Nexrad Wx Question...base vs. composite

killbilly

Vocals, Lyrics, Triangle, Washboard, Kittens
What's the difference between the base reflectivity and the composite reflectivity loops that I see on the NWS feeds?

I was just looking at the mess over San Antonio right now, flipping back and forth between the two loops. They seemed quite similar, although the composite seemed more comprehensive.

Base of what? Composite of what, what and what?

Thanks. Sorry if this is a dumb question.
 

killbilly

Vocals, Lyrics, Triangle, Washboard, Kittens
Thanks. I didn't see that link when I googled.

Any practical advice to be gleaned from looking at both composite and base?
 

meritflyer

Well-Known Member
When compared with Base Reflectivity, the Composite Reflectivity can reveal important storm structure, features, and intensity trends of storms. Although the Composite Reflectivity product is able to display maximum echo intensities 248 NM from the radar, the beam of the radar at this distance is at a very high altitude in the atmosphere (example picture). Thus, only the most intense convective storms and tropical systems will be detected at the longer distances. While the radar image may not indicate precipitation, it is quite possible that the radar beam is overshooting precipitation at lower levels, especially at greater distances.
Just from browsing the article, it'd appear CR may present more data that would indicate the trend of a storm system v. BR.
 

surreal1221

Well-Known Member
Base displays the highest reflectivities of the lowest elevation for a certain volume scan (.5 degree angle). Hence the name base, as in Latin for bottom (just kidding about the Latin part).

Composite displays the highest reflectivities of ALL the elevations combined within the volume scans. So an area has a base reflectivity of say 17dbz, but up at the elevation of 15.5, the reflectivity is 45dbz, and above that at 19.5 is 10dbz. Nothing is higher than 45dbz through that column, and as such that 45dbz will be splattered onto the radar depiction for that location since it trumps all the other reflectivity values for that location at the other elevation scans.

Practical applications, depends on what you're doing. As a pilot, sure. The composite shows you the strongest echos through the column vertically. But even that isn't a great insight because unfortunately the aftermarket public radar use sites do not provide the ability to view the different volume scans at each vertical level. So sure, while you know that hey, that's the highest reflectivity in that column vertically, you can only challenge it against the base elevation scan and say "Okay, well it's not on the base so it must be higher than .5, but hell if I know where it really is vertically." Common sense would lead you to use some basic met knowledge and say hell, it's a thunderstorm, it's probably in the 5,000 to 35,000 feet range. But it's also key to understand how far a radar can send out a pulse, and how that pulse, distance, and height all change with it's relative elevation that it is scanning. Whole 'nother story, won't even bother.

Base though, practically for a pilot, is really a quick glance tool to see where the surface reaching precip is occuring.

Now, practical applications for a meteorologist are a totally different ball game, and I won't venture into that arena again on this forum.

Now, who can tell me what the different VCPs are, and how frequently they update, and their designed use, along with where their "base" elevation is and their highest elevation that they will provide in the scan?
 

wheelsup

Well-Known Member
In my opinion only....as a pilot...

Short answer...I would be looking at the composite.

Storms that are in the their "updraft" or building phase can be holding a lot of liquid at altitude and none of it may be hitting the ground or even close to it, hence no return on the base or lowest sweep but they won't be pleasant to fly thru or under, as they could explode in downdrafts at any time.

That, and it gives you worst case scenario. I like worst case scenarios. It means I have all the info.
 

surreal1221

Well-Known Member
never heard of this updraft or building phase ;)

three stages of thundestorms :)

Sorry to bust your balls, just being an ####### now. ;)
 

wheelsup

Well-Known Member
I'm a pilot...not a meteorologist....:D.

There are 3 stages in my mind:

The don't go there soon phase, don't go there now phase, and I can go thru there in a bit phase :D.
 

BrewMaster

Well-Known Member
Compos....who?

No need to learn all that jazz, just remember how long it takes to turn 180 degrees......and how to make PIREP...:D
 

killbilly

Vocals, Lyrics, Triangle, Washboard, Kittens
Compos....who?

No need to learn all that jazz, just remember how long it takes to turn 180 degrees......and how to make PIREP...:D
Heh, I get what you're saying, but I'm really interested in the weather technology. There's no way to fly in TX in the summer without doing a bit of storm-dodging, and that radar picture is helpful in making the go-no-go decisions. Also - I've got XM Wx on my GPS, and I'm still learning to interpret the data it provides.

But mostly, I'm just trying to learn how to correllate what I see on the radar picture with what's out there in the sky. I'm pretty darn conservative at this stage of the game - a whopping 73 or so hours, so I figure any time I spend learning about my flying environment is well-spent, y'know?
 

BrewMaster

Well-Known Member
Crap, sorry, didn't mean at all for that to come off mean. Just trying to make a funny. You are probably doing ten times more than most students:rawk:...keep on truckin'

There is a fairly old, really boring, and very informative video out there that talks about onboard radar. It is a bit out-dated but provides more than a solid foundation to build on that knowledge.

http://www.sportys.com/acb/showdetl.cfm?&Product_ID=9700&DID=19

You probably wouldn't want to pay for this, but if you have other means of finding it, it is a very good video.

Good luck!!
 

surreal1221

Well-Known Member
There are some significant differences between the technology used, the processes, and the data received between on board weather radar systems and WSR-88D (NEXRAD) systems.

Not too sure if Killbilly is utilizing onboard radar, a true onboard radar system (not a NEXRAD overlay in G1000's).
 

killbilly

Vocals, Lyrics, Triangle, Washboard, Kittens
There are some significant differences between the technology used, the processes, and the data received between on board weather radar systems and WSR-88D (NEXRAD) systems.

Not too sure if Killbilly is utilizing onboard radar, a true onboard radar system (not a NEXRAD overlay in G1000's).
It's the NEXRAD feed that XM sells by subscription. I use an AnywhereMap ATC with the Wx Works box, so I'm assuming it's the WSR-88D feed. It's pretty good about updating.

I just use it as part of the total decisionmaking process. It's nice to have - especially when a cell pops up right in your path and you need to do a quick 180....
 

Boris Badenov

Let's get this thing on the hump!
At 73hrs if there's red within say 20 miles of your path, sit on the ground. If there's yellow go around it. Not super helpful with the technology, sorry, because I, too, rode the short bus, plus it's been my experience that "weather is where you find it" when it comes to operating an airplane (that is to say: sometimes things will look peachy and suck, sometimes you'll cinch the belt down so much it hurts and...fly through a gentle drizzle). Don't mean to detract from the conversation. Definitely some interesting reading from surreal...just remember that things always look a little different when it's your dumb ass doing the penetrating.
 

killbilly

Vocals, Lyrics, Triangle, Washboard, Kittens
At 73hrs if there's red within say 20 miles of your path, sit on the ground. If there's yellow go around it. Not super helpful with the technology, sorry, because I, too, rode the short bus, plus it's been my experience that "weather is where you find it" when it comes to operating an airplane (that is to say: sometimes things will look peachy and suck, sometimes you'll cinch the belt down so much it hurts and...fly through a gentle drizzle). Don't mean to detract from the conversation. Definitely some interesting reading from surreal...just remember that things always look a little different when it's your dumb ass doing the penetrating.
Yeah, 20mi is the rule of thumb I've got. For example, I fire up the plane and let the box start the weather download while I'm getting ATIS, calling departure, etc. Plan was fly from AUS up to Taylor to get some gas, then go tool around.

Well, I get to the runup area and the screen refreshes and there's a nice little red pocket sitting right over Bird's Nest - directly in my path to Taylor. So I look around at the sky, and to the south it's looking better, and the Wx says that other than a little green pocket 25 mi west of Lockhart (K50R), it's pretty clear.

Tower was gracious enough to re-do my departure clearance for me, sent me on m'way.

If I hadn't had that, I likely would have blundered into that pocket - I couldn't see it. Maybe I would have once at altitude, I dunno.

When I was returning to AUS, things looked good, ATIS was fine, and as they brought me in on a left downwind I see two massive rainshafts north of the airport, little south of Georgetown, I'd say. Sure enough, there they are on the screen - pockets of green and yellow. Plenty of space for me to swing in and land, though. Learned - on that approach - just how quick you can slow a 152 down, too. :)

The point being - and the reason I started this thread - is that I won't always HAVE the Wx device in the airplane, or it may crap out. So I'm trying to learn about what's really up there from the pre-flight routines, and using the NEXRAD stuff from the computer is a helpful tool. I just want to learn more. I'm starting to turn kind of obsessive about weather, and sometimes it's frustrating when I don't have all the understanding that someone like Surreal does. Thus, all the questions...
 

BrewMaster

Well-Known Member
There are some significant differences between the technology used, the processes, and the data received between on board weather radar systems and WSR-88D (NEXRAD) systems.

Not too sure if Killbilly is utilizing onboard radar, a true onboard radar system (not a NEXRAD overlay in G1000's).
I guess it was just food for thought, if I remember correctly there is also a lot of info about storms (sizes, speeds, etc.)

Go Cubbies!
 

Yank&BankmyRJ145

New Member
Killbilly,
Your on the right path, learn and learn read as much as possible about flying. When I started flying I too became a weather freak, always looking out the window at the clouds. I would start referencing the Wx maps, then go outside and see what that cloud really looks like. Clouds are your road signs as to what is going on around you.
Even on my days off, I still pull up the Wx and look at whats going on around the country. During my overnights, I turn on the weather channel as I get dressed. And yes I have caught myself watching the weather channel for hours on end. But keep on learning all you can about weather.

"When all else fails stay VFR"
 
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