The altitude of snow growth by riming and vapor deposition in mixed-phase orographic clouds

Oxygen isotopic ratios (δ18O) and sulfate concentrations were measured in cloud water and snow collected at Storm Peak Laboratory (SPL) during winter, 2007. The rimed mass fraction (RMF) was estimated as the ratio of sulfate concentration in snow to that in cloud water. A sharp increase in the RMF at mean droplet diameters above 10 μm confirmed the expected relationship between riming efficiency and cloud droplet size. The mass-weighted altitude of snow formation was inferred from δ18O in cloud water and snow and did not exceed 300 m above SPL. The mass-weighted altitude of snow growth by vapor deposition alone was no higher than 900 m above SPL. The results suggest that snow crystals nucleated under water-sub-saturated conditions at higher elevations attained a significant fraction of their water content as they grew by riming and vapor deposition in transit through the low-level orographic cloud near the mountain crest. This approach provides a direct means of validating model simulations of snow growth processes in cold mountain clouds.