SMAP Salinity V4 Validated Release

SMAP sea surface salinity (SSS) V4.0 validated data are available from Remote Sensing Systems. The near-polar orbit of SMAP allows for complete global coverage of the oceans in 3 days with a repeat cycle of 8 days. The SMAP SSS V4.0 validated release contains 4 products:

  • Level 2C swath data
  • Level 3 8-day running averages
  • Level 3 monthly averages
  • Level 3 monthly (RF) rain-filtered averages

The major achievement in going from Version 2.0 to Version 3.0 was consistency with the Aquarius Version 5 end of mission release (Meissner et al. 2017, 2018). SMAP V3.0 SSS also saw a reduction of spurious temporal and zonal biases over the open ocean that had been observed in the SMAP Version 2.0 release. Consistency with Aquarius Version 5 and the reduction of spurious temporal and zonal biases over the open ocean remain consistent in the SMAP Version 4.0 release.

The major change in Version 4.0 from Version 3.0 is an improved land correction, which allows for SMAP salinity retrievals closer to the coast.

Unlike in prior releases, the 40-km and 70-km fields in V4.0 have been consolidated as variables within a single data product rather than being made available as distinct datasets. The 70-km data are based on a smoothing of the 40-km product, which is retrieved directly from the geophysical model (see Meissner et al. 2018). The 70-km version should be considered the official product with the 40-km included for experimental purposes. For most open ocean applications, the 70-km fields are the best used since they have significantly lower noise than the 40-km data.

All files are in netCDF4 format and are CF/ACDD compliant.

For detailed documentation see:


Version Update Notes:

MAJOR UPDATES IN THE V4.0 RELEASE:

  1. Improved land correction: The land tables in V4.0 are derived at 1/8º resolution. In V3.0 the spatial resolution of the land tables resolution had been 1/2º. The land surface TB that is used in the derivation of the land tables in V4.0 is based on a monthly climatology of SMAP land TB measurements. In V3.0, the land surface TB was based on a land surface emission model.
  2. The sea-ice mask in V4.0 is taken from the RSS AMSR-2 sea-ice maps. In V3.0, the sea-ice mask was from NCEP. The threshold for sea-ice exclusion in V4.0 has been changed to gice > 0.003. This threshold was gice > 0.001 in V3.0.
  3. The sun-glint flag has been revised. In V4.0, the sun-glint exclusion is based on sun-glint angle and surface wind speed. The V4.0 sun-glint flag excludes less data then the V3.0 flag did.
  4. The Version 4.0 salinity retrieval algorithm is run solely on the 0.25º Earth grid using the 40-km spatial resolution Backus Gilbert Optimum Interpolation (OI). The resulting salinity product is called sss_smap_40km. From this 40-km product, a smoothed product with a spatial resolution of approximately 70km (called sss_smap) is derived using simple next-neighbor averaging. This smoothed 70-km sss_smap is to be regarded as the default (standard) salinity product. In Version 4, both sss_smap and sss_smap_40km are provided in the same file.

MAJOR UPDATES IN THE V3.0 RELEASE:

  1. Use of Version 4 L1B SMAP RFI filtered antenna temperatures.
  2. Implementation of the geophysical model function from Aquarius Version 5 adapted to SMAP (Meissner et al. 2018).
  3. Use of the near real time CCMP wind speed and direction data as ancillary input.
  4. Inclusion of IMERG rain rate for the atmospheric liquid cloud water correction and rain flagging.
  5. Improved computation of antenna weighted land fraction (gland) and enhanced correction for land radiation intrusion from antenna sidelobes.
  6. Improved correction for emissive SMAP mesh antenna.
  7. An error in the computation of the gain-weighted sea ice fraction (gice) during 2017 and 2018 in V2.0 has been corrected.
  8. The salty biases at low latitudes and the fresh biases at high N latitudes that were observed in the V2.0 release, have disappeared or are much reduced in V3.0.

KNOWN ISSUES:

  1. We observe a significant degradation in retrieval performance during the early months of the SMAP mission (APR 2015 – AUG 2015). This might be related to instrument calibration, but the exact cause is currently unknown and needs further investigation.
  2. There are several days where the ancillary RSS AMSR2 sea-ice mask is incomplete. In order to avoid possible contamination from sea-ice, no SMAP salinity retrievals are done in these cases. Consequently, the L3 products that include data from these dates contain reduced number of observations in the time average. The following dates have been identified as being affected by this issue:

    Year

    Month

    Day

    Day of year

    2015

    12

    3

    337

    2015

    12

    4

    338

    2016

    4

    15

    106

    2016

    4

    16

    107

    2017

    9

    27

    270

    2017

    9

    28

    271

    2017

    11

    25

    329

    2018

    12

    16

    350

MISSING DATA:

REFRENCES:

Meissner, T., F.J. Wentz, and D.M. Le Vine, 2018, The Salinity Retrieval Algorithms for the NASA Aquarius Version 5 and SMAP Version 3 Releases, Remote Sensing 10, 1121, doi:10.3390/rs10071121.

HOW TO CITE THE RSS SMAP SALINITY DATA:

As a condition of using these data, we require you to use the following citation:
Meissner, T., F. J. Wentz, A. Manaster, R. Lindsley, 2019: Remote Sensing Systems SMAP Ocean Surface Salinities [Level 2C, Level 3 Running 8-day, Level 3 Monthly], Version 4.0 validated release. Remote Sensing Systems, Santa Rosa, CA, USA. Available online at www.remss.com/missions/smap, doi: 10.5067/SMP40-xxxxx. In the doi, the string xxxxx is:

  1. 2SOCS for the L2C files.
  2. 3SPCS for the L3 8-day running maps.
  3. 3SMCS for the L3 monthly maps.


Continued production of this data set requires support from NASA. We need you to be sure to cite these data when used in your publications so that we can demonstrate the value of this data set to the scientific community. Please include the following statement in the acknowledgement section of your paper:

"SMAP salinity data are produced by Remote Sensing Systems and sponsored by the NASA Ocean Salinity Science Team. Data are available at www.remss.com."