question about NWS radar.

[JonathanBelles]

Wha is the differecne between composite and base refelectivity?

[senorpepr]

First, you need to understand how the radar works. To crunch all the algorithms that it does, the radar takes scans of the atmosphere in different angles. The 14 different angles are 0.5°, 1.5°, 2.4°, 3.35°, 4.3°, 5.25°, 6.2°, 7.5°, 8.7°, 10°, 12°, 14°, 16.7°, and 19.5°.

What angles are use, depends on what mode the radar is in.

VCP 11 is a precip mode that scans all 14 elevations in 5 minutes. It’s best for nearby severe weather.

VCP 12 is also a precip mode that only scans 9 elevations, but does it in 6 minutes. The slower scans allows better coverage and is considered the best for your typical precip situation.

VCP 31 and 32 are both clear air modes that scans only 5 elevations in 10 minutes. VCP 31 has a longer pulse for increased sensitivity while 32 has an increased velocity resolution.

Now… with that said…

Base reflectivity provides a display of reflectivity for the specific elevation angle displayed.

Typically, when you go to NWS’s website, the base reflectivity you are seeing is the 0.5° scan only. Here’s the problem with using base reflectivity—you’re only looking at one piece of the atmosphere—essentially the lowest parts of the storm. At about 40nm from the radar, you are looking at the cells roughly between 3,000-8,000ft. At about 80nm you’re looking roughly between 7,000-15,000ft.

Composite reflective lets us look at the entire volume of the radar coverage by combining all of the available elevation slices into one. Basically, say the radar picked up 30dbz, 45 dbz, and 40 dbz at 0.5°, 1.5°, and 2.4°, respectively. The NWS standard base reflectivity would only show 30dbz. Composite reflectively would find the highest reading over each spot. In this case, it would display 45dbz.

[senorpepr]

Here's an example of what I'm talking about:

I grabbed a copy of radar images.

The first one is the 0.5° base reflectivity scan over North Carolina. http://img.photobucket.com/albums/v606/ ... MHX-BR.jpg

The second one is from the same time, but is a composite of all 9 elevations (since the radar is in VCP 12). http://img.photobucket.com/albums/v606/ ... MHX-CR.jpg

[JonathanBelles]

so composite is more accurate?

[senorpepr]

so composite is more accurate?

I would say that composite is more complete. Base only gives you a small piece of the story and could be missing stronger parts of the storm.

[wxmann_91]

so composite is more accurate?

Up there, yes. Down here, no. Composite shows areas of virga that base can't pick up many times. Such as with thunderstorm anvils, if you've been paying close attention to the tornado outbreaks the past couple of days. However, for what's falling onto the ground, base reflectivity is usually the most accurate, more accurate as you get closer to the site, and growing in inaccuracies once you get farther out from the radar site.

In terms of severe wx, composite and higher elevation scans can be extremely useful because severe thunderstorms intensify and weaken from the top down. Base doesn't tell you a lot in those situations. Higher elevation reflectivities that are weaker than the base reflectivity shows a weakening storm, and vice versa. However, in mature severe wx situations, the base reflectivity is where you will find those classic signatures such as the hook echo, bow echo, etc.