General Weather/Earth Sciences Topics => Severe Weather => Topic started by: DaleReid on July 19, 2020, 01:37:18 PM

Title: Current NWS radar display
Post by: DaleReid on July 19, 2020, 01:37:18 PM
Last night as the severe weather moved through the Minneapolis, St. Paul area, the major TV stations had their updates on where the storms were, when the tornado was detected, etc.

Both were using what seems to be the radar feed from the NWS at Chanhassen for their live updates.  I don't think either WCCO or KARE have their own radar any more, or probably not co-located with the NWS.

In any event, as the high definition display was shown, as the radar sweep passed, there was a short period of time, a second or two, in which the really heavy rain and the tornado area (not the wind velocity display, the precip display) would change color, and fill back in to what it was just prior to the sweep.

I realize there must be some processing going on as new return data gets processed, but has anyone else noticed this, and if so does anyone know what is going on during that time?

I don't recall ever seeing this before, so it may be due to the update in radar there last fall, as part of the time the radar was off line.  At one point they were also showing the radar from Duluth/Superior for storms to the north and it did the same thing.

I seemed to be only when they were in high resolution, close up mode.  When they would scale back and show more of the state the sweep line went away, and the 'processing' that seemed to be going on with the display in the active areas no longer changed.

Any radar experts out there?

Title: Re: Current NWS radar display
Post by: chief-david on July 19, 2020, 01:46:35 PM
You would have to contact each station to see if they use them.

KARE still has one on Hwy 55. Dont know if it is used.

Fox 9 has one in EP. Pretty sure it is theirs.

Not sure about 5 or 4. I do not watch them.

The mets will communicate with you. If not, I can get a message to Belinda Jensen-a coworker is my neighbor.
Title: Re: Current NWS radar display
Post by: ocala on July 19, 2020, 03:06:28 PM
Not an expert but I'll take a stab.
First off I don't think there is a sweep anymore. Believe that it used just for eye candy. The NWS radars do take a 360 degree sweep of the sky but they do it at different levels. Base Reflectivity Level 3 is what you usually see when viewing radar data. It scans the lowest level at 0.5 degrees above the horizon so what that radar see's for a storm 30 miles away it's going to show different data for a storm it see's at 100 miles away because of the angle of the beam of the radar.
Composite Reflectivity Level 3 scans  all levels. I think they are 16 levels but not sure.It also takes longer to process one image because the radar is scanning all levels. So CR gives a better all around view of whats going on but Base Reflectivity has better resolution.
What you are talking about could have something to do with the zoom level of the radar. Most TV radars smooth the image to give viewers a better view then the blocky image the radar puts out. My guess is that during that zooming process they may have changed from Base Reflectivity to Composite Reflectivity, or visa versa, yielding slightly different data because of the different scan levels.
That's my uneducated guess. :grin:
Also if you want to see a real big difference in radar data check out Level 2 data. Much higher resolution. I believe one scan of Level 2 data is about 2 MB where one scan of Level 3 data is about 50 kbs. Check out L2 versus L3 this afternoon from Western NY.
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Title: Re: Current NWS radar display
Post by: ocala on July 19, 2020, 03:13:29 PM
Oops. Posted the same one twice.
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Title: Re: Current NWS radar display
Post by: DaleReid on July 19, 2020, 08:10:41 PM
I should have hit the 'record' button on the TV and then tried to photograph what I meant.

Hope we don't have the on-the-ground excitement very often, but it is weather.

I'll try to do some research, and Chief, if I can't get answers, perhaps see if it is Ok with your neighbor and friend if I formulate a question to send to her for consideration.

As far as the sweep, the antenna DOES rotate, and I know the wizards in the little chips do marvelous things digitally after the basic data are received.  But as you say, it is more like an old version of a PIP display, although i do know by watching the 'sweep' across an area with a fast moving rain front there is a little change in position as the sweep indicator passes.  Probably a combination of both.

In today's digital world, not much surprises me any more.  In the days of analog and the tremendous attenuation by rain and other precip the further you go out, the more of a chance of having what looks like a clear spot become very congested once you try to fly into it and find out that it was just being hidden.  Of course the aircraft radars have far smaller antennas and less power.

But think of ground clutter with evening temperature changes and try to filter that out with pure analog stuff.

I welcome more input from others and will let this thread know with an update if I can find anything more to share.

I have to leave with one of my favorite RADAR stories.  The DEW line up in Alaska and i think shared with Canada was just online when a few incoming Russian planes were seen.  Then a few minutes later, a lot more, and finally a whole slough of them.

Turns out the enormous antennas and megawatts of power were seeing the moon rise and the long delayed echo was falsing the displays.  Nowadays pulse encoding and other tricks would null out spurious returns but it made a few people very nervous for a few minutes.  A little astronomy lesson and things settled down.

And of course in early June, the LaCrosse, WI, radar picked up a huge storm, moving the wrong way in clear air.  Yup, the annual buffet for fishes, also known as the Mayfly swarm was a particularly vibrant color change for a bit.

I'm sure there are others, but fun for surprises.

Title: Re: Current NWS radar display
Post by: CW2274 on July 19, 2020, 08:39:03 PM
But think of ground clutter with evening temperature changes and try to filter that out with pure analog stuff.
Back in my ASR-5 days, during a certain time of year, we'd get huge splotches on the display as the sun was rising. MTI and other "tools" would also be use to to differentiate between a true radar paint and not was common. The ASR-5 also only had two levels of wx display, light and not so light. If it got bad enough, we'd switch from linear to circular polarization to help cut the "junk", which was actually precip. In 1998 we installed an digital ASR-9, no sweep perse, but the workforce complained enough that a "virtual" sweep was employed to keep us happy. We hate change.

Edited three times because it's been a long time ago to remember right. #-o