Ice–ice collisional breakup enhances secondary ice production but reduces surface hailfall in a simulated pulse hailstorm

Abstract. Secondary ice production (SIP) can substantially modify the ice-phase structure of mixed-phase clouds, but its role in short-lived pulse hailstorms remains unclear. Here, an ice–ice collisional breakup (BR) parameterization is implemented in the NSSL two-moment scheme within Cloud Model 1 (CM1), and seven sensitivity experiments are conducted for an idealized pulse hailstorm to examine the effects of droplet shattering during freezing (DS) and BR on ice-phase evolution and surface hailfall. The results show that BR has a much stronger effect than DS on the cloud-ice structure and surface hail mass. Relative to CNTL, the experiments including BR show a marked reduction in the peak total hail mass at the surface, with the largest decrease reaching 41.94 %. BR increases the ice crystal number concentration by about two orders of magnitude between approximately 5 and 9 km, accompanied by reduced supercooled liquid water above 4 km, lower graupel mixing ratio, and weakened graupel riming growth. These changes reduce the size of potential hail embryos, weaken graupel-to-hail conversion at some heights, and limit subsequent hail growth. The decomposition of BR collision pairs shows that, during the mature hail-producing stage, collisions of ice crystals with graupel and hail become the main sources of BR fragments. Overall, BR can substantially reduce surface hail mass in this idealized pulse-hailstorm case by restructuring the ice phase and enhancing competition for supercooled liquid water.