Atmospheric Rivers and Antarctic Peninsula Precipitation Phase Transitions

Atmospheric rivers (AR) are long, narrow, transient corridors of intense atmospheric moisture transport affecting many regions around the world including Antarctica, where they play an important role in surface mass and energy balance. Over the Antarctic Peninsula (AP), one of the most rapidly warming regions, ARs have been increasing in frequency and intensity causing major heatwaves, anomalous precipitation and surface melt [1,2]. The impact of ARs would not be as intense without global warming [3] and thus it is urgent to understand processes driving ARs and their impacts using observations and improve their representation in the models used for weather forecasts and for future climate projections. One of the most worrying impacts is the increasing frequency of rain both during summer and winter seasons over the AP, which can drastically change surface energy and mass balance as well as impact fragile ecosystems. Understanding processes driving the transitions from snowfall to rainfall in time, location and in vertical profile - during all-weather events and particularly during ARs - is one of our key goals for collecting precipitation and radiosonde measurements at King Sejong station on King George Island, north of the AP. Since February 2023, we have been operating MRR-PRO, a 24-GHz vertically profiling precipitation radar from which we can derive effective reflectivity, Doppler velocity, melting layer height and precipitation rates. King Sejong is also equipped with automatic weather stations providing near-surface meteorological parameters, broadband surface radiation, precipitation-gauge measurements and snow height. Cloud lidar measurements using miniMPL at Escudero station are available via NASA’s MPLnet [4]. Here we present the evolution of ARs and associated snowfall and rainfall properties during two years of observations (2023-2024). The spatial distribution of precipitation from ERA5 and high-resolution Polar-WRF simulations for specific events demonstrates a transition from rainfall in the northern AP to snowfall in its southern part with significant orographic enhancement over the western upwind side of the AP. Vertical profiling with MRR at King Sejong shows significant variability in the melting layer attaining higher altitudes (up to 3km) during AR events. Combining MRR and radiosonde observations during a selected AR in February 2024 showed strong temperature inversions in the first 3 km with a melting layer varying in height between 3 km and near surface, accompanied by a sharp transition from snowfall to rain. Observations are used to evaluate representation of precipitation in ERA5 and in Polar WRF.

Acknowledgements: We thank FCT (projects MAPS/MICROANT); PROPOLAR; KOPRI; ANR (project ARCA).

References:

[1] Wille, J.D., et al. (2019): West Antarctic surface melt triggered by atmospheric rivers. Nat. Geosci.

[2] Gorodetskaya et al. (2023): Record-high Antarctic Peninsula temperatures and surface melt in February 2022: a compound event with an intense atmospheric river. npj Clim Atmos Sci.

[3] González-Herrero et al. (2022): Climate warming amplified the 2020 record-breaking heatwave in the Antarctic Peninsula. Commun. Earth Env..

[4] Rowe et al. (2025) Observations of Clouds and Radiation Over King George Island and Implications for the Southern Ocean and Antarctica, JGR, in review.