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 > 9.36 OSCAR Currents

9.36 Visualization of Remotely Sensed Currents (from Winds and Sea Height) in IDV: OSCAR

  • Exercise Title:  Visualization of Remotely Sensed Currents (from Winds and Sea Height) in IDV: OSCAR

  • Abstract:  In this exercise you'll learn about the OSCAR ocean surface currents (nominally at "15-m") analysis system, based completely on satellite observations of sea height and winds.  Oddly, this important product is available by OPENDAP technology, but no current website lists the correct OPENDAP URL.  Fortunately it was recovered from an old MDL exercise, and published below in the exercise.

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

  • Required Software:

  • Other Resources: 

    • Developer's Original OSCAR Website - "...Project to calculate ocean surface velocities from satellite fields. Surface currents are provided on global grid every ~5 days, dating from 1992 to present day, with daily updates and near-real-time availability." [From the website]

    • Ocean Surface Currents Analysis - Real Time (OSCAR) - NOAA's presentation of the model and results.  "Processing system and data center to provide operational ocean surface velocity fields from satellite altimeter and vector wind data."  [From the website]  Essentially, the density-driven currents are provided from synoptic views of sea surface height (altimeters) and the wind-driven part of the total current is provided from scatterometers.  This is the only global system in place to measure surface currents; all other global systems for currents are numerical models.

  • Author:  Murray Brown

  • Version:  10-9-2015

1.  Open the Original OSCAR website, and spend some time reading through the explanatory materials to see how this unique product is created.  Here's a direct quote from the home page:

OSCAR

Ocean Surface Current Analyses Real-time

Principal Investigator

Kathleen Dohan
 

Co-Investigator

Gary Lagerloef

Overview

Ocean Surface Current Analyses Real-time (OSCAR) is a project to calculate ocean surface velocities from satellite fields. Surface currents are provided on global grid every ~5 days, dating from 1992 to present day, with daily updates and near-real-time availability. The data is freely available through two data centers operated by NOAA and NASA. ... The NOAA site (www.oscar.noaa.gov) provides data in both downloadable images and netcdf format. Validation statistics are also provided through this site. [Edited by MDL to remove out-of-date information.]

2.  This is the only direct satellite measurement system for total currents.  A good figure shows the older 1-degree version (left) and the new 1/3-degree version (right).

3.  Open the NOAA OSCAR website (cited in the text above) and see how the results are currently published.

4.  Look at the bottom of the page and you'll see these 3 methods to get the data.  They all sound interesting, but the third one says it has OPENDAP, which means online direct subsetting, so let's go that route.

5.  Click the third link [for OPENDAP/DODS], and here's the first page you see; it's a single target page in a very large NOAA catalog of satellite datasets.  It looks like we're on the right path.  Click on DATA ACCESS (just below the image) to continue.

6.  Here's what we see under DATA ACCESS:
  • FTP - Seems to lead directly to an FTP function
  • OPENDAP - Could lead to direct file access from specialty analysis/display software
  • LAS - Live Access Server; could lead to direct subsetting and display online

We'll explore all three and see what happens.

7.  Here's what you get with the FTP method.  It leads to a gigantic list of 5-day analyses, going back to 1992. 
  • If you need the entire NetCDF grid files, then this is the way to go, and the downloads are easy, if time-consuming
  • If you prefer to avoid downloads, then you need to look at OPENDAP and the LAS, the next 2 choices

 

8.  Click the OPENDAP choice (our second possibility), and you'll see this very long list of folders and files, going back to 1992 (the beginning of OSCAR).

 

 

 

 

 

 

 

 

 

 

 

 

Go all the way to the bottom of the list, and you'll see the link to information about the latest day, Number 8401 in the program (~365 days/yr * 23 yrs).  Open this link.

9.  Here you see the usual OPENDAP information sheet, which is quite standard.  Usualy, the DATA URL at the top can be used in IDV or other OPENDAP-enabled programs to get the data you want.  EVEN IF IT IS GZIPPED!
10.  But if you try to use this "URL" in IDV, you'll get this error message.  This is troubling.
11.  Our last possibility to get an OPENDAP approach is through the Live Access Server (LAS) the third choice above.  If there is a LAS, then usually it contains a "hidden" link to the OPENDAP URL, in the background information.

12.  But when you try to open the LAS, this is what you see.  This has been true for some while now, so apparently LAS is no longer provided for the OSCAR data.  That is very troubling, because it was the only way to find the real OPENDAP address. 

In the next few panels, highlighted in light blue, we show you how the LAS was working about a year ago, and we will be able to give you the real OPENDAP address, which we were able to save and has not been changed since.

13.  Open the Live Access Server.  Then when the menu appears, find the OSCAR currents, and select the vectors to view.
14.  Here's a typical visualization for Namibia, using the manual area selection tool.  This would be an ideal way to get these data, because the LAS usually has a subset and download function.

But if you check the latest available dates, you'll see that the products ended in early 2014.  Bad news, but it happens all the time.

So we can't use the LAS for operational purposes.  But there is one thing you should always do with valuable data that can be accessed in a LAS, it's a neat trick for you to learn.

15.  On almost all LAS data map pages there is a blue/white question mark, leading to further information.  Find it and click on it.

16.  You'll see something like this, although usually not so complete.  The PO-DAAC is congratulated on the quality of their work here.  These are the palace secrets on how to access the data on your own.  You could use IDV (which we will do below) or EDC (see Exercises 8.5 and 8.6).

17.  Click on SEE THE URL'S and this chart appears.  This chart is not really that complicated, because there really only 2 different URLs on it, the *.nc ones (3 instances) and the *.nc.jnl ones (2 instances).

Name The URL to access these data via OPeNDAP
x - Range y - Range t - Range Units
Ocean Surface Zonal Currents LAS F-TDS URL: http://thredds.jpl.nasa.gov/thredds/dodsC/las/PODAAC-OSCAR-03D01_LAS/data_thredds.jpl.nasa.gov_thredds_dodsC_ocean_circulation_FOR_LAS_ALL_OSCAR_L4_OC_third-deg_FOR_LAS.nc.jnl
Original OPeNDAP URL: http://thredds.jpl.nasa.gov/thredds/dodsC/ocean_circulation/FOR_LAS_ALL_OSCAR_L4_OC_third-deg_FOR_LAS.nc
-180 : 179.96 -80 : 79.98 21-Oct-1992 : 12-Mar-2014 meter/sec
Ocean Surface Meridional Currents LAS F-TDS URL: http://thredds.jpl.nasa.gov/thredds/dodsC/las/PODAAC-OSCAR-03D01_LAS/data_thredds.jpl.nasa.gov_thredds_dodsC_ocean_circulation_FOR_LAS_ALL_OSCAR_L4_OC_third-deg_FOR_LAS.nc.jnl
Original OPeNDAP URL: http://thredds.jpl.nasa.gov/thredds/dodsC/ocean_circulation/FOR_LAS_ALL_OSCAR_L4_OC_third-deg_FOR_LAS.nc
-180 : 179.96 -80 : 79.98 21-Oct-1992 : 12-Mar-2014 meter/sec
Vector of Ocean Surface Zonal Currents and Ocean Surface Meridional Currents LAS F-TDS URL: 
Original OPeNDAP URL: http://thredds.jpl.nasa.gov/thredds/dodsC/ocean_circulation/FOR_LAS_ALL_OSCAR_L4_OC_third-deg_FOR_LAS.nc
-180 : 179.96 -80 : 79.98 21-Oct-1992 : 12-Mar-2014 meter/sec
18.  OPENDAP URL's ending in NC.JNL would be quite unusual; we've tried them and they don't work.  So we'll ignore them for now.  We'll open Integrated Data Viewer, and try the URL ending in NC:

You'll need to save this URL permanently for all future OSCAR work.

19.  Run DIV.

20.  Let's try the URL in the DATA CHOOSER.  Enter it in the page shown here, and click ADD SOURCE.

21.  Hooray!  This field selector opens, and we're in familiar territory again.
22.  As usual with IDV you want to right-click on the data object and select PROPERTIES.
23.  For TIMES, select USE SELECTED and pick the latest image at the bottom.
24.  For SPATIAL SUBSET,  draw a small rectangle anywhere on the map (to activate the control).  Then enter the correct coordinates for your area of interest.  Liberia in this case.

You could also make some other choices, but for now just click OK.

25.  Now you can open the FIELDS controls to select the analysis product.  We'll begin with a speed image, where the color palette shows current speed.

Make these choices and click CREATE DISPLAY.

26.  You'll see a temporary image, but to get the best analysis, click on the WINDSPEED object in the LEGEND column.  Then make these refined adjustments.

27.  This is the final speed illustration, using the refinements mentioned above.  Of course there are dozens of changes you can make on your own, and you are urged to try all sorts of changes to see how IDV really works.

28.  Now let's add flow vectors to the above image.  Go back to the FIELDS selections and make these choices.  Then click CREATE DISPLAY.

29.  Here's the final figure, after a few small adjustments that you can discover on your own.  To see the options, always click the object label in the LEGEND column.

30.  This is the the beginning of the hundreds of adjustments and options you can take to display these data as you wish.  Don't be satisfied with just these products from the instructor.  Make us proud.
31.  So by using this URL (remembered from the HELP page of an extinct LAS server) you can access the data directly with IDV.  So you have 2 working routes from OSCAR to IDV:  FTP downloads and the OPENDAP URL ending in NC.
32.  And of course you will save your best visualizations in LIBERIA > PRODUCTS > IDV > XIDV with appropriate long, descriptive filenames.