Secondary ice production : An empirical formulation and organization of mechanisms among simulated cloud-types

Abstract: Clouds are essential elements within Earth's atmosphere, posing a challenge forcloud-resolving models in understanding the creation of new cloud ice particlesfrom existing ice and liquid phases. Such ice initiation determines cloudmicrophysical and radiative properties, influencing cloud phase, precipitation and cloud extent/properties. To address this challenge effectively, it proves beneficial to differentiate the fundamental microphysical properties of various cloud types, considering their basic classifications.A few historical experimental studies a few decades ago delved into how sublimation of ice crystals causes the emission of fragments. These fragmentssubsequently grow into crystals, and in some cases, may evolve into snow orgraupel. This sublimational breakup represents a form of secondary ice production, capable of causing ice multiplication in natural clouds. The origins of the high ice concentrations observed in clouds are becoming better understood, but still have some uncertainty.In this study, an empirical numerical formulation for sublimation breakup incloud models is introduced. This formulation is based on comprehensive laboratory data gathered from previous studies. By analyzing experiments that measured the number of ice fragments generated through sublimation, considering factors such as relative humidity and initial ice particle size, we derived essential parameters for a sublimation breakup scheme. The research findings highlight the prevalence of size dependency in smaller particles, while larger particles exhibit comparable dependencies.Ice initiation in clouds has primarily focused on specific cloud systems, revealingthat the majority of ice particles in the mixed-phase region result from secondary ice production mechanisms. However, these studies have been limited to individual cloud types. The objective of this thesis is to broaden the understanding of each secondary ice production mechanism's contribution across various fundamental cloud types with a more all-inclusive approach. To achieve this, numerical simulations are conducted utilizing our ‘Aerosol-Cloud model’ for different cloud categories. These simulations are then validated against in-situ cloud observations obtained from four distinct cloud observational campaigns, each representing a different cloud type for comprehensive analysis.In this study, the roles of various secondary ice production processes, includingthe HM process, ice-ice collisional breakup, raindrop-freezing fragmentation, and sublimational breakup were meticulously examined. These analyses are conducted through controlled simulations for different fundamental cloud types. Within warm cloud convective clouds, the HM process is particularly notable near the freezing level, making contributions within specific temperature ranges. Ice-ice collisional breakup emerges as the predominant secondary ice production mechanism across all cloud types, being the only one with appreciable activity in cold-based convection. Additionally, in slightly warm-based convective clouds, the breakup resulting from ice-ice collision takes precedence within the convective updrafts.