Marine Data Literacy 2.0

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Home > 9. Operational/Synoptic > 9.41 RapidScat Winds

9.41 Visualizing Satellite-Measured Winds in IDV: RapidScat

  • Exercise Title:  Visualizing Satellite-Measured Winds in IDV: RapidScat

  • Abstract: "Winds from the RapidScat scatterometer on the International Space Station (ISS) are available for OSI SAF users. The wind products are produced by the OSI SAF/KNMI and based on the level 2a backscatter data which are kindly provided by NASA/JPL. All products have status 'in development' and we [the KNMI] welcome users to provide feedback.  [The KNMI] delivers four wind products:

    • 25 km wind product with latency (delay from data collection to online data availability) of approximately 2 hours

    • 25 km wind product with latency of approximately 3 hours

    • 50 km wind product with latency of approximately 2 hours - Listed here; but not found [MDL editor]

    • 50 km wind product with latency of approximately 3 hours

    The above specified latencies are for the middle of one orbit file containing approximately 90 minutes of data. The 2 hour products are contain approximately 80% of the data available; 3 hour latency products contain approximately 98% of the data."  [From recent KNMI correspondence] 
  • Preliminary Reading (in OceanTeacher, unless otherwise indicated):

  • Required Software:

  • Other Resources: 

  • Authors:  Yuan Ho (IDV User Support) and Murray Brown

  • Version:  1-5-2014

1.  Open your FTP program (for example FileZilla) and set up a new account for the KNMI scatterometer products, as described above.
2.  Use your FTP program to open the KNMI scatterometer products.  Then select NETCDF.
3.  Here are the available products:
4.  And here are all the most recent RapidScat products, or similar.  The GZ files contain the zipped data products; the MD5 files contain quality control checksums for the zipfiles.  Files named with "250" are 25-km resolution; "500" means 50-km resolution.  Why the extra zero on the filename?  Who knows.

Select all the 25-km GZ files.

5.  Copy them to the folder LIBERIA > DATA > OCEAN > KNMI.  Then unzip them.  You should have these NC files (after deleting the GZ zips) or similar.

6.  As usual, we first check one of the NC files (actually the first one listed) in ncBrowse, as you see here.  You don't need to do this now, because as you can see there is an odd problem with the data.  They plot the expected global L2 swaths, but they also have a direct connector between the longitudes of 0 and 360.  We'll deal with this in IDV.
7.  Run IDV.  Make sure it is the latest version, and the standard setup steps in 9.1 have been taken.
8.  Use DATA CHOOSERS > GENERAL > FILES to select the desired NC files, and use ADD SOURCE to load them.

 9.  Let's look at the first file to see what happens.  Make these choices and click CREATE DISPLAY.

10.  Open the dashboard for the new map, and change the DEFAULT color palette to RADAR > DBZ as you see here.
11.  Also click on the DBZ control and select CHANGE RANGE > USE PREDEFINED > FROM ALL DATA to get the values you see here (or similar).

NOTE:  The range of wind value is left alone here, but you can see that the usual values range from 0 to about 27 m/sec.  We'll use this information below).

12.  You should see this map (or similar).  Everything looks fine, except the 0-to-360 connection (broad E-W band in the southern hemisphere) .  This may be due to a small glitch in the data, but we offer below a fix for this problem.

13.  Go to the main menu for the current map, and select FILE > NEW > VIEW WINDOW > GLOBE VIEW > ONE PANE.

This will open up the world of 3-D maps in IDV (similar to Google Earth).

14.  This new 3-D map appears, without any data on it yet.
15.  Remove the square frame with VIEW > SHOW > WIREFRAME BOX
16.  Now, you're ready to plot the data on this globe.  There will be no 0-to-360 connection, because they are already adjacent.

Why is this hard to read?  Both hemispheres are visible at the same time.  This view is actually pole-on.  Which pole is toward us?  You guess.

17.  Add the data from the first file again, just as you did before.

18.  Make the same color palette and value-range changes as above.
19.  And here is the 3-D map of the data swath.  You can already see that it is not easy to read such a map, because your eye has difficulty seeing what's "in front" and what's "in back" of the globe.


20.  If you are a real data explorer, you probably wondered about subsetting the data, as we usually do (as shown here).
21.  Sometimes, but not always, it is possible to do this successfully, as you seehere.  But these particular data don't allow that for Liberia, so that would not be a general solution to the problem, and we don't consider it further.
22. The best way to figure everything out is to place a solid earth in the middle of the 3-D map, so we can only see the data on our side.


23.  From the available choices, we suggest BLUE MARBLE - STATIC.
24.  Which gives you this map.
25.  Now add the first data file again, and you'll see something like this.  The data swath is broken up by complex algorithm effects which we need to correct.
26.  Return to the dashboard for the background map (i.e. the Blue Marble).  Use the VERTICAL POSITION control to move the Marble to -0.1.

You may have to experiment with other similar values, if this doesn't work!

27.  Now you can see the Marble has shrunk a little bit so the data swath is easy to see.
28.  Now you can add all the other data files to see the ensemble data product map.

NOTE 1.  If you don't like the green coastline map being separated from the Marble, then UNCHECK it in the maps part of the menu.

NOTE 2.  You should select one "universal" speed range for the winds, and use it on all of the swaths.  This is an important decision, especially if you plan on making many such maps over long periods of time.  Here we used 0-27 m/sec.


29.  And here's an example of a map without any coastline.  You can see large storms in the northern Pacific, chosen here because there weren't any swaths in the Liberia area during this time frame.
30.  Make sure, at this point, to use FILE > SAVE AS to save this great map in the folder PRODUCTS > IDV > XIDV with an appropriate, multi-part filename, for future reference.
31.  One of the authors (Brown) is going to check with the KNMI and the NetCDF folks to see if there is a simple fix to the data files that might make the swaths also visible in a planar view.