended to form at similar heights, whereas those larger than 25 mm in diameter exhibited at least one period of upward growth. Vertical trajectories derived from isotopic analysis were corroborated by radar hydrometeor observations.