Aquarius for the polar regions: a new gridded product and its analysis over the cryosphere

Microwave radiometers used to monitor the Earth's polar regions typically operate in the frequency range 6-150 GHz. Recent radiometers, like those onboard SMOS and Aquarius/SAC-D spacecrafts, provide measurements at a lower frequency (~1.4 GHz, L-band), bringing new capabilities to monitor the state of the ice sheets, sea ice cover, and polar oceans. We present a gridded weekly product of Aquarius measured brightness temperature (TB) and backscatter, and of retrieved Sea Surface Salinity (SSS), for the northern and southern high latitudes. This product, specifically designed for the polar regions, is distributed on the Equal-Area Scalable Earth Grid (EASE2.0) at 36-km resolution. This data set aims to increase the use of Aquarius measurements for cryospheric applications, and to improve our understanding of L-band measurements of ice sheet and sea ice. We describe it with a focus on the Greenland and Antarctic ice sheets. We also highlight the influence of the azimuth angle (~1 K for a 1.5o angle variation), and the variation within a grid cell (up to 1.5 K in locations where measurements are made 25+ times per one-week orbit cycle). This knowledge is of interest for geophysical property retrievals, and satellite intercalibration. In addition, we present an analysis of Aquarius measurements over the Antarctic Plateau, a potential target for intercalibration of spaceborne L-band radiometers. At Dome C, the mean annual TB is 181.2×0.7 K and 209.4×0.3 K for beam 3 at horizontal and vertical polarizations, respectively. While the annual standard deviation appears small, it is higher than the sensor accuracy of 0.2 K, especially at horizontal polarization. A careful analysis of the TB variations reveals an interesting correlation with the presence/absence of surface hoar (large grains) identified with autonomous daily infrared photographs of the snow surface. An additional correlation was found with the grain index retrieved from a combination of high microwave frequencies (89&150 GHz) recorded by AMSU-B. These results are important because they emphasize that part of the L-band measurement variability is explained by surface snow metamorphism. Therefore, despite the fact that L-band radiation has a deep penetration into the ice sheet, the horizontal polarization remains noticeably sensitive to surface snow properties, evolving quickly with atmospheric forcing. Aquarius SSS data are also examined to identify ocean freshening related to Greenland ice sheet melt water. Our investigations reveal off-shore Greenland SSS variations in agreement with the ice sheet melting period. Satellite SSS retrievals are examined in conjunction with buoy and ship measurements, and oceanic simulations. Of note, Aquarius retrievals are sensitive to the presence of sea ice in the field of view, requiring a cautious interpretation of derived SSS.