Ocean Surface Salinity from the SMAP Sensor

With the demise of Aquarius the ability of the SMAP sensor to measure ocean salinity has gained importance. Our presentation discusses the adaption of the Aquarius salinity retrieval algorithm to SMAP. It includes corrections for signals from the rough ocean surface, the atmosphere and ionosphere, cold space, galaxy, sun, moon as well as sidelobe and cross polarization effects from the SMAP antenna. Our presentation will also address several important differences between the Aquarius and SMAP sensors that impact the ocean salinity retrieval algorithm and its performance: 1. The full 360olook of SMAP makes it possible to take observations from the forward and backward looking direction basically at the same instance of time. This two-look capability strongly aids the salinity retrievals. It is possible to observe some of the spurious contamination sources such as the reflected galaxy or the reflected sun from different directions. 2. The spatial resolution of a SMAP radiometer measurement is about 40 km compared with 100 km for Aquarius. This higher resolution is associated with a higher noise for SMAP. Performing weekly or monthly time averaging is essential to reduce the noise in order to still achieve the accuracy goal of 0.2 psu. The 1000 km wide swath and 360oscan make this possible. 3. Due to the demise of the SMAP radar, SMAP does not provide valuable L-band scatterometer wind speeds at the same location and time as the radiometer observation as Aquarius did. Therefore the SMAP salinity retrieval algorithm needs to use wind speeds from WindSat and SSMIS for correcting the surface roughness effect. 4. Other than Aquarius the SMAP antenna is slightly emissive. The emissivity of 1% causes significant spurious biases in the SMAP salinity data that correlate with the physical temperature of the antenna, which depends on solar heating. It is therefore necessary to develop a correction for this emissivity signal.