Aerosol effects on Secondary Ice Production in Deep ConvectiveClouds: exploiting the synergistic benefit of observations andaerosol-aware cloud simulations

Secondary ice production (SIP) refers to a series of physical mechanisms that significantly increase ice number concentrations above those from primary ice production (PIP) via ice nucleating particles (INP). In-cloud observations have provided increasing evidence of SIP in mixed-phase stratiform and convective clouds at different latitudes. The presence of fragmented frozen drops and small columnar particles in measurements from holographic particle imaging systems is consistent with findings of laboratory experiments focused on rime splintering (RS), droplet shattering (DS), and ice-ice collisional breakup (IIBR) mechanisms. Since SIP rates are driven by the relative size of interacting hydrometeors, there is a need to understand which microphysical conditions trigger which mechanism in realistic atmospheric conditions where cloud micro physics are constrained by aerosols. Without describing aerosol-hydrometeor interactions, the majority of cloud modelling tools are limited to prescribed size distributions and process rates that may fail giving proper description of ice formation via primary and secondary pathways missing important links to others such as secondary activation and aerosol invigoration.
In this study we offer insights on aerosol-induced effects on SIP rates by coupling results from aerosol-aware large-eddy- simulations and in-cloud observations of a deep convective cloud case studied in the SPICULE campaign in the Southern Great Plains (USA) on June 05 2021. We employed UCLALES-SALSA, an LES model with sectional representation of aerosol microphysics to resolve rates for PIP via immersion freezing with time evolving contact angle distribution and SIP via droplet shattering (DS), and ice-ice collisional breakup (IIBR). After model initialization with observed atmospheric soundings and aerosol concentrations, we were able to reproduce observed trends in cloud properties including boundaries and vertical profiles of droplet and ice particle size distributions. The model was able to emulate the observed ice multiplication in the rising cloud tower indicating a positive feedback between SIP-DS and SIP-IIBR processes which in turn increased convection intensity through mixed-phase invigoration at the upper level and finally lead to glaciation and precipitation via seeder-feeder mechanism. Both, the convective available energy (CAPE) and the level of neutral buoyancy (LNB), were adjusted to reach model closure in the cloud tower. We also compared simulations differing in the aerosol number concentration in the accumulation mode used for model initialization and found that increasing fine particle concentrations increase ice formation and updraft strength above freezing level suggesting that mixed-phase invigoration has an role in cloud phase structure and glaciation of convective clouds.