Peter r. 2009-11-23 18:22:24
Thanks to Rod Machado’s article in May’s _AOPA Pilot_, which was a
recap of another aviation safety enthusiast’s lecture, I just learned
that there may be a big discrepancy in a storm’s intensity when
comparing its base reflectivity (what is actually falling out of the
cloud) to its composite reflectivity (what is going on inside the
The probability is greater, this article implied, that the composite
reflectivity will demonstrate more intense activity inside the cloud
than the base reflectivity depicts. This, in turn, implies that a
pilot of a small aircraft deciding to penetrate precipitation based on
base reflectivity may, in fact, be penetrating an area of stronger
Furthermore, the article mentioned that most (if not all?) current
downlinked/uplinked weather products only display base reflectivity due
to the attractiveness of the overlay on the display (less blocky in
appearance). This might lull some pilots into believing that
convective activity is non-existent or weaker than it really is.
Of course, those of you who fly aircraft equipped with active radar
knew this all along, but those of us low-time GA pilots being sold the
current uplinked/downlinked radar solutions may not have been aware of
First, a rather basic question: Why is it that all of the moisture
that appears in a composite reflectivity map is not all falling? Is
this due to the strength of the updrafts within the cell? It seems to
me that moisture that appears that heavy on radar would be heavy enough
to all fall out of the cloud.
Secondly, when ATC reports specific levels of precipitation, am I to
assume that they are giving us base reflectivity? What about when FSS
gives us precipitation levels en route? Base reflectivity only?
Should we be specifically asking for composite reflectivity?
Looking forward to learning more about this topic.
Mike rapoport 2009-11-23 18:22:26
Yes, the fact that the water is suspended high in the storm indicates that
there are strong updrafts. In a sense, the amount of water that can be
suspended aloft is a measure of the “strength” of the storm. It takes a lot
of energy to lift and suspend thousands (millions?) of tons of water miles
into the air. Once the water is dumped the storm is pretty much over. I
have flown through nasty looking CBs that showed only scattered green on
radar and there was little or no turbulence. It is diffucult to trust your
radar in such cases!
I don’t know what they use or even if it is consistant. I think that they
use the scale where a level three is a thunderstorm, four is a strong
thunderstorm and five and six are considered extreme.
Bob gardner 2009-11-23 18:22:37
If you are referring to Center when you ask about what ATC is seeing, the
reality is that they do not see anything like what you see. The Weather and
Radar Processing (WARP) system, which is installed in all Centers, has a
display unlike anything else…it shows three levels of blue (no green, red,
or yellow, as in cockpit displays), described as moderate, heavy, and heavy
(no fooling). At the recent NATCA conference, controllers expressed their
dislike of this display because it is hard to read.
IOW, they are not seeing reflectivity at all.
Bob gardner 2009-11-23 18:22:40
I might also note that the controller’s WARP display can be as much as 12
minutes later than real time…note the word “processing” in its name.
Stan gosnell 2009-11-23 18:22:45
The updrafts can be very intense. How do you think baseball-sized hail
occurs? Water drops get lifted so high they freeze, then eventuall fall,
only be lifted again after picking up more water, and the process repeats
until the ice ball gets large enough to finally fall through the updrafts
or else (more likely) gets thrown completely out of the updrafts
horizonally, and falls. If thunderstorm updrafts can lift huge chunks of
ice, they can certainly hold up water drops, and even airplanes can be
“They that can give up essential liberty to obtain a little temporary
safety deserve neither liberty nor safety.” B. Franklin
Jay beckman 2009-11-23 18:22:53
In light of the fact that summer thunderstorm activity is coming quickly
(and our traditional “monsoon” season) here in the desert, this subject is
In reading the descriptions of “Base” vs “Composite” at the NWS website, am
I taking away the right idea:
Base: Image derived from a single radar azimuth
Composite: Image derived from multiple radar azimuths then, well, composited
into one 2D top-down view?
John clonts 2009-11-23 18:22:59
I want the system that is installed in the plane that’s flying just east of Mullen!!
Icebound 2009-11-23 18:23:10
As I read it, Not exactly, but sort of. Depends on your definition of
Based on NOAA’s description, Composite is just *the strongest* of the echoes
from the scans at many elevations. In other words, the radar scans the
entire 360 degree azimuth scan at several elevations, say 0.5 degrees above
the horizon, 1.5 degrees above the horizon, 2.5, and 3.5, etc. etc.
Apparently up to 14 such different elevations may be used up to 19.5 degrees
above the horizon. What is displayed on the map is the *strongest* echo of
all of those scans. In some areas of the map the strongest echo may be from
the 1.5 degree scan, in another it might be from the 3.5 degree scan, etc.
hence *composite*. This would presumably assure you that the echo you see,
is the “most intense” echo of the storm in that area, no matter what height
it actually came from.
But the multiple scans (and the image processing) all take time, so the
images are updated only every 5 or 6 minutes.
Although their description makes it clear that “Base” reflectivity is the
display from just ONE scan (therefore it will not guarantee that you are
seeing the strongest reflection), but they did not say what at *what
elevation* that scan is normally done, or whether a “base” display is shown
for EVERY scan.
Icebound 2009-11-23 18:23:13
Well… from multiple radar elevations, not azimuths. The radar makes its
sweeps through the full 360-degrees of azimuth.
The radar processing system creates a “volume scan” from multiple 360-degree
azimuth scans at different elevation settings… 0.5 degrees above horizon,
then 1.5 degrees above horizon, 2.5, 3.5, etc. According to their
description, up to 14 such full-circle scans can be made, apparently from
0.5 up to 19.5 degrees above horizon.
The “composite” display, simply displays the strongest echo at any
particular point on the map, chosen from those 14 scans. Repeat for every
single pixel on the map.
Note that doing 9 or 14 circles with the radar antenna… plus the computer
processing to arrange the display… all take some time, so the composite
display is updated only every 5 or 6 minutes.
The “base” scan is the display from a single 360-degree scan at 0.5 degrees
above the horizon. So the farther away from the radar site, the higher
above the surface from which the echoes are displayed.
Those “volume-scans” can be computer-processed for other purposes, such as
determining max tops… which would help indicate the location of the most
Icebound 2009-11-23 18:23:16
My server rejected this post last night with an error… then magically it
appeared only after I posted a repeat. The repeat is somewhat more
accurate. Ignore this one.
Dave butler 2009-11-23 18:23:24
How about posting them at alt.binaries.pictures.aviation [hope that’s the right
name, my news server here at work doesn’t have the binaries groups] and then
posting a pointer to them back here?… or better yet, put them on a web server.
Mike rapoport 2009-11-23 18:23:27
No, like Iceman says, the composite shows the most intense echo over a range
of antenna tilts (elevations). The base image shows one elevation. Both
composite and base images show all azimuths. The antenna on a NEXRAD site
is so large (resulting in a very narrow beam) that it can easily scan above
or below the area of the storm holding the water. You could have a massive
supercell and the beam might pass right under the storm at low tilt angles
(elevation). The composite would show the storm unless it is directly over
Jay beckman 2009-11-23 18:23:32
Thanks Mike and “Ice” good explanations…
Nobody 2009-11-25 07:47:06
The comment that “even airplanes can be lifted uncontrollably”
deserves a real world example, for those of you that have never
During one flight in the south during the summer thunderstorm
season, I was using my Strikefinder and input from ground
controllers to fly through what appeared to be a 80 to 100nm
wide gap in a line of thunderstorms. I was at 6,000′ MSL and
got into the soup as I entered the area. A few minutes into it,
the rain picked up, it got dark, the rain got harder, it got darker,
etc. But the ride was smooth and I was nicely centered up the the
gap with cells off to both sides.
All of a sudden, the VSI pegged indicating a climb of greater than
4,000 FPM. With both throttles closed and a picth angle of about
20 degrees down, I was still going up at better than 4,000 FPM.
This was in a Piper Aztec that weighted in at about 4800 pounds
at that moment in time.
The updraft persisted about 30 seconds and resulted in a +2,000′
altitude deviation even after my best efforts to stop it, which I reported
to ATC as time permitted along with asking for an update on what he
was seeing on his radar. The response was that I was about through
the stuff and sure enough, I popped out into nice weather in another
couple of minutes.
Don’t underestimate the power of a building convective storm. It can
suck you up in a hurry. Likewise, understand the even more dangerous
downdrafts when the storm is dissapating. Flying along down low and
encountering a 4,000 FPM to 6,000 FPM downdraft would ruin your