Inter-Comparison of Aquarius and SMOS Calibration and Impact on Sea Surface Salinity Products
Abstract:NASA’s Aquarius/SAC-D and ESA’s Soil Moisture and Ocean Salinity (SMOS) missions have the objective of mapping global sea surface salinity (SSS) with an accuracy of 0.2 psu. They use L-band radiometers, fly on a 6 AM/PM sun synchronous orbit, and cover the globe in about a week. SMOS radiometers are calibrated against an internal warm target and deep space measurements at the cold end. Aquarius is calibrated against an internal warm target and a forward radiative transfer model (RTM) averaged over the global ocean weekly. Aquarius data are also validated looking at deep space once a month. Because large biases have been observed in the SMOS data over oceans, an additional correction (Ocean Target Transformation, OTT) is applied to radiometric measurements by comparison with an RTM similar to the one used for Aquarius. RTMs are therefore critical to both missions’ scientific objectives. There are some significant differences in the RTM used by SMOS and Aquarius, and this can lead to discrepancies in calibration between both instruments. Because it is desirable to use both datasets together to study salinity, and to use the brightness temperatures (TB) to study land and the cryosphere, the differences in calibration need to be identified. Also, in the very near future (in 2015), one expects to have an additional L-band dataset from NASA’s Soil Moisture Active-Passive (SMAP) mission.
We assess the impact of differences in the RTM and the way it is applied. We find that differences in sea water dielectric constant models result in differences in TB of the order of 0.2 K. Matching the dielectric constant models for both missions reduces differences in SSS by up to 0.4 psu, with most improvements in the range of temperatures 6°C to 17°C. Differences in the ancillary data used in the RTM differ can also be important. For example, Aquarius uses ancillary SSS from a model (HYCOM) and SMOS uses a climatology, leading to differences up to 0.1 psu over the area of the south Pacific Ocean used by SMOS for its calibration. The ancillary SST is also different. The differences are small globally, but exhibit localized differences of a few degrees. Because TB sensitivity to SST is usually small, most of the SST differences have a very small impact. However, in the cold waters of the high latitudes, SST differences lead to noticeable local SSS discrepancies.