The RSS/NASA SMAP Salinity Version 3 Release: Algorithm and Geophysical Model Function
[27-Aug-2018] Meissner, T., Wentz, F., and Manaster, A.
Presented at the 2018 Ocean Salinity Science Team and Salinity Continuity Processing Meeting
This presentation gives a detailed account of the RSS Version 3 SMAP SSS release, which is scheduled for summer 2018.
We discuss the major steps of the SMAP salinity retrieval algorithm, including updates and improvements from the Version 2 release and we compare with the algorithm of the Aquarius Version 5 release.
The main step in SMAP V3 is the use of the Aquarius V5 geophysical model function, which has been adapted to SMAP. This results in consistent geophysical salinity retrieval algorithms for the two sensors:
- The dielectric model of sea water: Aquarius V5 and SMAP V3 use both the Meissner and Wentz (2012) dielectric model.
- The atmospheric oxygen absorption model: Aquarius V5 and SMAP V3 use both the Liebe (1992) absorption model.
- The SST dependence of the wind induced emissivity model.
- The model for the reflected galaxy: Both Aquarius Version 5 and SMAP Version 3 use the geometric optics model together with empirical results that are based on an analysis of SMAP fore minus aft looks.
- The empirical derivations of the geophysical model functions for Aquarius V5 and SMAP V3 use both the analyzed Scripps ARGO salinity field as reference salinity field. This crucial, because the HYCOM field that has been used in previous releases, exhibits spurious zonal biases compared with ARGO and these spurious biases show up in the SMAP Version 2 release.
SMAP Version 3 also includes IMERG rain rates, which are used in the correction for atmospheric cloud water absorption. They can also be used for rain flagging in the validation to avoid mismatch due salinity stratification in the upper when comparing SMAP (measuring in the upper few cm) with ARGO or model SSS (measuring at 5m depth).
There are important differences between Aquarius and SMAP, which need to be taken into account when developing the SMAP V3 salinity retrieval algorithm.
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 an external ancillary wind speed for correcting the surface roughness effect. SMAP V3 uses wins speed from the near-real time Cross Calibrated Multi Platform (CCMP) Level 4 product. It is based on a variational assimilation of multiple Level satellite winds form the RSS Version 7 Ocean Suite (WindSat, ASMR2, SSMIS, GMI) together with a background field from 0.25° NCEP GDAS. The wind roughness correction of Aquarius V5 had used the Aquarius HHH wind speeds as ancillary input. There are small differences between these two ancillary wind products (Aquarius HHH winds and CCMP winds), which result in small differences between the wind emissivity models in Aquarius V5 and SMAP V3.
SMAP V3 also includes an improved correction for land contamination and is run at two different spatial resolutions (70 km and 40 km). These two aspects will be discussed in the presentation by Frank Wentz.
We will provide validation results for the RSS SMAP salinity against ground truth measurements from ARGO drifters. Most of the zonal and temporal biases that were observed in SMAP Version 2 are removed or significantly mitigated in the new Version 3.