Saturday, February 19, 2022

ASCAT News — The Good and the Bad.

Scatterometer wind measurements are the truth meter for weather work at sea. There are several sources, but the ASCAT data from EUMETSAT are the gold standard. We can see this data graphically at the STAR website—found easily with a Google search of "ASCAT." We have extensive coverage of this data and how to use it in Modern Marine Weather (MMWX), but the book links are out of date (being updated here!)—in part because the satellites have changed. A is now gone and we use Metop B  and C.

This article has been updated from its origin on May 2021, with new videos at the end.

So the first news is we have the updated links below, there are new data, and it is even better than before. The main bad news is the GRIB format of this data, which offered a super convenient and precise way to test model forecasts is at present no longer available from the Ocean Prediction Center (OPC). It is still listed at the OPC website, which might imply the loss is not permanent. The files look normal and are indeed updated as indicated, but they do not contain any data!

Granted, this wonderful source of digital data could only be conveniently viewed in two popular commercial programs, namely Expedition and LuckGrib—not to mention the free app Panoply, since that one takes extra work. Worse than that, even amongst the many users of these two leading programs, there were not many navigators actually taking advantage of this most important data, which might account for the lingering loss... ie there are not many people complaining. 

(For now, there is no GRIB source for ASCAT, but for those who want it enough to spend some extra work you can download the data in netcdf and then view it in Panoply, but this is not a practical, navigator's solution.)

*** Note added in July 2022: LuckGrib now offers ASCAT once again. They are doing the conversion from netcdf to grib on their own. To our knowledge this is the only grib source of ASCAT data.

Part of the problem in ASCAT popularity is the data are not plug and play like a GFS forecast is, for example. Once we did get the ASCAT grib, life was good, it showed the actual winds on the ocean, not a forecast, but real data as if we are looking at anemometers on a field buoys located every 25 km across the ocean. The problem is the data over some specific point, say 300 nmi in radius, might only be updated 2 or 3 times a day, and those times take some effort to predict. Refer to MMWX for these details.

The good news is, and the motivation of this article is, we have a new, free way to get georeferenced ASCAT data into a navigation program when underway using the program qtVlm. We already had one way described in MMWX, namely look up the file name for the data you want from the STAR site or from the table in MMWX, and then request that image from Saildocs. Once that is in hand, and you check the valid time of the satellite pass (MMWX), you can either just look at it to see the winds and compare to the forecasts, or in Expedition, OpenCPN, or qtVlm  you could manually georeference it and then overlay the model forecast at the corresponding tine right on top of it. In these programs you can store the georeference coordinates and then just periodically update to see if there is new data. The image size you are requesting is 20 to 60m kb. A sample is below.

ASCAT-A descending over Bermuda at 12:33z on May 2, 2021. It is May 2, and not May 3, because this image is from the 22-hr dataset that was most recently updated at 02:07 May 2. Had the small purple time at the bottom been between 00:00z and 02:07z then this would be May 2. (From the original article; A is no longer available.)

These files are always 10º Lat x 15º Lon so they are easy to georeference, plus the grid lines are clearly in view.

Below are samples georeferenced in Expedition and qtVlm with the GFS wind forecast at the corresponding time overlaid on it.

And then zoomed in below, we see the GFS is pretty good since it agrees with the scatterometer wind measurements. This is Metop A, descending. 

The same thing viewed in qtVlm looks like this:

The best comparison of overlaid data like this depends on our choices of transparency and wind barb  colors. This has not been optimized in either of these two programs.

The main new good news from qtVlm is their latest version allows us to enter a live internet link for the image, as opposed to a link to a static file on our hard drive, so that the data are updated automatically when we reload that image. The trick is to create an auto-updating KML file for image and then use that as the target file. This then not only updates automatically, it also brings with it all of the georeferencing information. With your computer linked to your satellite phone, this update should take place just as you do to obtain the latest model forecast.

We have videos online on how to do this.  We make the needed files in Google Earth, which is then another way to look at live, auto updating ASCAT images. The new step is just adding this functionality to a navigation program independent of Google Earth.

We made such KML files  some years ago for the Bermuda Race,  Transpac/Pacific Cup, and for Sydney-Hobart. Our next step will be to update the links and make a convenient place for mariners to download them.  Now that we have lost the GRIB format, these methods become of interest again.

Here is the generic image link using the Bermuda file as the base. Then the colored letters have to be changed for other locations, passes, and satellites.

Change A to B or C for the 3 ASCAT satellites, and change the blue file name to match the location you care about, and then change the d (descending) to a for the ascending pass. 

Thus for each location you care about, there will be 6 files that might be of interest, keeping in mind that A and B are fairly close in time so the earth has not rotated much between them. In rare cases, you might need B having seen A, but not often.  

You can get these files (once you know the name of the ones you want) from Saildocs. For example, send an otherwise blank email to query@saildocs with this as the message:


This should get you those two files by return email. Sample file names below.

Click the images above for a better view.

Below are a couple How-to videos:

How to create KML files of ASCAT wind data.

Viewing ASCAT wind data in qtVlm, and one with more timely data.
Links to the Bermuda Race KLM files are in the video description.

1 comment:

David Burch said...

Note that ASCAT A data is no longer available. we use B and C now.

Also look for a mid feb post to get the Bermuda Race file links that work in qtVlm and google earth