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Data Set ID:
AQ3_SSS

Aquarius L3 Weekly Polar-Gridded Sea Surface Salinity, Version 5

This data set consists of weekly gridded Level-3 products of Aquarius L-band radiometer Sea Surface Salinity (SSS) retrievals from the Aquarius/Satélite de Aplicaciones Científicas (SAC-D) mission, developed collaboratively between the U.S. National Aeronautics and Space Administration (NASA) and Argentina's space agency, Comisión Nacional de Actividades Espaciales (CONAE).

Version Summary:

Version 5 of Aquarius L3 Weekly Polar-Gridded Sea Surface Salinity utilizes Version 4 of the Level-2 Aquarius SSS as input data.

Geographic Coverage

Parameter(s):
  • Sea Ice > Sea Ice Concentration > Ice Fraction
  • Salinity/Density > Salinity > Sea Surface Salinity
Spatial Coverage:
  • N: -50, S: -79, E: 180, W: -180

  • N: 87.4, S: 50, E: 180, W: -180

Spatial Resolution:
  • 36 km x 36 km
Temporal Coverage:
  • 25 August 2011 to 28 May 2015
Temporal Resolution: 7 day
Data Format(s):
  • HDF5
Platform(s) SAC-D
Sensor(s): AQUARIUS_RADIOMETER
Version: V5
Data Contributor(s): Ludovic Brucker, Emmanuel Dinnat, Lora Koenig
Data Citation

As a condition of using these data, you must cite the use of this data set using the following citation. For more information, see our Use and Copyright Web page.

Brucker, L., E. Dinnat, and L. Koenig. 2015. Aquarius L3 Weekly Polar-Gridded Sea Surface Salinity, Version 5. [Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: http://dx.doi.org/10.5067/Aquarius/AQ3_SSS.005. [Date Accessed].

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Detailed Data Description

The data set consists of weekly gridded products of L-band (frequency ~1.4 GHz) radiometer Sea Surface Salinity (SSS) from the Aquarius/SAC-D mission. This product contains the average SSS retrieved from all three Aquarius radiometers. This is possible because SSS retrievals are independent of the beam (i.e. of the incidence angle) Each file contains the weekly gridded SSS, ice fraction, and associated standard deviations of the ascending orbits, descending orbits, and the two orbit types combined.

The term ice fraction refers to estimated sea ice concentration integrated over the sensor field of view and weighted by the antenna gain patterns. Similarly, land fraction data is integrated over the sensor field of view and weighted by the antenna gain patterns. The land fraction, distributed as an ancillary file, was obtained averaging all values per grid cell for the entire Aquarius period (August 2011 – June 2015), and keeping only the results when more than 35 values existed per grid cell.

This data set is designed for the monitoring of the polar oceans. For more advanced or detailed studies, SSS retrieved independently by each Aquarius radiometer is available at Aquarius Level-3 Weekly Polar-Gridded Brightness Temperature and Sea Surface Salinity.

Format

The data files are distributed in HDF5 format.

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File and Directory Structure

Data are available on the HTTPS site in the https://n5eil01u.ecs.nsidc.org/AQUARIUS/AQ3_SSS.005/ directory. Within this directory, the folders are organized by date, for example /2011.08.25/ to /2015.05.28/.

The three highest level directories in the HDF5 files contain the weekly-gridded data for all orbits, ascending orbits, and descending orbits as shown in Figure 1.

file structure
Figure 1. File Structure.

Ancillary data files include:

Northern hemisphere coordinates and land fraction of the Aquarius weekly polar-gridded product on the EASE-Grid 2.0 grid at 36 km resolution Coordinates_LandFraction_EASE2_36km_NH_v05.h5.

Southern hemisphere coordinates and land fraction of the Aquarius weekly polar-gridded product on the EASE-Grid 2.0 grid at 36 km resolution Coordinates_LandFraction_EASE2_36km_SH_v05.h5.

List of Aquarius orbit cycle numbers with the corresponding dates of cycle start and end. Cycle_Date.dat.

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File Naming Convention

Data files are named according to the following conventions and as described in Table 1:

Example file name:
SSS3b_ICEF_Aquarius_EASE2_36km_NH_20110825_20110901_001_v05.h5

SSS3b_ICEF_Aquarius_EASE2_36km_xH_YYYYMMDD_YYYYMMDD_CCL_vXX.h5

Where:

Table 1. File Naming Convention
Variable Description
SSS3b_ICEF_Aquarius Sea Surface Salinity 3 beam, Ice Fraction Aquarius data
EASE2_36km Version 2.0 Equal-Area Scalable Earth Grid cells at 36 km x 36 km
xH Hemisphere. NH = Northern Hemisphere; SH = Southern Hemisphere
YYYY Four-digit year of the first/last measurements in the given cycle
MM Two-digit month of the first/last measurements in the given cycle
DD Two-digit day of the first/last measurements in the given cycle
CCL Aquarius orbit cycle number, 3 digits
vXX Polar-gridded product version number.
.h5 Indicates HDF5 file format
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File Size

HDF files range from approximately 193 KB to 480 KB.

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Spatial Coverage

Northern Hemisphere: 
Southernmost Latitude: 50° N
Northernmost Latitude: 87.4° N
Westernmost Longitude: 180° W
Easternmost Longitude: 180° E

Southern Hemisphere: 
Southernmost Latitude: 79° S
Northernmost Latitude: 50° S
Westernmost Longitude: 180° W
Easternmost Longitude: 180° E

Spatial Resolution

The native spatial resolution of the radiometer footprint is 390 km (total crosstrack of three beams of 74 km along track x 94 km cross track, 84 x 120 km, and 96 x 156 km). Data are then gridded using the 36 km EASE-Grid 2.0 global projection.

Projection and Grid Description

The data are gridded to the Equal-Area Scalable Earth version 2.0 grid (Brodzik et al. 2012), with a grid cell resolution of 36 km. See also EASE-Grid 2.0 Format Description.

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Temporal Coverage

25 August 2011 to 28 May 2015.

Due to a power failure on the Satélite de Aplicaciones Científicas (SAC)-D spacecraft on 08 June 2015, data from NASA's Aquarius instrument are no longer being produced. For more information on this event, please refer to the official NASA announcement. The NASA National Snow and Ice Data Center Distributed Active Archive Center (NSIDC DAAC) will continue to distribute Aquarius soil moisture and polar-gridded data sets for the full duration of the mission, 25 August 2011 to 07 June 2015.

Temporal Resolution

Weekly averages

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Parameter or Variable

Aquarius Level-3 Weekly Polar-Gridded Sea Surface Salinity all orbit, ascending orbit, and descending orbit data parameters are described in Table 2.

Table 2. Parameters
Name Description Units
SSS3b Sea surface salinity (3 beam average) Practical Salinity Unit (psu)
SSS3b_STD Standard deviation of the sea surface salinity Practical Salinity Unit (psu)
ICEF_SSS3b Radiometer ice fraction n/a
ICEF_SSS3b_SDT Standard deviation of the radiometer ice fraction n/a
NFP_SSS3b Number of footprint measurements for radiometric retrievals n/a

The statistical significance of the standard deviation should be evaluated against the Number of Foot Print (NFP) measurements in the grid cell.

Sample Data Record

Figure 2 shows an image of Aquarius weekly polar-gridded SSS using retrievals from all three beams and all orbits in the Northern Hemisphere latitudes north of 50° N during September 26 - October 3, 2013.

Figure 2. Sample image of Version 4 SSS Northern Hemisphere data

Figure 3 shows an image of Aquarius weekly polar-gridded SSS using retrievals from all three beams and all orbits in the Southern Hemisphere latitudes south of 50° S during September 26 - October 3, 2013.

Figure 3. Sample image of Version SSS Southern Hemisphere data
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Software and Tools

MATLAB readers: For loading and mapping Aquarius Level-3 weekly polar-gridded radiometer and scatterometer data. Note: this tool was provided by the Principal Investigator "as-is" as a service to the user community in the hope that it will be useful. Please note that support for the program is limited. Bug reports, comments, and suggestions for improvement are welcome; please send to nsidc@nsidc.org.

The following external links provide access to software for reading and viewing HDF5 data files. Please be sure to review instructions on installing and running the programs.

HDFView: Visual tool for browsing and editing HDF4 and HDF5 files.

Panoply netCDF, HDF and GRIB Data Viewer: Cross-platform application. Plots geo-gridded arrays from netCDF, HDF and GRIB data sets.

For additional tools, see the HDF-EOS Tools and Information Center.

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Data Acquisition and Processing

Data Acquisition Methods

The following information is extracted from Brucker et al. 2014a, and Brucker et al. 2014b.

Aquarius Level-2 data used are distributed by NASA's Physical Oceanography Distributed Active Archive Center (PO.DAAC). The PO.DAAC Aquarius Level-2 product consists of the observations, retrievals, and ancillary data along the swath, including Brightness Temperature (TB), Normalized Radar Cross Section (NRCS), SSS, and ICEF. Neither new algorithms nor new observation processing were done. For further information on the Aquarius Level-2 data, see the Aquarius User Guide.

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Derivation Techniques and Algorithms

The Level-3 weekly polar-gridded SSS data are derived from the Aquarius Level-2 data and repackaged as weekly polar-gridded Level-3 products of SSS. Derivation occurs only in that Radio Frequency Interference (RFI) contaminated observations are excluded and a weekly average is calculated per EASE-Grid 2.0 cell.

Gridded data were produced using all the Aquarius SSS retrievals observations flagged as RFI free, and obtained during nominal operation of the spacecraft.

SSS retrievals at latitudes greater than 50 degrees in both hemispheres are averaged and gridded into a weekly product.

According to the orbit and sensor characteristics, the temporal resolution of the product was set to one week, corresponding to the time of revisit. Since the Aquarius sensors are in a push-broom alignment, a weekly-gridded product provides the largest spatial coverage.

SSS retrievals where the radiometer land fraction was ≥0.25 were not considered. If left uncorrected, a land fraction of as little as 0.001 can be detrimental to the SSS retrieval (~0.2 – 0.4 psu error; Dinnat and Le Vine, 2007). However, there is a first order correction for the effect of land contamination in the PO.DAAC Level-2 SSS retrieval that should remove most of the error (Wentz and Le Vine, 2012). In addition, the level of acceptable residual error in coastal studies will depend on specific applications of the product. For these reasons, the threshold for land contamination was set relatively high (Brucker et al. 2014).

A linear interpolation of the SSS averaged using all beams and orbits was applied in grid cells without observations during the cycle. However, where 25 or more continuous grid cells had no data, the interpolation was not applied, and a NaN value is provided.

The PO.DAAC Level-2 TB product is computed after empirical calibration of the measured antenna temperatures against a forward radiative transfer model over ocean surfaces (Le Vine et al. 2011a). More details about the Level-2 processing can be found in Wentz and Le Vine (2012)Le Vine et al. (2012), and Piepmeier et al. 2013.

The ICEF results from a combination of two elements: estimated sea ice concentration, and antenna characteristics. Sea ice concentration estimates are obtained from the analysis by NOAA's Marine Modeling and Analysis Branch (http://polar.ncep.noaa.gov/seaice/Analyses.shtml), available daily at a spatial resolution of 1/12 degrees, and distributed by the U.S. National Centers for Environmental Prediction (NCEP) as the Global Forecast System (GFS) Global Data Assimilation System (GDAS) sea ice product.

Processing Steps

Using the latest PO.DAAC Aquarius Level-2 data version 4.0:

  • Select data at latitudes greater than 50 degrees.
  • Reject data contaminated by RFI.
  • Reject data collected during spacecraft maneuvers or anomaly periods as reported on the Aquarius status Web page.
  • Grid and average data per seven-day cycle corresponding to the spacecraft time of revisit.
  • Where less than 25 contiguous grid cells do not have data, apply a Delaunay triangulation with linear interpolation to the weekly-gridded values to spatially interpolate them in grid cells without observations/retrievals during the cycle.
  • For a given one-week orbit cycle, all measurements within a same grid cell were averaged together, and the standard deviation calculated.

Version History

Version 5 of Aquarius L3 Weekly Polar-Gridded Sea Surface Salinity utilizes Version 4 of the Level-2 Aquarius SSS as input data.

Error Sources

The antenna temperatures are corrected for:

  • the emission of extraterrestrial sources (Sun, Moon, Celestial Sky), that directly reaches the antenna through the side and back lobes
  • the effect of the integration over the antenna gain patterns
  • the Faraday rotation
  • and atmospheric effects (upward emission, downward emission reflected at the surface, and attenuation of the signals from the surface)

These corrections provide the TBs at the Earth's surface. Over the oceans, the TB is corrected for the effects of surface roughness, including the reflected/scattered galaxy and Sun, using the scatterometer observations and a simulated wind speed. The remaining TB for a smooth surface is converted into SSS using ancillary data for the sea surface temperature, and a model for the sea water dielectric constant.

The requirement for the Aquarius SSS retrievals accuracy is 0.2 psu after temporal averaging over a month in global open oceans. While the algorithm used in the Aquarius Level-2 processing for retrieving SSS performs well in the tropics and mid latitudes (warmer) oceans (Lagerloef et al. 2013), L-band SSS retrieval in the polar (colder) oceans is challenging. SSS retrievals have not yet been specifically validated in cold water, and should be used with caution. L-band observations are less sensitive to salinity in cold waters. In addition, salinity retrieval is likely to be less accurate for very rough sea surfaces. For instance, in the Southern Ocean there are strong winds and the oceanic circulation is dominated by the Antarctic Circumpolar Current, which could reduce the quality of the SSS retrievals. Finally, the presence of sea ice and icebergs in the sensors field of view adds complexity to the retrieval of SSS in the high latitudes.

The weekly-gridded product contains SSS retrievals in ascending orbit and descending orbit. This distinction between the two orbit types may be required because differences have been identified (Lagerloef et al., 2013). The Aquarius Level-2 product version 3.0 has reduced the differences, though not completely eliminated them. While the origin of these differences has not been established yet, it is likely to be due in part to the reflected/scattered galaxy, and RFI contaminations. It is possible that residual RFI and sky contaminations impact the empirical calibration performed over the oceans, and therefore create biases dependent on the type of orbit. The correction for the reflected galaxy has been found insufficient, and an empirical adjustment was introduced for the source data from Level-2 version 3.0, and carried forward to the version 4.0 source data used for this current product. The galaxy contamination is very dependent on the type of orbit, because it is only significant when the contribution comes from a very limited region of the sky, for example the galactic plane.

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Sensor or Instrument Description

Aquarius/SAC-D is a collaboration between NASA and Argentina's space agency, Comisión Nacional de Actividades Espaciales (CONAE), with participation from Brazil, Canada, France and Italy. The Aquarius instrument was built jointly by NASA's Jet Propulsion Laboratory and NASA's Goddard Space Flight Center.

The Aquarius instrument includes three radiometers and one scatterometer. The SSS data are collected by the radiometers. The radiometers measure brightness temperature at 1.414 GHz in the horizontal and vertical polarizations (TBH and TBV). The scatterometer is a microwave radar sensor that measures backscatter for surface roughness corrections. A product containing the scatterometer observations (Normalized Radar Cross Section, NRCS) is available at Aquarius Level-3 Weekly Polar-Gridded Normalized Radar Cross Section.

Table 3 summarizes instrument characteristics.

Table 3. Aquarius Instrument Characteristics
Instrument Characteristics
3 radiometers in push-broom alignment
  • Frequency: 1.413 GHz (L-band)
  • Band width: less than or equal to 26 MHz
  • Swath Width: 390 km
  • Science data block period: 1.44 sec
  • Footprints for the beams are: 74 km along track x 94 km cross track, 84 x 120 km, and 96 x 156 km, yielding a total cross track of 390 km.
  • Beam incidence angles of 29.36, 38.49, and 46.29 degrees incident to the surface. Beams point away from the sun.
Scatterometer
  • Frequency: 1.26 GHz
  • Band Width: 4 MHz
  • Swath Width: 390 km
  • Science data block period: 1.44 sec

SAC-D spacecraft Orbit Parameters:

  • 98 minute sun-synchronous
  • 6 PM ascending orbit, 6 AM descending orbit
  • 657 km equatorial altitude (655 km minimum, 685 km maximum over the orbit)
  • Ground-track repeat interval: Weekly
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References and Related Publications

Contacts and Acknowledgments

Ludovic Brucker, Emmanuel Dinnat, and Lora Koenig
NASA Goddard Space Flight Center
Cryospheric Sciences Laboratory, Code 615
Greenbelt, MD 20771

Document Information

DOCUMENT CREATION DATE

21 July 2014

DOCUMENT REVISION DATE

14 December 2015

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