Romanos Foskinis

Closed-to-open cell mixed-phase cloud transitions within marine cold air outbreaks remain poorly understood despite their importance to high-latitude climate. The Cold-Air outbreak Experiment in the Sub-Arctic Region (CAESAR) research aircraft sampled a cold and polluted closed-to-open cell transition with cloud-top temperatures below −20 °C and upstream cloud condensation nuclei concentrations surpassing 600 cm ⁻³ . The aerosol likely originates from Siberian industrial emissions. As the boundary layer deepens prior to and across the transition, driven by strong turbulent surface fluxes and aided by a weakening cloud-top inversion, aerosol number concentrations decrease and supercooled droplet sizes increase, causing ice particles to shift from primarily vapor depositional growth towards riming. Precipitation shafts within open-cells contain abundant large graupel (>5 mm diameter) with liquid-equivalent precipitation intensities reaching 3 mm h ⁻¹ . Ice precipitation co-exists with lidar-observed surface cold pools, modulated by entrainment events, juxtaposed with surface-based plumes of warm moist air. Updrafts support liquid water production along convergence boundaries at the upwind cold pool edge. Nonetheless, strong buoyancy fluxes exceeding 200 W m ⁻² prevent sub-cloud decoupling. Ice particle number concentrations increase tenfold across the transition, surpassing 10 L ⁻¹ within open-cells, more than 1000× ice nucleating particle number concentrations. Given that cloud temperatures are too cold for Hallett-Mossop rime splintering, this indicates the importance of other secondary ice production mechanisms. This case expands the observations needed to better understand mixed-phase Arctic cloud processes.

Knowledge gaps in the source and parameterization of ice-nucleating particles (INPs) remain a major uncertainty in quantifying the properties and climate impacts of mixed-phase clouds (MPCs). Bioaerosols are increasingly recognized as important INPs for MPCs, yet it is unclear whether their contribution is critical through modelling studies. We investigate this using field observations at a semi-rural site in the central Europe, combining INP and aerosol measurements, remote sensing, and air-mass source analysis. We synergically use the results of in-situ and offline measurements to identify INPs originated from different sources, including biological aerosols, dust, and biomass burning aerosols, and to quantify their abundance and relative contributions to total INPs. More than 85% of immersion-mode INPs (>−24°C) are heat-labile and significantly correlated with fluorescent biological aerosols particles and pollen, while heat-resistant INPs (<~10%) are likely mineral dust, while biomass burning is an insignificant source. The proposed bioaerosol-ware INP parameterization reproduces observations across multi-regional datasets, showing that neglecting bioaerosol-INPs results in an average ~50% (~32%) bias in predictions of immersion-mode INPs active warmer than −15°C (−24°C), with frequent deviations of up to an order of magnitude from observations.

Knowledge gaps in the source and parameterization of ice-nucleating particles (INPs) remain a major uncertainty in quantifying the properties and climate impacts of mixed-phase clouds (MPCs). Bioaerosols are increasingly recognized as important INPs for MPCs, yet it is unclear whether their contribution is critical through modelling studies. We investigate this using field observations at a semi-rural site in the central Europe, combining INP and aerosol measurements, remote sensing, and air-mass source analysis. We synergically use the results of in-situ and offline measurements to identify INPs originated from different sources, including biological aerosols, dust, and biomass burning aerosols, and to quantify their abundance and relative contributions to total INPs. More than 85% of immersion-mode INPs (>−24°C) are heat-labile and significantly correlated with fluorescent biological aerosols particles and pollen, while heat-resistant INPs (<~10%) are likely mineral dust, while biomass burning is an insignificant source. The proposed bioaerosol-ware INP parameterization reproduces observations across multi-regional datasets, showing that neglecting bioaerosol-INPs results in an average ~50% (~32%) bias in predictions of immersion-mode INPs active warmer than −15°C (−24°C), with frequent deviations of up to an order of magnitude from observations.

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.

This paper presents a dataset obtained from a ground-based W-band (94 GHz) Doppler spectral polarimetric cloud radar (WProf) during the CLeancloud Arctic VIllum ExpeRiment (Clavier) campaign at Villum Research Station, Northeast Greenland, from 23 March to 13 April 2024. The campaign aimed to investigate Arctic cloud microphysical processes and aerosol–cloud interactions under pristine and Arctic haze conditions. Here, we describe the WProf radar measurements and the full processing chain, including noise removal and quality-control filtering, ensuring complete traceability from raw spectra to higher-level products. To resolve the complex microphysical structure of Arctic clouds, the peakTree algorithm was applied to Doppler spectra to identify and characterize multimodal velocity distributions, enabling the detection of coexisting hydrometeor populations within individual radar volumes. Peak-resolved spectral parameters are subsequently used within a hydrometeor classification framework implemented in pyLARDA enabling the identification of liquid droplets, pristine ice crystals, aggregates and rimed particles. The dataset captures a wide range of Arctic spring cloud regimes, including persistent mixed-phase clouds, deep and shallow ice-precipitating systems, and vertically structured seeder–feeder configurations. The observations indicate that multimodality is a common feature of Arctic clouds, particularly during periods of active ice growth, aggregation, and cloud-layer interaction. As dynamic forcing weakens, cloud systems frequently evolve toward shallower and more homogeneous stratiform states with reduced spectral complexity. Overall, the dataset provides a solid basis for studying Arctic aerosol-cloud interactions and cloud microphysics for evaluating the cloud and precipitation parameterizations in models.

Closed-to-open cell mixed-phase cloud transitions within marine cold air outbreaks subjected to strong turbulent surface fluxes remain poorly understood despite their importance to high-latitude climate. The Cold-Air outbreak Experiment in the Sub-Arctic Region (CAESAR) research aircraft sampled closed-cells with cloud condensation nuclei concentrations surpassing 680 cm^-3, decreasing to 90 cm^-3 across a transition to open-cells. The aerosol likely originated from Siberian industrial emissions. With fetch, liquid water paths (LWPs) increase from 120 g m^-2 to 270 g m^-2 and cloud-top effective diameters increase from 10 μm to 16 μm, coincident with more riming. Ice particle number concentrations (N_i) are generally 2 L^-1 or less, but exceed ice nucleating particle number concentrations by 100x. As the cloud-top inversion weakens and the boundary layer deepens further, ice precipitation co-exists with lidar-observed surface cold pools, modulated by entrainment events, juxtaposed with surface-based plumes of warm moist air. Open-cells contain isolated LWP peaks surpassing 500 g m^-2 collocated with strong updrafts, adjacent to glaciated cloud. N_i surpasses 10 L^-1 at cloud temperatures < -15 °C. Precipitation shafts contain abundant large graupel (> 5 mm diameter) with liquid-equivalent precipitation intensities reaching 3 mm hr^-1 developing cold pools with virtual potential temperature depressions reaching 1.3 K. Nonetheless, buoyancy fluxes of 200-250 W m^-2 prevent sub-cloud decoupling. The updrafts supporting liquid water production occur at the upwind edge of the cold pools. This case expands the observations needed to better understand mixed-phase Arctic cloud processes.

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.

Closed-to-open cell mixed-phase cloud transitions within marine cold air outbreaks subjected to strong turbulent surface fluxes remain poorly understood despite their importance to high-latitude climate. The Cold-Air outbreak Experiment in the Sub-Arctic Region (CAESAR) research aircraft sampled closed-cells with cloud condensation nuclei concentrations surpassing 680 cm^-3, decreasing to 90 cm^-3 across a transition to open-cells. The aerosol likely originated from Siberian industrial emissions. With fetch, liquid water paths (LWPs) increase from 120 g m^-2 to 270 g m^-2 and cloud-top effective diameters increase from 10 μm to 16 μm, coincident with more riming. Ice particle number concentrations (N_i) are generally 2 L^-1 or less, but exceed ice nucleating particle number concentrations by 100x. As the cloud-top inversion weakens and the boundary layer deepens further, ice precipitation co-exists with lidar-observed surface cold pools, modulated by entrainment events, juxtaposed with surface-based plumes of warm moist air. Open-cells contain isolated LWP peaks surpassing 500 g m^-2 collocated with strong updrafts, adjacent to glaciated cloud. N_i surpasses 10 L^-1 at cloud temperatures < -15 °C. Precipitation shafts contain abundant large graupel (> 5 mm diameter) with liquid-equivalent precipitation intensities reaching 3 mm hr^-1 developing cold pools with virtual potential temperature depressions reaching 1.3 K. Nonetheless, buoyancy fluxes of 200-250 W m^-2 prevent sub-cloud decoupling. The updrafts supporting liquid water production occur at the upwind edge of the cold pools. This case expands the observations needed to better understand mixed-phase Arctic cloud processes.