Aerosol Influences on Enhanced Riming Events in Low-Level Arctic Mixed-Phase Clouds of the North Slope of Alaska and the Central Arctic

Riming is a highly efficient process for removing water mass, cloud condensation nuclei (CCN), and ice nuclei (IN) aerosols from ubiquitous low-level mixed-phase clouds in the Arctic. Measurements along the North Slope of Alaska (NSA; Fitch & Garrett, 2022a & 2022b), and more casual, anecdotal observations during summer expeditions into the Central Arctic (Tjernström, 2020) indicate that riming is a common occurrence in this region. Counterintuitively, this is most commonly observed with clouds that have low liquid water content (LWP < 50 g/m2)—at least on the NSA (Fitch & Garrett, 2022a & 2022b). Thus, the traditional model of a continuous collection kernel – falling in still air through a cloud of supercooled liquid water droplets – is insufficient to explain such “enhanced” riming. Furthermore, it is not clear which aerosol types (e.g., marine vs. anthropogenic) and sources (local vs. long-range transport) are most favorable for this enhanced riming process (Heutte et al., 2023; Maahn et al., 2017). Here we use U.S. Department of Energy Atmospheric Radiation Measurement (DOE ARM) measurements from two NSA sites (2017-2024) and the Multidisciplinary Drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition (2019-2020) to better constrain the relationships between enhanced riming events and their associated aerosols. Initial results indicate that enhanced riming is more likely to be observed in cases where “polluted” (i.e., anthropogenic) aerosols dominate. Two competing hypotheses are presented to suggest potential modelling strategies to help explain the connection between aerosol types and enhanced riming processes, with the hope that better parameterizations can lead to more accurate prediction of low-level Arctic mixed-phase cloud sustainment or dissipation.

Fitch, K. E., & Garrett, T. J. (2022a). Measurement and analysis of the microphysical properties of Arctic precipitation showing frequent occurrence of riming. Journal of Geophysical Research: Atmospheres, 127(7), https://doi.org/10.1029/2021JD035980.

Fitch, K. E., & Garrett, T. J. (2022b). Graupel precipitating from thin Arctic clouds with liquid water paths less than 50 g m⁻². Geophysical Research Letters, 49(1), https://doi.org/10.1029/2021GL094075.

Heutte, B., Bergner, N., Beck, I., Angot, H., Dada, L., Quéléver, L. L., ... & Schmale, J. (2023). Measurements of aerosol microphysical and chemical properties in the central Arctic atmosphere during MOSAiC. Scientific data, 10(1), 690, https://doi.org/10.1038/s41597-023-02586-1.

Maahn, M., de Boer, G., Creamean, J. M., Feingold, G., McFarquhar, G. M., Wu, W., & Mei, F. (2017). The observed influence of local anthropogenic pollution on northern Alaskan cloud properties. Atmospheric Chemistry and Physics, 17(23), 14709-14726, https://doi.org/10.5194/acp-17-14709-2017.

Tjernström, M., personal communication, July 2020.