ALBERT

Description: UCLALES-SALSA is a detailed high-resolution model for studying aerosol-cloud interactions in shallow clouds. The model is based on a large eddy simulator UCLALES which was coupled with a sectional aerosol-cloud microphysics module SALSA. The model was recently coupled with a Secondary Organic Aerosol (SOA) formation module. The SOA module contains lumped Volatile Organic Compound (VOC) categories such as biogenic monoterpenes and isoprene, and anthropogenic aromatics. Their oxidation produces Semi-Volatile Organic Compounds (SVOCs), which partitioning between vapor and particle phases. The partitioning is modelled using a Volatility Basis Set (VBS) approach. The module also contains aqueous-phase SOA (aqSOA) formation.The model was tested using a LES setup based on in-situ stratocumulus cloud observations from Kuopio, Finland. The observed low aerosol number concentrations and high aerosol organic factions indicated that SOA formation could have a significant impact on cloud properties. First, by allowing the partitioning of SVOCs the condensed-phase organic aerosol (SOA) concentration becomes temperature dependent. In this case the temperature decreased due to radiative cooling, which increased the SOA mass and the cloud droplet number concentration (CDNC). Surface VOC emissions are important for maintaining SOA production as the reactive VOCs are otherwise consumed rapidly. Thanks to the effective vertical mixing, surface emissions had direct impact on CDNC. Finally, the impact of aqSOA formation depends mainly on anthropogenic emissions because VOC emissions from local boreal forests are dominated by monoterpenes which are not effective aqSOA precursors.

Description: Deliberate injection, or seeding, of large hygroscopic particles into clouds has been suggested as a potential approach for artificial enhancement of rainfall. The particles are expected to accelerate coalescence growth of droplets, leading to more rapid production of rain drops. However, in continental convective clouds rain formation is significantly controlled by mixed-phase microphysical processes. Accordingly, the interaction between hygroscopic seeding and ice particle growth has been highlighted in recent studies. In our previous work we used the large-eddy simulation model UCLALES-SALSA to study convective clouds in conditions typically observed at the United Arab Emirates (UAE). The results showed that hygroscopic seeding increased the rime fraction of ice particles, which was identified as the main source for subsequently increased surface rainfall. In our current work, we continue this line of research further, now focusing on the importance of secondary ice production. Secondary ice production is expected to promote cloud glaciation and growth of ice particles. By the token of our previous model studies, such changes would act to increase surface rainfall. Since field observations suggest the naturally occurring secondary ice production to be weak in clouds with continental characteristics, such as those at UAE, we hypothesize that hygroscopic seeding could trigger or enhance secondary ice production where it otherwise wouldn’t occur, providing a so far insufficiently studied pathway for rain enhancement. We will present results from UCLALES-SALSA model experiments, where our preliminary simulations indeed suggest an increased sensitivity of the clouds to seeding injection when secondary ice is present.

Description: Extreme weather conditions and widespread drying induced by climate change will increase the risk and severity of wildfires increasing the importance of the wildfire emissions in the climate system. Aerosol emissions from the wildfires may affect the cloud formation by increasing the concentration of cloud condensation nuclei (CCN) and by affecting the composition and hygroscopicity (k) of the aerosol population. In this study, we investigate the effect of long range transported (originated from South-Eastern Europe) wildfire plume on cloud microphysics at two sites: Puijo SMEAR IV in Eastern Finland, and Zeppelin Observatory in Svalbard, high Arctic. We use both in-situ and satellite observations to investigate the changes in aerosol population, cloud activation and cloud properties.During the wildfire plume period, the aerosol hygroscopicity slightly increased compared to clean periods at Puijo station, but decreased at Zeppelin. A substantial increase in aerosol number concentration in the accumulation mode size range was observed at both stations. Despite the increase in k, the increase in critical diameter for activation was observed as the water supersaturation was decreased due to increased aerosol concentration at Puijo station. A substantial increase in CCN concentration and cloud droplet number concentration (CDNC) was observed based on in-situ observations at both stations during the wildfire plume period. Also satellite observations revealed a comparable change in CDNC and cloud optical thickness over Puijo station. Our results demonstrate that the long range transported (3-5 days) wildfire plume can significantly affect cloud formation in environments where the background concentrations are relatively low.

Description: A proper description of the in-cloud stratification/structure and microphysics in mixed-phase clouds (MPC) is essential to improve modeled estimates of cloud shortwave cooling and longwave warming effects as well as changes in precipation in climate simulations. Even with significant progress in the understanding of primary ice formation from ice nucleating particles, the dynamics of ice multiplication from pre-existing ice particles or secondary ice production (SIP) remains a challenge in atmospheric modelling. Ice multiplication factors can reach up to four order of magnitude. SIP can accelerate ice aggregation processes leading to rapid cloud glaciation, precipitation and finally to cloud dissipation. Cloud schemes in atmospheric models just consider the Hallet-Mossop or rime-splintering mechanism, although there are SIP parametrizations via fragmentation during drop freezing or after ice-ice collisions. Recent modelling studies have addressed this issue and shown that estimates from different parameterizations differ by orders of magnitude even when functions show similar variable dependencies. It is particularly difficult to distinguish which SIP mechanism occurs in a grid point of a model domain since variable dependencies overlap among parameterizations.We worked with UCLALES-SALSA, a state-of-the-art large eddy simulation model, for a realistic representation of aerosol-hydrometeor interactions in turbulent conditions to account for SIP via rime splintering, frozen drop fragmentation, and breakup fracture after ice-ice collision. The model is used to simulate an Artic MPC case observed during the Ny-Ålesund Aerosol Cloud Experiment (NASCENT). In-cloud microphysical properties allowed us to constrain model parameters and define ranges of action for SIP mechanisms.