Visualization of Remotely Sensed Currents (from Winds and Sea Height) in
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):
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]
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
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
Surface Current Analyses
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
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
|3. Open the NOAA
OSCAR website (cited in the text above) and see how the results are
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
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
- FTP - Seems to lead directly to an FTP function
- OPENDAP - Could lead to direct file access from specialty
- LAS - Live Access Server; could lead to direct subsetting and
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
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
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
|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.
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.