Gate-to-Gate

[Chris Lott]

I have heard Meteorologists use this term when they have identified a tornado on radar, but exactly what do they mean by Gate-to-Gate?

 

[Steve Miller TX]

My explanation is if you are looking at the velocity radar image (specifically storm-relative velocity), each pixel is a "gate". When you have two pixels next to each other with one showing outbound (usually red color) and the other showing inbound (usually green color), that is pretty much gate-to-gate shear and also known as a velocity couplet. Here are some radar images:

http://www4.ncsu.edu/~nwsfo/storage/cases/...2/1944z.srm.jpg

http://newweb.wrh.noaa.gov/wrh/98TAs/9812/12fig3.jpg

http://newweb.wrh.noaa.gov/wrh/HNXFIG5.GIF

 

[Glen Romine]

... each pixel is a "gate". When you have two pixels next to each other with one showing outbound (usually red color) and the other showing inbound (usually green color), that is pretty much gate-to-gate shear and also known as a velocity couplet....

I'd add to that the importance of making this assessment only with true level II radar data (not the stuff you typically find available on the net, which is level III) and recognizing if the velocity "gates of interest" are parallel (shows divergence/convergence) or perpendicular (shows clockwise or counterclockwise rotation) to the the radar beam, or perhaps some combination. Gate terminology comes from breaking up the radar return signal into small samples (gates) in time as the radar beam pulse returns back to the radar receiver.

I'll try below to offer a simplified description of how this works. Ignore for a second the radar is moving, and assume it sends out a single pulse of energy in a given direction. Immediately after sending the pulse the radar "listens" for an echo as the energy passes through the atmosphere interacting with whatever might be in the path, and "reflecting" some of the energy pulse back towards the radar receiver (more reflection as more matter interacts with the pulse). Since the energy is moving at the speed of light, we can determine how far away the interaction occurred by how long it took the energy pulse from the time it left the radar to reach the object and return back to the radar. For processing this information, the reciever will listen for a given time period for any return signal, and this represents a region in space along the beam path - a.k.a. a gate. Processing of the intensity of the return signal gives information on how much substance is there (radar reflectivity), and can also be processed (checking for amount of phase shift in the pulse) to determine what the motion (radial velocity) was if there was enough of a signal. Allowing now for the radar to rotate a small amount and sample along an adjacent beam path, if two gates an equal distance from the radar in adjacent beams show a difference in radial velocity, then the difference in velocity is called gate-to-gate shear.

Glen

 

[Steve Miller TX]

I'd add to that the importance of making this assessment only with true level II radar data (not the stuff you typically find available on the net, which is level III)

Well, maybe I can learn something then. Why can't you make such a determination with L3 data? It seems to me that gate-to-gate would be the same with either level except that L2 shows greater detail. Could you explain?

 

[Glen Romine]

Why can't you make such a determination with L3 data?

I guess maybe it is not appropriate to say that you can't, but just that the value you get will be different. Level III is reduced data to make it easier to ship around (smaller files), with a radial resolution of 1 km, whereas level II velocity gates are 1/4 that size (or 1/8 with some newer algorithms coming out these days). In other words, you really aren't looking at the real gate velocity values in a level III product. So, if a velocity couplet is small, it is easily smeared in the coarse resolution level III product. Further, the data values are reduced into large bins (5 knot intervals). Also, some radar displays then interpolate the level III product into a different coordinate set, which can lead to even further distortion/smearing of the original data, making the level III radial velocity product difficult to use in some cases where fine detail is needed (such as looking at shear across adjacent gates). It is still ok though for finding a mesocyclone scale vortex - but pretty unreliable for finding shear signatures associated with tornado cyclones that are very far away from the radar. Here is an example image comparing the two I found online:

http://www.wxc.com/level2/images/vel_comp.jpg

Probably other (better) comparisons out there.

Glen

 

[Steve Miller TX]

I figured it had to do with resolution, but seeing examples of that really drives the point home. Now I know why I'm scratching me head when a TOR is issued and I don't see a distinct couplet on my L3 data....vie versa when I think they should issue a TOR and don't. I was also unaware of the additional "smearing" of the data that you mentioned. Definitely something to seriously consider from now on personally. Thanks for the explanation!

 

[Greg Stumpf]

Glen touched on this issue, but typically most Level II viewers (Gibson Ridge notwithstanding) transform the data from the native polar coordinates to a cartesian grid. Once that is done, the notion of radial velocity no longer exists, as there are no more radials. Also, when data are resampled to another coordinate system, lots of the original data points will be tossed where the data are oversampled.

It helps to reduce the velocity data to a scalar quantity, such as azimuthal shear (similar to the NSSL WDSSII "rotation" product), which can then be transformed into a Cartesian coordinate system. But you stil suffer from the oversampling issue - any strong gate-to-gate shears close to the radar may end up being "smoothed away" (as in that NIDS example).

 

[Chris Lott]

Back in October of 2001, what looked to be a fairly large tornado takin shape on radar, was approaching my house at 10:00 at night. While I'm listening to the Meteorologist on t.v., he used the term gate-to-gate, saying that the wind speed measured from the radar and toward the radar were 158 mph( I lost electricity after that). So, is this basically what it means, that the two measurements the radar picked up were both 158 mph, both toward and away from the radar, therefore indicating the gate-to-gate shear within the storm?

 

[Robert Dewey]

So, is this basically what it means, that the two measurements the radar picked up were both 158 mph, both toward and away from the radar, therefore indicating the gate-to-gate shear within the storm?

You usually add the two numbers together. So, that would mean you could have had a couplet of 79mph towards and 79mph away, or 85mph towards and 73mph away, etc..

 

[Chris Lott]

Thanks guys, you've been a big help.

 

[Jarrod Cook]

Here is a great example of gate-to-gate shear. It is a SRM image of the July 13, 2004 Roanoke, IL F4 tornado. Notice the bright green pixel right next to the bright red pixel.

http://www.crh.noaa.gov/ilx/events/jul1320...lx-srm1941z.png

Jarrod Cook
KC9CJY

 

[Mike Johnston]

So, in general, would it be advisable to turn off the "smoothing" function on level 3 viewer when looking at velocities? It looks nicer to the eye, but perhaps lose some resolution?

 

[Jeff Snyder]

So, in general, would it be advisable to turn off the "smoothing" function on level 3 viewer when looking at velocities? It looks nicer to the eye, but perhaps lose some resolution?

Personally, I'd turn off smoothing for all products if you're doing serious radar interpretation. Smoothing tends to smear tight gradients, which is exactly NOT what you want to do when looking for circulations on velocity images. I guess it just matters how important the data is to you. For the casual user, the "nicer to the eye" lets one get a nice glimpse of what's gonig on, though it's usually entirely unwanted when doing anything more than casual radar analysis.

 

[rdale]

"Personally, I'd turn off smoothing for all products if you're doing serious radar interpretation."

In Level III yes - I think it does add value without any sacrifice with 1/4km velocity in L2.

 

[Jeff Snyder]

"Personally, I'd turn off smoothing for all products if you're doing serious radar interpretation."

In Level III yes - I think it does add value without any sacrifice with 1/4km velocity in L2.

Rob is correct -- I should have specified that I was referring to the more coarse Level III / NIDS resolution...