Marine Data Literacy 2.0

Providing instruction for managing, converting, analyzing and displaying oceanographic station data, marine meteorological data, GIS-compatible marine and coastal data or model simulations, and mapped remote sensing imagery

 

 

 

 

Home > 9. Operational/Synoptic > 9.8 AVISO SSH

9.8 Visualizing Satellite-Measured Sea Surface Height & Geostrophic Current Estimates in IDV: AVISO GDET

  • Exercise Title:  Visualizing Satellite-Measured Sea Surface Height & Geostrophic Current Estimates in IDV: AVISO Gridded Data Extraction Tool (GDET)

  • Abstract:  This exercise shows you how to work with two very interesting aspects of altimeter data:  sea surface height (SSH) alone, and the U and V geostrophic current vector component grids that can be derived from SSH.  The SSH grids are treated as scalars, just as you have done with SST, etc. in other IDV exercises.  The U and V grids are used to calculate "geostrophic" current vectors.  Previous versions of this exercise used global NetCDF file downloads, followed by regional subsetting in IDV.  This latest version uses the Gridded Data Extraction Tool at AVISO, to speed the process.  Streamlines are demonstrated (as an option to usual vectors), and an animation is created.

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

  • Required Software:

  • Other Resources: 

  • Author:  Murray Brown

  • Version:  11-9-2014

1.  Open the main AVISO website.  [There are English, French and Spanish versions; see the bottom left corner for the links]

Take some time to read though this very powerful site, and find out what sorts of data and data products it offers.  Results range from highly detailed, single transect satellite passes, to very generalized geographic grids of top-level products.

2.  Select DATA > ACCESS to see these 7 items.  They offer an extremely broad set of access methods, which you should explore later on your own.  They differ in the timeliness of the of the data, and other technical details.

NOTE:  You can alternately choose the DATA > PRODUCTS tabs to go through a more generalized introduction to the whole product suite at AVISO

Select GRIDDED DATA EXTRACTION TOOL (at the bottom)

3.   You'll see a very large page, with this at the top:

This is the point where you can SUBSCRIBE to get these data.  This is the same registration process that you may have used previously to use Marine Data Literacy exercises for AVISO data; if you're not sure, then try to pass this step and see what happens below.  You'll have opportunities there to register, if necessary.

4.  Lower down on this same large page, you'll see these available data types:

5.  This is what you'll see in the MSLA/MADT > GLOBAL section:

We'll need data from both of the bottom sources, MAPS OF ABSOLUTE DYNAMIC TOPOGRAPHY (MADT) and ABSOLUTE GEOSTROPHIC VELOCITIES.  In both cases we want the NEAR REAL TIME (NRT) data (column 1), not the DELAYED TIME (DT) data (columns 2 and 3). 

The strange label "> 30 days" means the latest 30 days, and not data over 30 days old!

6.  At this point, you must have already registered with AVISO for FTP and/or gridded data extraction services at the address listed above.  Either way, this is what you need:

  • User Name

  • Password

Enter these items to continue, or take the time to register now.

7.  GETTING THE DYNAMIC HEIGHT  
8.  Click on MADT > NRT > DATA REQUEST FORM.  Then enter your data to log in.
9.  This is the opening screen.
  • Select Output - NetCDF
  • Selection Region - Don't touch
  • Select Time Range - Set a reasonable recent span of a few days, for example the most recent week
    • Available data appear to range from 2009-9 to the present
  • NSEW - Enter appropriate values for your area of interest; in the case of Liberia, they are:
    • Top = 9
    • Bottom = -6
    • Left = -23
    • Right = -3
  • Variable Download - Check the data you want; for this exercise you can check only ADT (for absolute dynamic topography)

On your own time later, you can explore the other fields.

10.  Click DOWNLOAD to begin the process.
11.  This helpful message appears, giving you an alternative download method, if you have problems. Your download should begin shortly. If it does not, try dataset-duacs-nrt-over30d-global-allsat-madt-h_1415469209930.nc
12.  However you get the data, navigate to the folder LIBERIA > DATA > OCEAN >AVISO and save with the filename dataset-duacs-nrt-over30d-global-allsat-madt-h_1415469209930.nc

OPTION:  Use -liberia- instead of -global- in the filename if you want to be exactly correct.

13.  The "gold standard" of NetCDF format is ncBrowse.  Here you can see the downloaded file opened in it.
14.  Here is the ADT object visualized as an image in ncBrowse, showing that we have the right sort of data in the right place.  This is always a good check to make with NC data.

The data values range from about 0.3 to 0.5, which is about right for open sea dynamic height in meters.

15.  GETTING THE U AND V CURRENT COMPONENTS
16.  Go back to Panel 5 above and get the geostrophic current vectors (U and V) in the same way.  This same helpful message should also appear. Your download should begin shortly. If it does not, try dataset-duacs-nrt-over30d-global-allsat-madt-uv_1415537131875.nc
17.  Navigate to the folder LIBERIA > DATA > OCEAN >AVISO and save the resulting U/V data with the filename dataset-duacs-nrt-over30d-global-allsat-madt-uv_1415537131875.nc.

OPTION:  Use -liberia- instead of -global- in the filename if you want to be exactly correct.

If you're interested, view both grids in ncBrowse to see if they make sense.  The U grid should be much more energetic than the V, with noticeable E-W strips of very strong current regimes, especially in the tropics.

18.  Now, we're ready to view the data in IDV.  Run the latest version.
19.  In IDV, use the standard DATA > DATA CHOOSERS.  Make sure the DATA SOURCE TYPE is set to GRID FILES (NETCDF, ETC.).  Then navigate to and select dataset-duacs-nrt-over30d-global-allsat-madt-h_1415469209930.nc

20.  Make these FIELDS and DISPLAY choices.  Then click CREATE DISPLAY.

21.  Here is the first draft of your SSH map.  The data unit is meters, which you can see if you open the properties for the color-shaded plan view under LEGEND.

22.  To get a better palette, use DISPLAYS > COLOR TABLE > RADAR > DBZ to obtain a better match with the value range (0.302-0.537 for these data).

The eddies are very easy to see, both warm core (red) and cold-core (blue)

 

23.  You can save this analysis with FILE > SAVE AS and use the filename seaheight_nrt_aviso_areaname_merged_madt_h_20141101_20141108.idx (where you can substitute the appropriate area name and dates for your own data) or something similar according to your own tastes.
24.  So much for the sea surface height, which gives us a very general idea of circulation.  Now let's use the U and V surface current vectors from AVISO to estimate the actual current vectors. 

Load the UV file, dataset-duacs-nrt-over30d-global-allsat-madt-uv_1415537131875.nc, as you did above.

25.  Immediately select FIELDS > 2D GRID > DERIVED > MAKE FLOW VECTORS FROM U AND V DATA

And for DISPLAYS, select VECTOR PLAN VIEW

Then click CREATE DISPLAY.

26.  At this point you may be asked to identify the U and V components specifically, if IDV does not recognize the names from the actual data file.  Do so if needed.  But in this case IDV easily recognizes these from such well-formulated NetCDF files.  So you are not asked.
27.  This control panel for the vectors appears.  On your own time later, you can explore the options. But here are the most important right now.
  • VECTORS > SIZE - Increases the length of vector arrows; leave as is
  • SHOW > STREAMLINES - Changes the display from vector arrows to lines of equal flow velocity; don't check now
  • SKIP - Decreases the number of arrows by eliminating them, e.g. 2 = every second arrow; leave as is
28.  Here the vectors have been drawn on top of the existing map of sea surface height.  You can easily see the expected relationships.

North of the equator, water flows clockwise around highs (red) and counter-clockwise around lows (blue).  The opposite is true south of the equator.  Because the equator runs horizontally through the approximate middle of the map, you should see both behaviors.

29.  Now you can save all your vector-drawing work with FILE > SAVE AS and use the filename currents_nrt_liberia_aviso_merged_uv_20141101_20141108.xidv for this IDV project (or something similar you like).
30.  Obviously, you just passed by some other very interesting display and analysis choices.  Take the time now to go back and explore these options.

For example, you can select STREAMLINES on the DISPLAYS page (see above), to get this interesting view of the currents in the area.  Typically, closer-packed lines indicate stronger currents.  Also note the reversal of direction for the cold-core features above and below the equator (about 2/3 down from the top.

31.  Because there are multiple times available (1 week in this case), you can use VIEW > CAPTURE > MOVIE (along top left margin of map) to make and save an animation.  If this case it would consist of 8 days. 2
32.  seaheight_liberia_aviso_20141101_20141108.gif - Click this link to see this same data sequence as a movie (GIF format).