Isotopic fractionation during ice growth by riming and its effect on the d -excess of precipitation

We have investigated the impact of riming on the deuterium excess ( d -excess =  δ ² H − 8 ⋅  δ ¹⁸ O) of precipitation. In mixed-phase clouds, precipitation forms by vapor deposition, where supercooled liquid droplets do not come in contact with ice particles, and by riming, where droplets freeze directly on particle surfaces. While vapor deposited ice generally has higher d -excess than liquid, riming is assumed to occur without isotopic fractionation. We correlated radar-observed mean Doppler velocity (MDV), an independent indicator of riming, with precipitation d -excess (−23 ‰ to +45 ‰) at polar (Summit, Greenland; Ny-Ålesund and Andenes, Norway, Dumont d'Urville, Antarctica), mid-latitude (Cazadero, California) and tropical (Rio Claro, Brazil) sites. The d -excess decreases with increasing MDV (or riming intensity) at all locations, except for winter precipitation at Summit. The low d -excess of rimed ice is consistent with the evaporation of accreted liquid on particle surface before freezing is complete. For Summit winter, cold temperatures and low δ ¹⁸ O of vapor suppress d -excess independently of riming. Calculations show that mixtures of ice growing by riming and vapor deposition (including diamond dust) can produce the observed range of precipitation d -excess in this study. We conclude that riming-driven lowering of d -excess can explain, at least in part, the spatial gradients in surface snow d -excess across Greenland and Antarctica previously attributed to changes in source moisture origin, and the very low d -excess of rainfall attributed solely to sub-cloud evaporation. Precipitation d -excess can be used to estimate the rimed mass fraction, providing observational constraints for improving microphysics schemes in climate models.