Snowfall particle size distribution and precipitation observations in the Southern Ocean and coastal Antarctica

Snowfall is a key component to the Antarctic region, contributing significantly to the surface mass balance and influencing mean sea level changes. The intricate nature of ice particle microphysics, encompassing type, size, and structure, presents a great challenge in comprehending the processes of solid precipitation in Antarctica. The characteristics of individual ice crystals as they fall from clouds are crucial for understanding their formation and evolution along the vertical profile. Mechanisms such as aggregation, fragmentation, and riming play a pivotal role in accurately representing precipitation in numerical weather prediction models [1]. Despite their importance, the scarcity of observations for evaluating and validating these processes, particularly in the Southern Ocean and Antarctica, adds complexity. To address this gap, a comprehensive set of precipitation observations occurred during the Antarctic Circumnavigation Expedition (ACE) in the austral summer of 2016-2017 was carried out, utilizing diverse sensors aboard the research vessel Akademik Tryoshnikov. The observational toolkit included a snow particle counter (SPC), two total particle counters (Wenglors), vertical precipitation profiles from 24-GHz micro rain radar (MRR) observations, and manually collected Formvar samples. The Formvar technique, preserving ice particle shapes, offers insights into microphysical properties of ice crystals and snowflakes. SPC and Formvar were employed for particle size distribution (PSD) characterization and quantitative precipitation estimations (QPE) [2]. Precipitation was derived from MRR using the existing reflectivity (Ze)-snowfall (S) relationship for Antarctica [3,4,5]. During ACE, primary observations related to snowfall were near the coasts of the Antarctic Peninsula, Western Antarctica, and Adélie Land (Eastern Antarctica). In the last region, a large-scale event was observed by both the ACE expedition and a Multi-angle Snowflake Camera (MASC) at Dumont d’Urville station. Results showed good agreement between Formvar, SPC (size < 500µm), and MASC (size > 500µm) PSDs. Notably, the 20-µm resolution Formvar images exhibited significantly better performance for particles smaller than 500µm compared to MASC (35-µm resolution). Regarding QPE, all sources exhibited a large spread, particularly MRR estimations, sensitive to Ze-S relationship parameters. The use of PSD observations proved useful in making informed choices about these parameters. In monitoring snowfall precipitation, developing a multi-instrumental approach to overcome individual system limitations is crucial, reducing uncertainty.

References:

[1] Grazioli, J. et al. MASCDB, a database of images, descriptors and microphysical properties of individual snowflakes in free fall. Sci Data 9, 186 (2022).

[2] Sugiura, K. et al., Application of a snow particle counter to solid precipitation measurements under Arctic conditions. CRST, 58: 77-83, 2009.

[3] Grazioli, J. et al., Measurements of precipitation in Dumont d'Urville, Adélie Land, East Antarctica. TC 11, 1797–1811, 2017.

[4] Souverijns, N. et al., Estimating radar reflectivity – snowfall rate relationships and their uncertainties over Antarctica by combining disdrometer and radar observations. AR, 196: 211–223, 2017.

[5] M.S. Kulie and R. Bennartz, Utilizing Spaceborne Radars to Retrieve Dry Snowfall. JAMC, 48, 2564-2580.

Acknowledgements: PROPOLAR APMAR-2024, FCT ATLACE (CIRCNA/CAC/0273/2019) and ANR-APRES3. ACE was made possible by funding from the Swiss Polar Institute and Ferring Pharmaceuticals.