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Home > 9. Operational/Synoptic > 9.22 Aquarius SSS

9.22 Visualizing Satellite-Measured Sea Surface Salinity in IDV: AQUARIUS via THREDDS

  • Exercise Title:  Visualizing Satellite-Measured Sea Surface Salinity (SSS) in IDV: AQUARIUS via THREDDS

  • Abstract:  In this exercise you'll visit the improved data download service for the new AQUARIUS mission, providing Level 3 sea surface salinity data both via Live Access Server access, and via IDV from a THREDDS service, instead of downloading HDF files.

  • Preliminary Reading (in OceanTeacher, unless otherwise indicated):

  • Required Software:

  • Other Resources: 

    • Aquarius Website - "Aquarius is a focused effort to measure Sea Surface Salinity and will provide the global view of salinity variability needed for climate studies. The mission is a collaboration between NASA and the Space Agency of Argentina (Comisión Nacional de Actividades Espaciales)."  This is the main website for obtaining the data.

    • Compare with 2.31 for an alternate method

  • Author:  Murray Brown

  • Version:  1-8-2015
IMPORTANT NEW INFORMATION:  Recently the Saga GIS software has been shown to deal quite adequately with most NetCDF (NC) data grids and also GRIB files.  This should be viewed as a useful addition to the rapid, direct NC analysis and display capabilities of IDV (see below).  If you are also interested in this route for your data, then please check Viewing and Managing Well-Formed NetCDF Grids in Saga.

1.  Open the main Aquarius website to read more about the mission, the sensor and the available data products.  [Don't be surprised if the actual index page looks different from this; apparently NASA's contractor is very busy with website design.]
2.  Click on DATA to see a page containing these important items.
  • We will focus on the PO.DAAC DATASET DISCOVERY tool (in the upper group), because it provides access to standard, easy-to-use NASA NetCDF products.
  • We will not focus on the GSFC products (the lower group), because they are HDF behemoths from the ColorWeb factory, and not easy to assimilate into multi-parameter marine data ensembles.

After studying the page, click on the DATASET DISCOVERY link.

Data

Aquarius data are available from two locations.

The archive of record for Aquarius archived data products is NASA's Physical Oceanography Distributed Active Archive Center (PO.DAAC), located at the Jet Propulsion Laboratory. In addition to the Level-1a orbital source data products which are updated daily, refined Level 2 orbital swath data and Level 3 gridded salinity and wind speed products (1 degree spatial resolution for daily, 7 day, monthly, seasonal, and annual time intervals) are updated monthly.

"Quick-look" products, along with the above-described refined data products, are available from NASA's Goddard Space Flight Center (GSFC) within 1-2 days of acquisition by the Aquarius instrument.

  • GSFC's Aquarius Level-1 and Level-2 Visual Browser/Data Download Tool

  • GSFC's Aquarius Level-3 Salinity Visual Browser/Data Download Tool (V3.0)

  • GSFC's Aquarius Level-3 Wind Speed Product Visual Browser/Data Download Tool (V3.0)

  • Aquarius Data Access (GSFC's source, refined and evaluation data products)

  • Ancillary Data Access (CUSS and CODS files and meteorological files used in Aquarius data processing)

  • Large Salinity Maps Created From Aquarius Level-2 Data

3.  This is the main menu for DATASET DISCOVERY.  You can reach this search engine from the location above, or from many other pages in NASA.  The very long list of "filters" on the left can be expanded to cover a huge list of choices.  And after some choices are made then new items will appear in the filter list, as appropriate.  The initial choices, shown on the right below, will quickly change as filters are edited on the left.
4.  Here is a partial set of the initial filters, edited for simplicity.  Only about 1/3 of the actual choices are shown here.  When some items are selected, then new groups of items may also appear, as necessary.  Keep your eyes open.
5.  In the next few panels we're going to search for operational salinity data, but you will naturally be able to use the same methods for any other datasets.  You can repeat any searches with new settings on your own time.
6.  For PARAMETER, select SALINITY/DENSITY.
7.  For PROCESSING LEVELS, select LEVEL-3 GRID.
8.  For TEMPORAL RESOLUTION, select 7DAY.
9.  This should narrow down the possibilities from hundreds of datasets to these few.  Read through them to see the similarities and differences.

10.  For our work below, just click on the top one to see this detailed description:
11.  Click on the title link (the top line) and this technical description opens.

12.  In the top line, click on DATA ACCESS to see this 3-part menu of methods.  We will focus on the TOOLS AND SERVICES section to use NetCDF for easy access/use.  We will avoid the needless burden of HDF files in DIRECT ACCESS, and the steep learning curves involved in the use of READ SOFTWARE.
13.  We're interested in the TOOLS AND SERVICES section, where we can first try the Live Access Server (LAS) method, and second, we'll try the Thematic Realtime Environmental Distributed Data Services (THREDDS)  method.
14.  Click on the LAS link to see this familiar page.
15.  This list of dataset categories should open automatically, covering most of the major PODAAC data products.

Navigate down to the AQUARIUS DAILY SSS item and select it.

16.  This is an excellent way to see the latest data product available.  In the menu on the left, select the latest possible date, then click on UPDATE PLOT in the top line.  This test was made on 1-7-2015, so you can see that for daily products there is a time lag of about 40 days.  The other products should have lags of roughly the same or greater.
17.  Please take note that you can download the globe or any viewed region in the LAS with the SAVE AS control.  Format choices include NetCDF, so this is one easy and powerful pathway to IDV or Saga, if you want to use downloaded files.
18.  Now we'll move on to the THREDDS methodology, using IDV for the access and display functions.
19.  To use a THREDDS server, take the first part of the THREDDS URL (up to the HTML part) and edit it as you see here:

The HTML extension becomes XML. 

Things to remember:

  • Usually, when you see ...CATALOG.HTML, you have something that is almost a THREDDS catalog. Try changing the extension.
  • Usually, when you see ...CATALOG.XML, you have a THREDDS catalog.  See if it opens in IDV
  • If the URL includes a ? followed by something, then the URL will usually take you to a lower level in the catalog
  • If you drop the ? section, then you will usually see the top-level THREDDS catalog
20.  Run the latest version of IDV, and make sure it is set up properly (9.1 Integrated Data Viewer (IDV) Preliminary Setup)

21.  Select DATA CHOOSERS > GENERAL > CATALOGS and insert the XML link you obtained above.  Hit RETURN to activate the process, and you will quickly see the AQUARIUS items list, as you see below.

NOTE:  The number of available data products here in IDV is 12, compared to the 10 items shown in the LAS method above.  Examine both product lists to see what's different.
22.  Select the non-CAP standard 7-day product, as you see here.  Then click ADD SOURCE.
23.  Here we see that the 7DAY products are available for visualizing.  You can see that they go back into 2011, at weekly intervals.
24.  In the DATA SOURCES panel, right-click on the 7DAY products, and select PROPERTIES.
25.  On the TIMES tab, select USE SELECTED, and scroll down to select the very last date. 

NOTE:  This exercise is months old, so you will find much later dates to use.
26.  On the SPATIAL SUBSET tab, draw a small rectangle anywhere near Liberia (it need not be exact).

NOTE:  Spatial subsetting is mandatory for most large grids in IDV, and these GlobColour files are included, so make sure to do this before clicking on CREATE DISPLAY
27.  Then enter the correct coordinates of the Liberia area of interest in the spaces provided.

Then click OK.
28.  Now you have the times and location set up.  In the DISPLAYS panel, select COLOR-SHADED PLAN VIEW.

Then click CREATE DISPLAY.
29.  This is your first view of the surface salinity offshore Liberia, from satellite.  The color palette is somewhat strange, but the value range is a reasonable 33.7-36.2.

Notice the half-cells along the margins, indicating the original data grids from Aquarius are vertex-centered.
30  On the dashboard, Click the DEFAULT control beside the color palette, and select the new palette RADAR > DBZ (rainbow).
31.  Here's the final Liberia figure, using the default value range.

32  You should save this with FILE > SAVE AS and the filename sssal_20130122_liberia_aquarius_l3.xidv (appropriately modified to match your own specific dates).
33.  Of course it would be interesting to see the whole globe, so in the next few panels we'll repeat the steps without any spatial subsetting.
34.  Here is the first global view, using the same color palette as above.

The value range is strongly affected by the presence of a very few low values at the poles and near large rivers.  To see more structure in the open, deep sea, we need a range that is typically "oceanic".
35.  With the same control, select CHANGE RANGE
36.  Change the min/max values to something you think is reasonable for the global ocean.  Here we chose 30-37.

You can do some experiments later to get the best range for the globe or for your area.
37.  And here is the result.  You can see very interesting structures in all the oceans.

38.  You can use FILE > SAVE AS to save this product in PRODUCTS > IDV with the filename sssal_20130122_global_aquarius_l3.xidv (or similar).  We have checked this method, and the XIDV file does include the color settings you made above.
39.  Some users have expressed an interest in "smoothing" the above images to see the patterns better.  To explore this option, you can use the SMOOTHING control among the displays settings.  You'll find that some settings give you much better pictures of the data in some areas, in other areas the data are missing due to low data density.  It's all up to you.
40.  Of course, we have skipped over a mass of interesting data products above.  It's your job to go back and try them to see how they differ, and to read about them at the source website.  If SALINITY is your topic of concern, then dig into this exercise and cover everything.