The Soil Moisture Active Passive (SMAP) platform and mission were launched on January 29, 2015. The onboard instruments began observations soon after in April 2015. Although SMAP was designed to measure soil moisture, its L-band radiometer may also be used to measure sea surface salinity (SSS). With the fatal anomaly of the Aquarius (SAC/D) platform, the ability of the SMAP sensor to also retrieve ocean salinity has gained importance. SMAP now allows us to extend the data record of the highly successful 3-year Aquarius salinity mission into the future.
Using our experience with Aquarius [Meissner et al., 2014a; Meissner et al, 2014b], scientists at Remote Sensing Systems are developing an algorithm that retrieves SMAP radiometer brightness temperature measurements into ocean surface salinity. The SMAP antenna temperatures are provided by the SMAP radiometer team [Piepmeier et al., 2015].
There are several important differences between Aquarius and SMAP that affect the ocean salinity retrievals from SMAP:
1. SMAP observes the Earth in nearly full 360-degree scans, whereas Aquarius looked at the Earth at three fixed angles. This difference results in a 1000 km wide swath from SMAP, compared to the narrow 350 km Aquarius swath, resulting in greater Earth coverage in less time. We also believe that the ability of SMAP to observe the same location at the Earth surface from different directions/viewing angles helps to eliminate various spurious signals that are a function of viewing direction (an-isotropic signatures), such as signals from the sun, galaxy, moon, or the Earth's ionosphere intruding on the antenna.
2. SMAP observations have a footprint size of 40 km which is higher resolution than the 100 km footprint of Aquarius. As a consequence, the SMAP data are noisier than Aquarius, but we find the noise can be reduced by temporal averaging the data over days or weeks.
The animation below shows this type of averaging with SMAP data: 7-day averages of ocean salinity centered on each day from the beginning of April to the beginning of July 2015. There are a couple brief data gaps during which the instrument was turned off. Incomplete maps for the affected days are thus produced.
Several interesting features to note in the salinity maps include the ocean freshwater fluxes visible in the animation:
1. The plume of the Amazon River with its varying outflow.
2. Tropical instability waves of sea surface salinity in the equatorial Pacific and Atlantic.
3. The peak salinity values in the subtropical North and South Atlantic.
The first version of the SMAP ocean surface salinity data set will be released by the NASA Ocean Salinity Science Team later this year (2015).
Brown, D., R. Gran, A. Buis, 2015. International Spacecraft Carrying NASA’s Aquarius Instrument Ends Operations. NASA press release 15-126, June 17, 2015. http://www.nasa.gov/press-release/international-spacecraft-carrying-nasa...
Meissner, T, FJ Wentz, D LeVine, J. Scott, 2014a, Aquarius Salinity Retrieval Algorithm Theoretical Basis Document (ATBD), Addendum 3, report number 060414, Remote Sensing Systems, Santa Rosa, CA, 24 pp.
Meissner, T, FJ Wentz, L Ricciardulli, 2014b, The emission and scattering of L-band microwave radiation from rough ocean surfaces and wind speed measurements from Aquarius, Journal of Geophysical Research: Oceans, 119, doi:10.1002/2014JC009837.
NASA SMAP Handbook, http://smap.jpl.nasa.gov/system/internal_resources/details/original/178_....
Piepmeier, J. R., P. N. Mohammed, J. Peng, E. J. Kim, G. De Amici, and C. Ruf, 2015. SMAP L1B Radiometer Half-Orbit Time-Ordered Brightness Temperatures. [data used: antenna temperatures after RFI mitigation]. Boulder, Colorado USA: NASA National Snow and Ice Data Center Distributed Active Archive Center. http://dx.doi.org/10.5067/1V33MVRRLCCT.