Ocean Products from the SMAP Radiometer: Surface Salinity and Wind Speeds

Abstract:

Though designed to measure soil moisture, the SMAP radiometer is an excellent sensor to measure sea surface salinity and sea surface wind speed. It is possible to retrieve both quantities from passive SMAP observations without using the SMAP radar, whose transmitter likely failed in July 2015.

With the demise of Aquarius the ability of the SMAP sensor to measure ocean salinity has gained importance. The main part of our presentation discusses the adaption of the Aquarius salinity retrieval algorithm to SMAP. It includes corrections for spurious signals coming from cold space, the galaxy, the sun and the moon as well as sidelobe and cross polarization effects from the SMAP antenna. Based on Aquarius observations, we have developed a radiative transfer model that characterizes the surface emission of a wind roughened ocean. Our surface roughness correction for the SMAP salinity retrieval will use match-ups of SMAP radiometer observations and surface wind speeds from WindSat or SSMIS.

Our presentation will also address several important differences between the Aquarius and SMAP sensors that impact the ocean salinity retrievals. The full 360^o look capability of SMAP makes it possible to take observations from the forward and backward looking direction basically at the same instance of time. We expect that this two-look capability will strongly aid the salinity retrievals. It will be possible to observe some of the spurious contamination sources like the reflected galaxy or the reflected sun from two different directions.

Finally, we will address the capability to measure ocean surface wind speed with the SMAP radiometer. As it has been demonstrated with Aquarius and SMOS the L-band passive ocean surface emission exhibits very good sensitivity to surface wind speeds to at least 35 m/s and it is very little affected by precipitation. This allows the retrieval of ocean surface winds, in particular in storms and even under rainy conditions.