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Cloud properties are strongly influenced by ice formation, hence we need to understand the sources of ice-nucleating particles (INPs) around the globe. Boreal forests are known as sources of bioaerosol and recent work indicates that these dominate the INP spectra above -24 °C. To quantify the INP population at temperatures below -24 °C, we deployed a portable cloud expansion chamber (PINE) in a Finnish boreal forest from March 13, 2018 to May 11, 2018. Using the 6 min time resolution PINE data, we present several lines of evidence that INPs below -24 °C in this location are also from biological sources: an INP parameterization developed for a pine forest site in Colorado, where many INPs were shown to be biological, produced a good fit to our measurements; a moderate correlation of INP with aerosol concentration larger than 0.5 μm and the fluorescent bioaerosol concentration; a negative correlation with relative humidity that may relate to enhanced release of bioaerosol at low humidity from local sources such as the prolific lichen population in boreal forests. The absence of correlation with ultrafine particles (3.5 to 50 nm) indicates that new particle formation events are not sources of INP. This study should motivate further work to establish if the commonality in bioaerosol ice nucleating properties between spring in Finland and summer in Colorado is more generally applicable to different coniferous forest locations and times, and also to determine to what extent these bioaerosols are transported to locations where they may affect clouds. [ABSTRACT FROM AUTHOR]

Ice nucleating particles (INPs) facilitate heterogeneous freezing of cloud droplets and thus modify cloud properties. Hence, it is important to understand the sources of INPs. During the HyICE-2018 campaign, which took place in the boreal forest of Hyytiälä, substantial concentrations of biological INPs were observed despite many potential biological sources of INPs being snow covered. A potential source of INP that were not covered in snow were lichens that grow on trees, hence we investigated these lichens as a potential source of biological INPs in this boreal forest environment. INPs derived from lichen sampled during HyICE-2018 are shown to nucleate ice at temperatures as warm as -5 °C in aqueous solutions extracted from 0.03 g ml-1 lichen. In particular, we identify two species of INPs active at -16 and -18 °C and smaller than 2 µm in size, which makes them good candidates for atmospheric INPs. These particles are smaller than whole lichen spores. In species specific tests we show that the four investigated lichen species harbour differently sized INPs, which might suggest that some species of lichen are more important as a source of INPs than others. This study shows that lichen from a European Boreal forest harbour INPs, which may be especially important in habitats where few other biological INP sources are present, such as in a snow covered environment. The great terrestrial abundance of lichen INPs calls for further research to combine their ice nucleating ability with dispersal studies to evaluate the flux of lichenous INPs into the atmosphere as well as to what extent these particles reach heights and locations where they might influence cloud properties. [ABSTRACT FROM AUTHOR]

Aerosol particles influence cloud formation and properties. Hence climate models that aim for a physical representation of the climate system include aerosol modules. In order to represent more and more processes and aerosol species, their representation has grown increasingly detailed. However, depending on one's modeling purpose, the increased model complexity may not be beneficial, for example because it hinders understanding of model behaviour. Hence we develop a simplification in the form of a climatology of aerosol concentrations. In one approach, the climatology prescribes properties important for cloud droplet and ice crystal formation, the gateways for aerosols to enter the model cloud microphysics scheme. Another approach prescribes aerosol mass and number concentrations in general. Both climatologies are derived from full ECHAM-HAM simulations and can serve to replace the HAM aerosol module and thus drastically simplify the aerosol treatment. The first simplification reduces computational model time by roughly 65 %. However, the naive mean climatological treatment needs improvement to give results that are satisfyingly close to the full model. We find that mean CCN concentrations yield an underestimation of CDNC in the Southern Ocean, which we can reduce by allowing only CCN at cloud base (which have experienced hygroscopic growth in these conditions) to enter the climatology. This highlights the value of the simplification approach in pointing to unexpected model behaviour and providing a new perspective for its study and model development. [ABSTRACT FROM AUTHOR]

The ice phase impacts many cloud properties as well as cloud lifetime. Ice particles that sediment into a lower cloud from an upper cloud (external seeder-feeder process) or into the mixed-phase region of a deep cloud from cirrus levels (internal seeder-feeder process) can influence the ice phase of the lower cloud, amplify cloud glaciation and enhance surface precipitation. Recently, numerical weather prediction modeling studies have aimed at representing the ice crystal number concentration in mixed-phase clouds more accurately by including secondary ice formation processes. The increase in the ice crystal number concentration can impact the number of ice particles that sediment into the lower cloud and alter its composition and precipitation formation. In the Swiss Alps, the orography permits the formation of orographic clouds, making it ideal for studying the occurrence of multi-layered clouds and the seeder-feeder process. We present results from a case study on May 18, 2016, showing the occurrence frequency of multi-layered clouds and the seeder-feeder process. About half of all observed clouds were categorized as multi-layered, and the external seeder-feeder process occurred in 10 % of these clouds. In between cloud layers, ≈ 60 % of the ice particle mass was lost due to sublimation or melting. The external seeder-feeder process was found to be more important than the internal seeder-feeder process with regard to the impact on precipitation. In the case where the external seeder-feeder process was inhibited, the average surface precipitation and riming rate over the domain were both reduced by 8.5 % and 3.9 %, respectively. When ice-graupel collisions were allowed, further large reductions were seen in the liquid water fraction and riming rate. Inhibiting the internal seeder-feeder process enhanced the liquid water fraction by 6 % compared to a reduction of 5.8 % in the cloud condensate and, therefore, pointing towards the deamplification in cloud glaciation and a reduction in surface precipitation. Adding to the observational evidence of frequent seeder-feeder situations at least over Switzerland Proske et al. (2021), our study highlights the extensive influence of sedimenting ice particles on the properties of feeder clouds as well as on precipitation formation. [ABSTRACT FROM AUTHOR]