Extreme increases in snow grain size on the Antarctic Plateau from Satellite Observations and Ice Sheet-Atmosphere Interactions

Grain size variations impact the albedo and have consequence for the energy budget of the surface.  The snow grain size in Antarctica follows a clear seasonal pattern: a summer increase and a winter decrease, which are conditioned by atmospheric processes —namely temperature, wind, snowfall— and by mechanisms acting inside the snowpack leading to water vapour transport thus causing the coarsening of the grains. This study focuses on the evolution of the grain size in the interior part of East Antarctica, where dry metamorphism occurs, by using satellite observations. For this, we use, as proxy for the snow grain size, the Grain Size Index (GSI) inferred from the 89 and 150 GHz radiometer observations collected by the Advanced Microwave Sounding Unit-B (AMSU-B) from 2000 to 2022. Four extreme increase in GSI have been identified over the Antarctic Plateau, along the highest ice divide. In these cases, the ERA5 reanalysis revealed an atmospheric blocking/ridge situation around the onsets of the summer growing of the grain size, conveying the relatively warm and moist air coming from the mid latitudes, often associated with atmospheric rivers. The snow dry metamorphism is facilitated conditions of weak wind, low temperature and low snowfall conditions during the following weeks, leading to grain growth. These conditions determine anomalous high value of the snow grain size at the end of summer. Theoretical analysis have been performed to investigate in detail the extreme snow grain size event happened near Dome Fuji during the summer 2019-2020. The simulations of the AMSU-B observations  confirm that this extreme variation is mainly related to an increase in snow grain size. Results also highlighted  a decrease in snow density during this event. This is supported by independent satellite observations at 1.4 and 36 GHz (from Soil Moisture and Ocean Salinity SMOS and Advanced Microwave Scanning Radiometer 2 AMSR-2, respectively), which showed synchronized variations related to an unusual change in surface snow density.