The role of secondary ice production in shaping the microphysical evolution of orographic clouds: A RAMS-ICLAMS modeling study

Abstract. Secondary ice production (SIP) strongly influences ice crystal number concentrations (ICNCs) and cloud microphysical properties, with important implications for radiative forcing, thermodynamics, cloud evolution, and precipitation. Here, we investigate four SIP mechanisms—rime splintering, collisional breakup, rain droplet shattering during freezing, and sublimation breakup—in the Integrated Community Limited Area Modeling System, a specialized version of the Regional Atmospheric Modeling System (RAMS-ICLAMS), during winter storms observed in November–December 2024 as part of the Cleancloud Helmos OrograPhic sIte experimeNt (CHOPIN). Activating SIP increases ICNCs by two to three orders of magnitude and enhances ice water content by up to a factor of 2–3, substantially modifying cloud structure, including a 25–35 % reduction in liquid water path and a doubling of ice water path. These changes improve agreement with observed radar signatures and precipitation patterns and alter the amount of precipitation reaching the ground. Among the represented mechanisms, collisional breakup is dominant, followed by rime splintering. Sublimation breakup is locally intense but at least one order of magnitude weaker, while droplet shattering is the weakest and is mainly activated in regions rich in raindrops. The simulated systems encompass synoptic-scale, convective, and stratiform clouds, including snowfall-producing and seeder–feeder configurations. These results highlight the importance of representing SIP—particularly collisional breakup and rime splintering—for realistic simulations of diverse orographic clouds and precipitation in weather- and climate-scale models.