NEXRAD Level 3 data

[Stephen Locke]

When the atmosphere is generally clear
I've noticed the radar site reports a big glob of the entire colour spectrum even during the day. Does this mean they power up? Stations in the vicinity of legitimate precip appear to have the (gain?) turned down and the map is readable and individual storm cells can be discerned. Question; Is this a normal behaviour of all Level 3 data programs? And, who or what decides to turn the gain down when real conditions warrant so we get a clear view.

 

[rdale]

Ground clutter is a fact of life -- do a quick search on that phrase and "anomalous propagation" for more info. Nothing is "turned up" or "turned down" in the radar's power.

 

[Greg Higgins]

Also need to look at what VCP mode they are using.

http://www.srh.noaa.gov/jetstream/doppler/vcp_max.htm

Greg Higgins

 

[Stephen Locke]

Also need to look at what VCP mode they are using.

http://www.srh.noaa.gov/jetstream/doppler/vcp_max.htm

Greg Higgins

Thanks that link was informative. I guess I am still wondering who makes the decision to switch from clear air to precipitaion mode. And, what is the precip/amount threshold for that decision. Or, do they switch based on a forecast.

 

[rdale]

Clear air to precip is automated based on settings inside the software, which look at how intense and how widespread the precip is, although I think they can manually make that happen too. VCP settings are decided by the meteorologist at the NWS office.

 

[Chris Bowman]

There was a recent update that automated switching from clear air to precip and back. It is based on a dbz value detected over and an area in km^2. The option is still there to run either partially or fully manual and this needs to be done some mornings when AP is very bad and trips (tricks) the computer algorithms. Wichita uses 35 dbz over 50 km^2. It works well with switching for convection but sometimes a manual switch to precip mode is needed for light stratiform precip. We have also been known to force a precip VCP in anticipation of convection firing up shortly so as not to halt a volume in the middle of scan or wait 10 minutes for it switch.

Also if that threshold is met, like during a snowstorm and the mets want the sensitivity that comes with VCP32 for detecting something like heavy banded snow, they have that option to force the radar to stay in clear air mode. There are trade offs to that higher sensitivty in that a volume scan takes 10 minutes to complete. Which is fine for snow storms but not so much for convection.

 

[Scott Roberts]

Wichita uses 35 dbz over 50 km^2. It works well with switching for convection but sometimes a manual switch to precip mode is needed for light stratiform precip. We have also been known to force a precip VCP in anticipation of convection firing up shortly so as not to halt a volume in the middle of scan or wait 10 minutes for it switch.

Nice to know what the values are...and, I had no idea it took so long to effect a switch. But I guess it makes sense, given the technology. I'm one who like Clear-Air mode on just as long as practical, because of the extra data it provides -- I'm thinking particularly of those subtle boundaries that disappear when the mode switches.

 

[Stephen Locke]

I'm one who like Clear-Air mode on just as long as practical, because of the extra data it provides -- I'm thinking particularly of those subtle boundaries that disappear when the mode switches.

Does clear air mode increase distance as well?

 

[rdale]

Clear-air is a much slower rotation rate so updates are only every 10 minutes - a tradeoff...

 

[Aaron Wesson]

It all depends on what the clutter suppression is set to.

Clear Air is good for picking up snow but that is about it.

 

[Joe Dorn]

It all depends on what the clutter suppression is set to.

Clear Air is good for picking up snow but that is about it.

Also picks up bats as they come out of their daytime homes and birds as they migrate. Look at the radars around central Texas and to the west at or around sunset. They usually appear as an expanding circle as they start their nightly search for things that like to bite me. The exit becomes a little elongated if the surface winds are high.

If you are expecting some build-ups this can be disconcerting the first few times you see it but you soon learn where the locations are and go on about more serious business. I don't know if the bats have headed south yet since we have had so little significant weather lately and I have not been watching radar.

The best display I have seen is on IH-35 in Round Rock, Texas, just north of Austin. The second best is in south Austin where Congress Avenue crosses the Colorado River. They love the overpasses and Texas DOT takes extra effort to make them welcome.

If the clutter is covering up some weather activity I have adjustments to my GRLx color tables to disable the display of less intense returns...

 

[rdale]

Clear Air is good for picking up snow but that is about it.

I disagree... It's GREAT for picking up boundaries - some of which would initiate convection later on.

 

[Aaron Wesson]

Agree, you can also use spectrum width to find boundaries.

Good point rdale...I was not yet awake when posting.

 

[rdale]

I'm still amazed at lack of SW studies... I think there's a lot of usefulness which hasn't been developed.

 

[Danny Neal]

Has there ever been any talks of adding radar sites where there are currently holes? Is there not enough funding? Not enough of a need? I don't recall reading anything about it.