Initial stages of the riming process on ice crystals

The characteristics of the capture and freezing of supercooled water droplets by falling ice crystals were quantitatively investigated in a vertical supercooled cloud chamber of 10 m length at temperatures from −5 to −25°C. We present evidence that ice crystals can start the riming process at smaller sizes than previously reported. The results obtained show that the minimum dimension of ice crystals involved in riming is around 60 μm in diameter for hexagonal plates and 30 μm width and 60 μm length for columnar ice crystals. No substantial difference has been observed between the size distribution of the droplets accreted on the crystals and the cloud droplet size distribution, indicating that this process should not produce significant changes in the cloud droplet spectrum. The time elapsed between the ice crystal nucleation and the collection of crystals with droplet accretion was typically around 60 seconds.