Effect of Snow Surface Metamorphism on Aquarius L-Band Radiometer Observations at Dome C, Antarctica

The Antarctic Plateau presents ideal characteristics to study the relationship between microwave observations and snow/ice properties. It is also a promising target for radiometer calibration and sensor intercalibration, which are critical for applications requiring subkelvin accuracy, such as sea surface salinity retrievals. This paper presents the spaceborne Aquarius L-band radiometric observations collected since August 2011 over the Antarctic Plateau, and it focuses on their temporal evolutions at Dome C (75.1° S, 123.35° E). Aquarius operates three radiometers with a sensitivity of 0.15 K (over the oceans), allowing us to analyze small variations in brightness temperature (TB) and changes with incidence angles. Over the Antarctic Plateau, Aquarius TBs have a relatively low annual standard deviation (0.2-0.9 K) where melting never occurs. However, the analysis of the TB time series at Dome C revealed significant variations (up to 2.5 K) in summer. First, these variations are compared with a remote sensing grain index (GI) based on high-frequency (89 and 150 GHz) shallow-penetration TB channels. Variations in the ratio of TBs observed at horizontal and vertical polarizations are synchronous with GI changes. Second, Aquarius TB variations are compared with the presence of hoar crystals on the surface identified using surface-based near-infrared photographs. The largest and longest changes in TBs correspond to periods with hoar crystals on the surface. Therefore, in spite of the deep penetration of the L-band radiation, evolutions of the snow properties near the surface, which usually change rapidly and irregularly, do influence L-band observations. Collection of accurate snow surface measurements and thorough analyses of the L-band observations are thus needed to use the Antarctic Plateau as a calibration/inter-calibration target.