Why do NEXRAD's have so much more false return?

[sjcscanner]

You can clearly see it on this shot of the mosiac I took this morning. Almost every NEXRAD staion is like that. The news doesn't have it on their radars, does anyone know why it's on the NEXRAD's?

 

[bezking]

They're old? NOAA Radars are called WSR-88Ds and the 88 is for 1988, when they went into service. However I don't know the lifespan should be for a radar, so I couldn't say for sure that this is why...

 

[rdale]

Absolutely has nothing to do with age... An upgrade just completed this summer greatly increased their sensitivity. What's happening is that during the overnight, the air near the surface cools faster than the air aloft. The inversion (same thing that lets you hear distant radio transmissions) does the same thing to radar beams, so they bounce off the inversion, hit the ground, come back, and the radar thinks it is light rain.

It's not really a big deal, as most radar software displays have a way to filter it out.

 

[n5ims]

The news radars often use software to remove these "false" echos so their viewers are not confused. Also, the TV station radars normally are on different bands than the weather service radars and may pick up things differently. The mode selected for the radar can also affect what is seen. We used to see planes and flocks of birds on one of the TV station radars when certain modes were selected.

 

[poltergeisty]

Believe it's called clutter. Changer your settings.

 

[rdale]

No, it's called AP (anomalous propagation.) Ground clutter is right around the radarsite, from buildings / etc in the direct path of the beam.

 

[DaveNF2G]

Just curious about the filtering software. Does it reject the AP by comparing the round-trip time for the signal? I would think that the extra bounce would add a tiny but measurable delay to the return signal.

 

[rdale]

The NWS runs clutter suppression at the radar site... In a nutshell - it looks for returns that have no velocity but high spectrum width.

When it looks at velocity, it averages all returns over the 1/4km bin. If the average is zero, but the SW (think of it as uncertainty) is high then it rejects as non-meteorological.

For outside software users, basically we just turn off those very low returns since we know they aren't precip. Worse case - it won't show some sprinkles that evaporate before hitting the ground.

 

[poltergeisty]

Interesting! http://en.wikipedia.org/wiki/Anomalous_propagation

 

[RayAir]

They're old? NOAA Radars are called WSR-88Ds and the 88 is for 1988, when they went into service. However I don't know the lifespan should be for a radar, so I couldn't say for sure that this is why...

They are still called WSR-88's but they are NOT 1988 technology. They have been updated many times. What you see on the radar pic you have there is called backscatter.

 

[rdale]

Backscatter is something different - not sure that the 88D's would show that on a screen, but in any case it's definitely not what he is seeing.

The core of the radar is still 1988's technology though.