Mombu the Aviation Forum sponsored links

Go Back   Mombu the Aviation Forum > Aviation > Base reflectivity versus composite reflectivity
User Name
Password
REGISTER NOW! Mark Forums Read

sponsored links


Reply
 
1 23rd November 18:22
peter r.
External User
 
Posts: 1
Default Base reflectivity versus composite reflectivity


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
cloud).

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
activity.

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
the differences.

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.

--
Peter
  Reply With Quote


  sponsored links


2 23rd November 18:22
mike rapoport
External User
 
Posts: 1
Default Base reflectivity versus composite reflectivity


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.

Mike
MU-2
  Reply With Quote
3 23rd November 18:22
bob gardner
External User
 
Posts: 1
Default Base reflectivity versus composite reflectivity


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 ****pit 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
  Reply With Quote
4 23rd November 18:22
bob gardner
External User
 
Posts: 1
Default Base reflectivity versus composite reflectivity


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.

Bob Gardner
  Reply With Quote
5 23rd November 18:22
stan gosnell
External User
 
Posts: 1
Default Base reflectivity versus composite reflectivity


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
lifted uncontrollably.

--
Regards,

Stan

"They that can give up essential liberty to obtain a little temporary
safety deserve neither liberty nor safety." B. Franklin
  Reply With Quote
6 23rd November 18:22
jay beckman
External User
 
Posts: 1
Default Base reflectivity versus composite reflectivity


In light of the fact that summer thunderstorm activity is coming quickly
(and our traditional "monsoon" season) here in the desert, this subject is
very interesting.

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?

Jay Beckman
PP-ASEL
Chandler, AZ
  Reply With Quote
7 23rd November 18:22
john clonts
External User
 
Posts: 1
Default Base reflectivity versus composite reflectivity


I want the system that is installed in the plane that's flying just east of Mullen!!
--
Cheers,
John Clonts
Temple, Texas
N7NZ
  Reply With Quote
8 23rd November 18:23
icebound
External User
 
Posts: 1
Default Base reflectivity versus composite reflectivity


As I read it, Not exactly, but sort of. Depends on your definition of
"composited".

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.
  Reply With Quote
9 23rd November 18:23
icebound
External User
 
Posts: 1
Default Base reflectivity versus composite reflectivity


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
severe storms.
  Reply With Quote
10 23rd November 18:23
icebound
External User
 
Posts: 1
Default Base reflectivity versus composite reflectivity


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.
  Reply With Quote
Reply


Thread Tools
Display Modes




Copyright 2006 SmartyDevil.com - Dies Mies Jeschet Boenedoesef Douvema Enitemaus -
666