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A bin-microphysics parcel model investigation of secondary ice formation in an idealised shallow convective cloud

. We provide the first systematic study of ice formation in idealised shallow clouds from collisions of supercooled water drops with ice particles (‘mode 2’). Using the University of Manchester bin-microphysics parcel model, we investigated the sensitivity of ice formation due to mode 2 for a wide range of parameters: aerosol particle size distribution, updraft speed, cloud base temperature, cloud depth, ice-nucleating particle concentration and freezing fraction of mode 2. We provide context to our results with other secondary ice production mechanisms as single mechanisms and combinations (rime-splintering, 5 spherical freezing fragmentation of drops [‘mode 1’] and ice-ice collisions). There was a significant sensitivity to aerosol particle size distribution when updraft speeds were low (0.5 m s-1); secondary ice formation did not occur when the aerosol particle size distribution mimicked polluted environments. Where secondary ice formation did occur in simulated clouds, significant ice formation in the shallower clouds (1.3 km deep) was due to mode 2 or a combination which included mode 2. The deeper clouds (2.4 km deep) also had significant contributions from rime-splintering or ice-ice collisions SIP mechanisms. 10 While simulations with cloud base temperatures of 7 °C were relatively insensitive to ice-nucleating particle concentrations, there was a sensitivity in simulations cloud base temperatures of 0 °C. Increasing the ice-nucleating particle concentration delayed ice formation. Our results suggest that collisions of supercooled water drops with ice particles may be a significant ice formation mechanism within shallow convective clouds where rime-splintering is not active. and Research Forecasting Model simulations of summer clouds over the Antarctic coast. Their results suggested that ice-ice collisional breakup alone could be responsible for observed ICNCs in Antarctic clouds, despite including the rime-splintering 60 mechanism in their simulations. Georgakaki et al. found that a combination of ice-ice collisional breakup and external ice seeding could account for the observed ICNCs in wintertime alpine mixed-phase clouds observed during the CLACE campaign. Our results suggest that M2 may be a significant ice enhancement mechanism in shallow convective clouds where large drops 580 are present especially when cloud tops are warmer than -15 °C, the thermal peak of M1 and M2. It may also be significant for clouds where large drops do not reside within the RS region. However, there are still many areas within the M2 parameterisation that need to be addressed via further laboratory work as detailed in James et al. (2021), a theme which can be applied to all SIP parameterisations. Our results also show that the parameterisation of INPs can have an effect on SIP mechanisms, and certainly with modelling studies these should be varied.