Influence of Surface Snow Properties on an 89-GHz Brightness Temperature Extreme Event at Dome Fuji, Antarctica

Microwave brightness temperatures observed in Antarctica at 89 GHz from the advanced microwave sounding unit B (AMSU-B) point out an exceptional decrease of 57 K at Dome Fuji ( 77.31°S , 39.70°E ) during the 2019–2020 summer. The grain size index (GSI) based on 89 and 150 GHz from AMSU-B and independent observations at 89 GHz from the advanced microwave scanning radiometer 2 (AMSR-2) also show concurrent unusual values. To explain such event, a theoretical analysis was carried out by means of a radiative transfer model. We explore the sensitivity of brightness temperature to surface snow properties focusing on December, just before the decrease, and April, at its end. Results confirm that this variation is mainly related to an increase in snow grain size. A decrease in snow density is also involved as suggested by the increase in brightness temperature at 1.4 GHz from soil moisture and ocean salinity (SMOS) and in the polarization ratio at 36 GHz from AMSR-2. Extreme values observed at multiple frequencies, as well as peculiar atmospheric conditions explored in a previous study, confirm the uniqueness of this event at least on decennial scale.