Cold air outbreaks (CAOs) are phenomena that occur across high latitudes during winter months and favor the development of extensive boundary layer clouds. As the boundary layer evolves, changes in cloud morphology often result in a stratocumulus to cumulus transition (SCT). The onset of precipitation is considered to be a key factor that leads to the break-up of the stratocumulus deck. In this modeling study we investigate the additional role that aerosols have on the SCT within a CAO event in the North Atlantic, by using prognostic fields for both aerosols and cloud droplet number concentrations (Nd). By using two chemical/aerosol schemes we assess and quantify the impact of aerosols on the SCT evolution. Our results indicate that the aerosol load and its chemical composition affect the timing of precipitation initiation and its magnitude and thus the break-up. However, the two schemes reveal contradictory results, which are mainly associated with different aerosol size and chemical composition partitioning between modes and bins. The simulations with the aerosol scheme, which considers the modal approach, show that the reduction of Nd across the SCT is driven by changes in the cloud liquid water content, the sulfate availability, and the fine sea-spray availability in the cumulus region, which suppresses sulfate activation. The Nd decreases mostly follow the decrease in accumulation-mode aerosols. For the scheme that considers the sectional approach, both the stratiform and the cumulus clouds appear sensitive to new particles formation and their competition for water. However, in the cumulus region, the higher updrafts and the greater availability of fine sea salt particles become critical for the activation of small particles. New particle formation and background sulfate concentrations are critical in this pristine environment, while sea salt particles have a significant impact on SCT in both sets of simulations.

The accurate representation of mixed-phase monsoon clouds and their phase distribution is of great importance for numerical models used to predict monsoon rainfall. Therefore, it is essential for these models to correctly capture the phase fraction of clouds, which includes the proportions of liquid and ice. Ice particle formation in clouds occurs through primary ice production and secondary ice production (SIP). Most weather and climate models tend to overlook secondary SIP mechanisms, often only including rime-splintering. This oversight can introduce biases in the phase partitioning of mixed-phase clouds and monsoon rainfall predictions. In this study, we investigate the roles of three major SIP mechanisms: Hallett-Mossop (HM), droplet shattering (DS), and ice-ice collision (IIC) in mixed-phase monsoon clouds. This investigation is the first of its kind and was conducted using high-resolution simulations of mixed-phase convective clouds observed during the fourth phase of the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) over a rain shadow region of India. The default cloud microphysical scheme, which originally included only the HM process, was modified to incorporate additional SIP mechanisms such as DS and IIC. The simulated cloud parameters, including liquid and ice water content and ice number concentration, showed good agreement with airborne measurements. Our findings indicate that IIC is the predominant SIP mechanism, contributing 90 % to the total ice production through SIP. The inclusion of the three SIP mechanisms resulted in an enhancement of ice concentration by three to four orders of magnitude at temperatures warmer than -20 °C. SIP significantly influenced various cloud parameters between 0 to −20 °C, including total ice number concentration, ice crystal mass, rimed mass, liquid water content, and phase fraction. It also influenced the Ice Water Path (IWP), Liquid Water Path (LWP), and cloud top temperature. The rates of several mixed-phase processes were also affected by the SIP mechanisms. Overall, SIP led to a 15 % reduction in accumulated surface precipitation.

Summary Mixed-phase clouds are ubiquitous in the Arctic and play a critical role in Earth's energy budget at the surface and top-of-the-atmosphere. These clouds typically occupy the lower and mid-level troposphere and are composed of purely supercooled liquid droplets or mixtures of supercooled liquid water droplets and ice crystals. Here, we review progress in our understanding of the factors that control the formation and dissipation of Arctic mixed-phase clouds, including the thermodynamic structure of the lower troposphere, warm and moist air intrusions into the Arctic, large-scale subsidence, and aerosol particles. We then provide a brief survey of numerous Arctic field campaigns that targeted local cloud-controlling factors and follow this with specific examples of how the Arctic Cloud Observations Using airborne measurements during polar Day (ACLOUD)/ Physical feedback of Arctic PBL, Sea ice, Cloud And AerosoL (PASCAL) and Airborne measurements of radiative and turbulent FLUXes of energy and momentum in the Arctic boundary layer (AFLUX) field campaigns that took place in the vicinity of Svalbard in 2019 were able to advance our understanding on this topic to demonstrate the value of field campaigns. Finally, we conclude with a discussion of the outlook of future research in the study of Arctic cloud-controlling factors and provide several recommendations for the observational and modeling community to advance our understanding of the role of Arctic mixed-phase clouds in a rapidly changing climate.