Radial Velocity question

[L.B. LaForce]

Yep, I agree on the aliased data. In GR2 or GR2AE, there is also an option to dealias the velocity. That is pretty neat to see that embedded supercell on the reflectivity.

Also, he shows radial velocity, and base velocity..I thought those were the same? I know SRV takes into account storm motion, and subtracts it from the base velocity.

 

[Jeff Duda]

radial velocity is just the normal velocity product that is displayed. It is the component of velocity that is along the radial vector (directly towards or away from the radar site). It means the same thing as base velocity.

 

[Joel Wright]

So basically the best radar mode for determining severe winds is storm relative? It appears that radial and base velocity modes have their issues.

 

[Jeff Duda]

So basically the best radar mode for determining severe winds is storm relative? It appears that radial and base velocity modes have their issues.

Not really. You'd want to use both products. What storm relative velocity shows you is the vector winds when the storm motion vector is subtracted from the base velocity wind vectors. The equation to describe this (if it helps you to see) is V(storm-relative) = V(actual...what base velocity shows) - V(storm). (the Vs should have arrows above them, as they represent vectors, not scalars)

The advantage that storm relative velocity can unmask circulations that might otherwise be hidden. For example, if you had a supercell moving away from a radar at 50 kts, but the mesocyclone only had a rotational velocity of 40 kts (i.e., (|max outbound| + |max inbound|) / 2 = 40), then, on base velocity, the mesocyclone would appear as an area of very high outbound velocities next to an area of low outbound velocities. That is to say, you would not see red against green. You would only see green. To the untrained eye, it would then appear that there was no rotation present. Yet, if you applied that storm motion vector (away from the radar at 50 kts), suddenly you'd see reds show up as 50 kts was subtracted off the rotation. (I know this isn't the best way to describe it, and I have pictures, so if you'd like to see them I could dig them up and PM you).

You would want to use base velocity for things such as bow echoes and squall lines, though, to determine if you should expect severe winds in an area. When it comes to straight line winds, base velocity is usually preferable.

 

[John Wetter]

In storm relative velocity you also need to be cautious of storms with deviant motion from the mean storm motion. In Level3, the storm motion is the mean of the volume, and in Level2, it is usually an assignable value, but you need to be aware of what SRV is using as the storm motion and what the storm you're interrogating is actually doing. They CAN be different!

To the original question though, it looks like it has been answered but it really is just aliased data, another limitation to be aware of!

 

[Jeff Duda]

Very true, John. But in GR3 at least, the storm motion vector will be displayed in the information, so you can at least determine whether or not the storm motion vector matches that of the actual storms.

 

[John Farley]

BTW, these radar archives are from this website.. http://www.ncdc.noaa.gov/nexradinv/
I'm sure many of you know about this site, but for those who don't it's a very useful tool for looking up past events. The database goes back to when the 88Ds first became operational.

I don't mean to hijack this thread, but I have a question. I was pretty excited when I saw this, because I did not know there was a site that went back so far. So I went back and downloaded radar images for my first tornado intercept, back in December (yes, really), 1996. Don't laugh, but all the radar I have for this storm from back in this lower-tech era is a videotape of the TV weather that night where the OCM showed a radar loop from the tornadic storm.

I downloaded the images fine, but I now have these files with a .z extension that I can't seem to decompress, much less actually view. So two questions - is there any free Windows software available to decompress these or do I have to purchase Winzip, and second, what additional software do I need in order to view the files from Windows?

 

[Jeff Duda]

I don't mean to hijack this thread, but I have a question. I was pretty excited when I saw this, because I did not know there was a site that went back so far. So I went back and downloaded radar images for my first tornado intercept, back in December (yes, really), 1996. Don't laugh, but all the radar I have for this storm from back in this lower-tech era is a videotape of the TV weather that night where the OCM showed a radar loop from the tornadic storm.

I downloaded the images fine, but I now have these files with a .z extension that I can't seem to decompress, much less actually view. So two questions - is there any free Windows software available to decompress these or do I have to purchase Winzip, and second, what additional software do I need in order to view the files from Windows?

Did you download level 2 or level 3 data?

 

[Joel Wright]

...I downloaded the images fine, but I now have these files with a .z extension that I can't seem to decompress, much less actually view. So two questions - is there any free Windows software available to decompress these or do I have to purchase Winzip, and second, what additional software do I need in order to view the files from Windows?

I downloaded a toolkit from this site http://www.ncdc.noaa.gov/oa/wct/

You can load the HAS file into this and it will display all the individual files from each radar scan. You can pick which ones you want to look at or animate by selecting them. Let me know if you need any help.

 

[John Farley]

Thanks. That toolkit did the trick, Joel. Here are the images I was after:

Reflectivity:

Base Velocity:

I am guessing most of you can tell where the tornado was. These images were from the time while the tornado was on the ground (about 2055-2058z). I got an animation, too. Thanks for the tip on the toolkit, Joel!

Actually, getting back to an earlier issue discussed in this thread, the velocity image above is a rare case where base velocity and storm-relative velocity give would likely give similar results, since at that point the storm was moving almost exactly at a 90 degree angle to the radar site, neither toward nor away from it. In the velocity animation, the difference in colors is most evident right at this point, and appears and fades as the storm crosses the location where it was moving at this angle. Earlier, there was more of a "toward" component, and later, more of an "away" component. I am sure in part the result is also because this is when the tornado occurred, but had the storm been moving toward or away from the radar, I doubt the velocity couplet would have been this obvious.