Fragmentation in Collisions of Snow with Graupel/Hail: New Formulation fromField Observations

Secondary ice production (SIP) is believed to be responsible for the majority of ice particles observed in precipitating clouds with temperatures above −36°C, based on various field observations from both aircraft and ground-based studies worldwide. One known mechanism of SIP is the fragmentation of ice particles during collisions. This process has been explored using a theoretical model, which has been incorporated into the microphysical schemes of some atmospheric models, where it has been shown to significantly influence cloud glaciation and radiative properties. However, there has been a lack of experimental field studies, particularly those involving naturally falling snowflakes, to better understand this specific SIP mechanism. This study presents the first field measurements of fragmentation during collisions between naturally falling snowflakes and graupel/hail particles, using a newly designed portable chamber deployed outdoors in northern Sweden. Based on these field observations, we refined the existing model for predicting the number of fragments produced by collisions between snow and graupel/hail. The data revealed that, on average, dendritic snowflakes (3–12 mm) produced about 12 fragments per collision, while nondendritic snowflakes (1–3 mm) produced around 1 fragment. This represents an increase in predicted fragment numbers compared to our original model published in 2017. The updated fragmentation model for ice–ice collisions can now be integrated into atmospheric models’ microphysical schemes.