Status of Aquarius and Salinity Continuity
Abstract
:1. Introduction
2. Background
3. Results: Aquarius Version 5.0
3.1. Changes in the Retrieval Algorithm
- The ancillary sea surface temperature (SST) field was changed from the National Oceanographic and Atmospheric Administration (NOAA) Optimally Interpolated (OI) SST to the SST field from the Canadian Meteorological Center (CMC);
- The reference sea surface salinity (SSS) field used in the sensor calibration and in the derivation of expected antenna temperature, TA_expected, in the forward algorithm was changed from SSS obtained from the Hybrid Coordinate Ocean Model (HYCOM) to the analyzed monthly Scripps Argo SSS;
- The model for the celestial radiation reflected from the surface into the radiometer antenna was changed to values derived from fore and aft observations of the SMAP radiometer. The advantage of this approach is that it includes the effects of surface roughness;
- The empirical symmetrization correction that corrects asc/dsc differences [1] was re-derived to reflect the improvements that resulted from the improved model for the reflected celestial radiation (above).
- The surface roughness correction was updated (i.e., compared to that described in Meissner, Wentz and Riciardulli [19]):
- ၀
- The SST dependence was adjusted.
- ၀
- The dependence on significant wave height (SWH) was omitted.
- ၀
- In addition, the correction table for dependence on wind speed and radar backscatter was updated, and as a consequence, the initial guess for the SSS field used to derive the final wind speed (i.e., “HHH” wind speed) was also updated.
- Observations at vertical and horizontal polarization are given equal weight in the retrieval of salinity (i.e., in the maximum likelihood estimate used in the last step in the retrieval);
- The L2 files include instantaneous rain rates based on the NOAA rain product, CMORPH (Climate Prediction Center Morphing). They are used to filter data for rain in the calibration and also for validating the Aquarius salinity versus in situ measurements.
3.2. Evaluation of the Version 5.0 Salinity
3.3. Work Remaining to Improve the Salinity Product
- Determining the physical reason for an SST-dependent bias (which is empirically removed in Version 5; See Section 4.3 below);
- Identifying and correcting a remaining small annual cycle (not due to changes in salinity);
- Merging Aquarius, SMOS, and SMAP salinity maps into a single product;
- Improving the theory for the effect of surface roughness on emission and the correction for the reflection of signals such as the galactic background;
- Improving the level of missed detection in the RFI algorithm;
- Improving the performance in cold water;
- Addressing regional biases (e.g., North Pacific and southern Indian Ocean);
- Improving calibration over the full range of expected targets (i.e., cold sky, ocean, and land).
4. Discussion: Remaining Issues
4.1. Background
4.1.1. Calibration
4.1.2. Retrieval of Salinity
4.2. Example Issue: Drift and Wiggles
4.3. Example Issue: SST Dependence
4.4. Example Issue: Whole Range Calibration
4.4.1. Introduction
4.4.2. Whole Range Calibration V5.WR
- At the cold end, using the difference between the mean of the TA measured by Aquarius for the 30 cold sky calibrations and the mean of the corresponding TA_expected for the cold sky look computed from radiative transport theory;
- Over the ocean, using the mean of the TA measured by Aquarius globally for the year 2012 (filtered for RFI, and with a water fraction of ≥99.9% ) and the mean of the corresponding TA_expected.
5. Conclusions: Future of SSS Remote Sensing
Author Contributions
Funding
Acknowledgments
Conflicts of Interest
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V-Pol | H-Pol | |||
---|---|---|---|---|
Beam 1 | 1.003350568406014 | −3.592857280825468 | 1.007405352181248 | −6.595668007611077 |
Beam 2 | 1.008337848581688 | −9.655610862597533 | 1.003498234086013 | −2.948722716952730 |
Beam 3 | 1.017695610594212 | −2.233911609523380 | 1.001311195384693 | −1.075651639210478 |
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Le Vine, D.M.; Dinnat, E.P.; Meissner, T.; Wentz, F.J.; Kao, H.-Y.; Lagerloef, G.; Lee, T. Status of Aquarius and Salinity Continuity. Remote Sens. 2018, 10, 1585. https://doi.org/10.3390/rs10101585
Le Vine DM, Dinnat EP, Meissner T, Wentz FJ, Kao H-Y, Lagerloef G, Lee T. Status of Aquarius and Salinity Continuity. Remote Sensing. 2018; 10(10):1585. https://doi.org/10.3390/rs10101585
Chicago/Turabian StyleLe Vine, David M., Emmanuel P. Dinnat, Thomas Meissner, Frank J. Wentz, Hsun-Ying Kao, Gary Lagerloef, and Tong Lee. 2018. "Status of Aquarius and Salinity Continuity" Remote Sensing 10, no. 10: 1585. https://doi.org/10.3390/rs10101585