Improved Formulation of Fragmentation of Snow during Collision with Graupel/Hail based on Observations at Jungfraujoch: Cold Non-Dendritic Regime of Temperature

Abstract. Much of the initiation of ice particles in deep precipitating clouds has been attributed to Secondary Ice Production (SIP). Fragmentation during collisions among particles of ice precipitation is one of the known SIP processes. Some recent studies have used our theoretical formulation of this SIP process in the cloud microphysics scheme of numerical atmospheric models published in 2017. However, there has been a lack of observational data for better understanding of the SIP process. The focus of the present study is on fragmentation of naturally falling snowflakes during their collisions with graupel/hail particles, based on observations conducted at Jungfraujoch, a mountain pass in the Alps and located about 3.6 km above Mean Sea Level. The cloud-top was at about −25° to −32 °C. The study used a portable chamber specially designed to observe the fragmentation of snow particles outdoors. Fixed ice spheres in the chamber were used to mimic graupel or hail. Based on the observational study, we optimised the theoretical formulation for prediction of the number of fragments arising from collisions between snow and graupel/hail. The observations reveal an average number of fragments per collision of about 5. The study improved the prediction of SIP by this type of fragmentation compared to our original theoretical formulation, for snow consisting of mostly aggregates of crystals from the ‘non-dendritic habit regime’ of temperatures colder than −17 °C.