GRlevel 3 SW 32

[John Cameron]

What is spectrum width data used to determine?

 

[Greg Blumberg]

Spectrum width is the measurement of the distribution of all the values returned from a certain pixel displayed by the radar. If radial velocity is the average of all these values, then spectrum width can tell you how narrow or broad the values returned are. If the values are broad, then one can guess that the pixel being scanned is pretty turbulent. A more mathematical approach to describing spectrum width would be standard deviation.

 

[John Cameron]

I'm also tryin to figure out what the difference is between MAXDV and MAXSHR when looking at TVS attributes

 

[Greg Blumberg]

Ah. Someone asked this question a while back and I can't find the thread, but here's the thread I referenced.

http://www.stormtrack.org/forum/archive/index.php/t-3252.html

 

[John Cameron]

then one can guess that the pixel being scanned is pretty turbulent.

Then you would give that pixel credence or not?

 

[Greg Blumberg]

I would, but only if there was a consistent pattern of incredibly broad spectrum widths all across the general area. I'd post a snapshot I got of a supercell near the radar, but it's not entirely showing what I'd like to show in terms of turbulent downdrafts.

 

[Jason McClintock]

So in spectrum width the higher the speed in knots the more turbulent that pixel is?

 

[Mike Smith]

So in spectrum width the higher the speed in knots the more turbulent that pixel is?

Yes. In fact, I did an involuntary test several years ago.

I was a passenger flying from ICT to DEN. The pilots boarded and there was some delay. It was 45 minutes from their boarding to the passenger boarding. I was monitoring the wx at the gate and during that interval the NWS issued a severe thunderstorm watch for most of western KS and the storms had really intensified. When I boarded, I took my laptop up to the cockpit and offered to show it to the captain and FO. They dismissively said they didn't need to see it. With a sinking feeling, I said OK and took my seat.

We were in clouds near HYS when the captain came on the PA and said, "flight attendants take your seats!" in a rather urgent tone and the seatbelt light came on. The FAs were in the aisle with the drink cart. Seconds later, we were in severe turbulence, definition here: http://bathursted.ccnb.nb.ca/vatcan/fir/moncton/WeeklyTopics/Archives/20031207/CurrentTopic.html .

Even though I am a very frequent flier, this is the only time I have been in severe turbulence. The water in my glass rose as a globule, things were thrown about, and most of the overhead compartments opened. The flight attendants, who are trained to lie down in the aisle if they get into unexpected severe turbulence, levitated. The passengers had to try to hold them down. It went on for what seemed like 5-6 minutes, but it was probably shorter.

Once we were out of the turbulence, I used the Airfone to call back to WeatherData® and have them pull the ASD data (aircraft position) and radar data, including spectrum width.

Through a stroke of luck, the GLD -88D was pointing at the correct altitude and location at the time of the encounter and, as you would expect, it showed high values of spectrum width where we penetrated the thunderstorm. After not caring about the weather before we took off, the flight crew "got religion" as the ASD data showed the rest of the flight took a meandering path all the way up to around LBF as we avoided even the slightest rain shower on the rest of our flight to DEN.

So, I know that spectrum width works in practice as well as in theory!

 

[Greg Blumberg]

That's an excellent story, Mike. I don't think I have any better example than what you just mentioned there. There are plenty other applications to spectrum width, such as identifying especially turbulent downdrafts and three-body scatter spikes (hail spikes.)

Les Lemon has a small paper on spectrum width online:

http://www.stormchasers.au.com/lemon13.htm