Meetings: Documents

Salinity Remote Sensing from SMAP
[25-May-17] Meissner, T., Wentz, F., Manaster, A., and Lee, T.
Presented at the Global Ocean Salinity and the Water Cycle Workshop
Remote Sensing System’s (RSS) SMAP Version 2 sea surface salinity (SSS) data have been released on September 13, 2016. The release contains a Level 2 swath product and Level 3 maps of 8-day running averages and monthly averages. Our talk discusses the major steps of the SMAP salinity retrieval algorithm, including updates and improvements from the Version 1 (BETA release). Though designed for measuring soil moisture, the SMAP radiometer has excellent capabilities to retrieve SSS with a similar accuracy as Aquarius. However, the calibration accuracy of the SMAP brightness temperatures on which the SMAP soil moisture product is based, is itself not sufficient for retrieving SSS, but additional steps need to be taken. These steps will be discussed in our presentation. Other than Aquarius, the SMAP antenna is slightly emissive. The value of the emissivity is approximately 1%, which is 4 times as large as anticipated from ground calibration. This 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 necessary to develop a correction for this spurious emissivity signal. 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 F17 SSMIS for correcting the surface roughness effect. The full 360-deg look 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 from different directions and thus determine the size of these spurious contamination signals. We will provide validation results for the RSS SMAP salinity against ground truth measurements from ARGO drifters. Finally, we will present our plans for upcoming SMAP SSS releases. After the algorithm for the Aquarius Version 5 release has been finalized, we need to make the retrieval algorithms for SMAP and Aquarius as consistent as possible. The correction for emission from land surfaces that is currently used in SMAP Version 2 needs to be improved. We also plan to include uncertainty estimates with all of the SMAP SSS retrievals. SMAP has a much better filter for Radio Frequency Interference (RFI) than Aquarius or SMOS had. Nevertheless, likely intrusion of undetected RFI is observed in the Western Pacific near China, Korea and Japan and in the Gulf of Bengal, which manifests in fresh biases on the SMAP SSS in these regions. The RFI intrusion varies with looking direction. A mitigation for this undetected RFI can involve analyzing and comparing the SMAP SSS that are observed from different look directions.

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