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Aquarius L3 Gridded 1-Degree Monthly Soil Moisture, Version 4
This data set contains Level-3 gridded monthly global soil moisture estimates derived from the NASA Aquarius passive microwave radiometer on the Satélite de Aplicaciones Científicas (SAC-D).
Changes in the Version 4 Aquarius Soil Moisture data include: use of the most recent version (Version 4) of Aquarius Brightness Temperatures as input.
|Temporal Resolution:||1 month|
|Data Contributor(s):||Rajat Bindlish, Thomas Jackson|
|Metadata XML:||View Metadata Record|
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.Bindlish, R. and T. Jackson. 2015. Aquarius L3 Gridded 1-Degree Monthly Soil Moisture, Version 4. [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_MOSM.004. [Date Accessed].
Detailed Data Description
The Aquarius L3 Gridded 1-Degree Soil Moisture Data are produced by NASA Goddard Space Flight Center's Aquarius Data Processing Segment (ADPS).
The data files are in Hierarchical Data Format 5 (HDF5). The values are stored as bytes, 2-byte integers, or 4-byte floats. Soil moisture data are 32-bit float, palette is 8-bit unsigned integer. Each data file is paired with an associated XML file. XML files contain file level metadata and location, platform, and campaign information.
Data are available on the HTTPS site in the
https://n5eil01u.ecs.nsidc.org/AQUARIUS/ directory. Data files are organized into directories by time period: Annual, Daily, Monthly, Seasonal, and Weekly:
Within each directory, folders are organized by date, for example:
Folders contain HDF5 and XML files.
File names correspond to those of their parent Aquarius binned data products, indicating the binning periods as part of the names.
Soil moisture files are named according to the following conventions and as described in Table 1:
||Indicates Aquarius instrument|
||Year climatology start|
||Day climatology start|
||Year climatology end|
||Day climatology end|
Binning period length, where:
||Geophysical parameter: SOILM = soil moisture|
||Data version, example: V4.0|
||1-degree radiometer soil moisture|
Each data file is paired with an XML file of the same name with .XML extension. The XML file contains metadata associated with the data file.
Data files are approximately 262 KB each.
XML files are approximately 3 KB each.
Data Volume for Aquarius L3 Daily Soil Moisture is approximately 476 MB.
Data Volume for Aquarius L3 Weekly Soil Moisture is approximately 70 MB.
Data Volume for Aquarius L3 Monthly Soil Moisture is approximately 17 MB.
Data Volume for Aquarius L3 Seasonal Soil Moisture is approximately 5.6 MB.
Data Volume for Aquarius L3 Annual Soil Moisture is approximately 1.8 MB.
Spatial coverage is global.
Spatial resolution of the L3 data is 1 degree.
Projection and Grid Description
l3m_data object is a two-dimensional array (180 rows, 360 columns) of an Equidistant Cylindrical (also known as Plate Carrée) projection of the globe.
25 August 2011 to 07 June 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.
The weekly data start from August 20, 2011 in 7 day intervals until the present, yeilding a weekly data file every 7 days. The first weekly data have a time period of August 20 to August 26, 2011. The file name specifies the start date and end dates for Aquarius coverage. For example, the weekly file named:
Q20142392014245.L3m_7D_SOILM_V4.0_rad_sm_1deg contains data from Day Of Year (DOY) 239, 2014 to DOY 245, 2014.
The month data for August 2011 is a partial coverage only, comprised of observations from August 25 to August 31, 2011.
Similarly, the yearly data for 2011 has observations after August 25, 2011 only.
The seasons are defined between equinox and solstice. The file names contain the exact DOY for each season (Autumn: March 21 to June 21; Summer: June 22 to September 22; Fall: September 23 to December 21; and Winter: December 21 to March 20). For example: the file
Q20141722014263.L3m_SNSU_SOILM_V4.0_rad_sm_1deg contains the data for summer 2014 ranging from DOY 172 (June 21, 2014) to DOY 263 (September 20, 2014).
Daily, Weekly, Monthly, Seasonal, and Annual
The Level-3 products are representations of binned data products generated from Aquarius data. The data object,
l3m_data, represents a mean Soil Moisture at each grid point. The grid resolution is 1 degree.
Each Level-3 soil moisture product contains the
l3m_data object, with attributes described in Table 2.
|Scaling Equation||(Slope*l3m_data) + Intercept = Parameter value|
The palette object included in the data file is the color palette used in the graphics generated from the mapped files. The user can either use this palette or any palette of their choice.
Metadata are included as global attributes with the Level-3 data files, as described for data file
Q20112442011273.L3m_MO_SOILM_V4.0_rad_sm_1deg in Table 3. Values that vary from granule to granule are noted.
|Title||Aquarius Level-3 Standard Mapped Image|
|Data Center||NASA/GSFC OBPG|
|Mission Characteristics||Nominal orbit: inclination=98.0 (Sun-synchronous); node=6PM (ascending); eccentricity=<0.002; altitude=657 km;
ground speed=6.825 km/sec
|Sensor Characteristics||Number of beams=3; channels per receiver=4; frequency 1.413 GHz; bits per sample=16; instatntaneous field of view=6.5 degrees;
science data block period=1.44 sec
|Processing Time||2015166163059000 (varies)|
|Input Files||Q20112442011273.L3b_MO_SOILM_V4.0.main (varies)|
|Processing Control||smigen par=Q20112442011273.L3m_MO_SOILM_V4.0_rad_sm_1deg.param (varies)|
|Input Parameters||ifile = Q20112442011273.L3b_MO_SOILM_V4.0.main|ofile = Q20112442011273.L3m_MO_SOILM_V4.0_rad_sm_1deg|prod = rad_sm|palfile =
/sdps/sdpsoper/Science/OCSSW/V2015.2/data/common/palette/sm.pal|processing version = V4.0|meas = 1|stype = 1|datamin = 0.000000|datamax =
0.400000|lonwest = -180.000000|loneast = 180.000000|latsouth = -90.000000|latnorth = 90.000000|resolution = 1deg|projection = RECT|gap_fill = 0|seam_lon =
-180.000000|minobs = 0|deflate = 4|oformat = HDF5|precision = F|
|L2 Flag Names||POINTING,NAV,LANDRED,ICERED,REFL_1STOKESMOONRED,REFL_1STOKESGAL,TFTADIFFRED,RFI_REGION,SAOVERFLOW,
|Period Start Year||2011|
|Period Start Day||244 (varies)|
|Period End Year||2011 (varies)|
|Period End Day||274 (varies)|
|Start Time||2011244021854088 (varies)|
|End Time||2011274002912490 (varies)|
|Start Year||2011 (varies)|
|Start Day||244 (varies)|
|Start Millisec||1752490 (varies)|
|End Year||2011 (varies)|
|End Day||274 (varies)|
|End Millisec||1752490 (varies)|
|Map Projection||Equidistant Cylindrical|
|Latitude Units||degrees North|
|Longitude Units||degrees East|
|SW Point Latitude||-89.5|
|SW Point Longitude||-179.5|
|Number of Lines||180|
|Number of Columns||360|
|Scaling Equation||(Slope*l3m_data) + Intercept = Parameter value|
|Suggested Image Scaling Minimum||0.0|
|Suggested Image Scaling Maximum||0.4|
|Suggested Image Scaling Type||LINEAR|
|Suggested Image Scaling Applied||No|
Sample Data Record
Below is a sample of the l3m_data soil moisture data array from the Level-3 Aquarius soil moisture file:
Figure 1 shows the average soil moisture estimates for the month of July 2012.
For additional images, see: NASA Aquarius Gallery: Soil Moisture - monthly soil moisture map images.
Software and Tools
HDF-aware software must be used to read the Aquarius soil moisture files. 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 datasets.
For additional tools, see the HDF5 Tools and Software Web site.
Data Acquisition and Processing
The Aquarius SCA algorithm uses the L-band horizontally polarized (h-pol) brightness temperature observations due to the higher sensitivity of this channel to soil moisture. The Aquarius SCA approach is based on the simplified radiative transfer model developed under the assumption that the canopy and soil temperatures are the same (Jackson 1993). The SCA is applied to the individual Aquarius footprint Level-2 brightness temperature observations to produce a swath-based time-order product. (Bindlish and Jackson, 2013; Bindlish et al, 2013). Details on these steps are provided in the Aquarius L2 Soil Moisture documentation.
The Version 4 Aquarius Level-3 Soil Moisture product is generated from measurements derived from the NASA Aquarius Level-2 Sea Surface Salinity & Wind Speed Data V4.0 product. Each Level-3 product contains data from one time period (daily, weekly, monthly, seasonally or yearly) of Aquarius data. The best quality data are selected for each orbit during Level-0 to Level-1A data processing and are then used to create the Level-2 file that is input to the Level-3 science file.
The Aquarius Level-3 gridding algorithm uses local polynomial fitting to grid the Level-2 soil moisture retrievals on a 1 degree grid (Fan and Gijbels, 1996; Lilly and Lagerloef, 2008). The Level-3 processing of Aquarius satellite data takes measurements at the boresight locations of the three radiometer beams, which have been already converted into physical units of soil moisture, and maps these onto a 1 degree grid.
This method fits a Pth-order polynomial at each grid point xm. For data values gn observed at locations xn, n=1, 2,…N, this corresponds to minimizing
at every grid point x=xm, where
is a decaying weighting function which depends upon the bandwidth h, with K(x) being a probability distribution function.
The regression coefficients
p=1, 2,…P vary with spatial location, and are estimated at all grid point locations.
The function g(x) is estimated by the lowest order coefficient,
while higher-order regression coefficients estimate the derivatives of the field through
The above discussion focuses on a 1-dimensional application, but can be extended to a 2-dimensional application. A complete description for the 2-dimensional problem is available in Fan and Gijbels, 1996; Lilly and Lagerloef, 2008.
Each product represents data binned over the period covered by the original Aquarius product. The mean for the observation period is used to obtain the values for the grid points from the binned data products. Each product contains one soil moisture image and is stored in one physical HDF file. The data are not filtered during the gridding process. The user is advised to refer to the flags in the Aquarius L2 Swath Single Orbit Soil Moisture Data.
Changes in the Version 4 Aquarius L3 Gridded 1-Degree Soil Moisture Data data include: use of the most recent version (Version 4) of Aquarius Brightness Temperatures as input.
The Aquarius L3 Gridded 1-Degree Soil Moisture Data, Version 3 are processed from the Aquarius L2 Swath Single Orbit Soil Moisture Data, Version 3. Changes in the Aquarius L2 Swath Single Orbit Soil Moisture Data, Version 3 data included: use of the most recent version (Version 3) of Aquarius Brightness Temperatures as input; Aquarius Brightness Temperatures are no longer re-calibrated before soil moisture retrievals as was done for Version 2 data; soil moisture observations are valid over a wider range of brightness temperatures compared to Version 2 data; updates to the soil moisture model parameters (b and ω).
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 soil moisture data are collected by the radiometers. The radiometers measure brightness temperature at 1.414 GHz in the horizontal and vertical polarizations (TH and TV). The scatterometer is a microwave radar sensor that measures backscatter for surface roughness corrections. Table 4 summarizes instrument characteristics.
|3 radiometers in push-broom alignment||
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, 103 orbits
References and Related Publications
Contacts and Acknowledgments
NASA Goddard Space Flight Center
Hydrological Sciences Laboratory
Code 617, Bldg 33, G216
Greenbelt, MD 20771 USA
United States Department of Agriculture
Agricultural Research Service
Hydrology and Remote Sensing Laboratory
Beltsville, MD 20705 USA
This work was funded by NASA under the Interagency agreement NNH10AN10I. Tianjie Zhao helped with development of the soil moisture algorithm. The support provided by Michael Cosh, Peggy O'Neill, Thomas Holmes and Wade Crow is acknowledged. We acknowledge the support provided by Gary Lagerloef, David Le Vine, Gene Feldman and the Aquarius Data Processing Segment (ADPS) group in the implementation of the Aquarius Soil moisture algorithm.
DOCUMENT CREATION DATE
02 December 2013
DOCUMENT REVISION DATE
03 June 2014
14 November 2014
01 October 2015