GDL-49 and Levels of Radar Returns

"O. Sami Saydjari" wrote:

Tim,

Chapter 1, section 1 of the AIM would seem to be most appropriate for
this question, and I have found no references to radar echoes. If you
have a pointer to the information that you think answers my question,
that would be helpful.

Please forgive my ignorance, but I do not know the acronym "IHB."
Please expand.

FAA Instrument Handbook



I have also tried Circular AC-00-6A, and did not find it.

I would also note that note of these sources is likley to answer the
questions of color mapping on a GDL-49/Garmin 430.

That is Garmin's responsibility to explain what their equipment does and
presents. Having said that, a pilot cannot be expected to assess the levels of
intensity to the same degree of discernment that a severe weather expert
sitting at a desk with a giant NEXRAD display does.

Green, yellow, and red where chosen for color radar displays based on the same
human factors used for traffic lights, and other safety signals.

Some folks used monochrome radar displays very successfully for many years.

Gerry Caron wrote:
Rod Machado's column in the latest AOPA Pilot covers the subject very well.


I haven't seen the referenced article, but I remember from seeing a R. Machado
presentation that he had some interesting graphics relating echo intensity to
expected turbulence. IIRC his presentation included pointers to the literature.
Seems like Rod has done a lot of work in this area.

Dave

Newps wrote:

O. Sami Saydjari wrote:

As I was coming into Grand Rapids (KGRR) yesterday, the controller
announced that there was a "level 5" return (or perhaps she said "cell")
on final to runway 8, so I was vectored to 35 where there were mostly
"just level 4" stuff.

That's pretty funny because even the airlines ain't flying thru a level
four return on final.

Well, Eastern did at JFK in 1976 (OTA) and Delta did at DFW in 1985 (OTA).

"O. Sami Saydjari" wrote:



That is pretty odd. I am sure she said level 4. The rain seemed only
moderate to me and visibility seemed like it was 2-3 miles. Perhaps I
got lucky. How sure are you that airlines do not land in level 4? If
so, it seems that they would advise a light aircraft to divert or hold.

ATC does not provide advice as to whether you should divert or hold because
of thunderstorms in the area.

Start by doing a search on "Radar VIP level". Basically VIP levels measure
reflectivity and go from 1 to 6. Anything from 3 on up is probably a
thunderstorm in North America although in the tropics you can get level
three without a thunderstorm. A level 5 is considered an "extreme" echo.
Your Garmin displays a VIP 5 as orange. A level 5 echo will produce about
6-8" of rain per hour (I forget the exact figure) so you are looking a a
"wrath of God" type storm and if you fly in the side you will probably come
raining out the bottom in little pieces There are a lot of general
observations like "a level five echo in FL will probably be less severe than
a level five echo in the Great Plains", but since these observations always
include a "probably" they are not too useful. Some pilots think that they
can tell the level of a thunderstorm by looking at it, they are idiots.

Mike
MU-2

"O. Sami Saydjari" wrote in message
...
Tim,

Chapter 1, section 1 of the AIM would seem to be most appropriate for this
question, and I have found no references to radar echoes. If you have a
pointer to the information that you think answers my question, that would
be helpful.

Please forgive my ignorance, but I do not know the acronym "IHB." Please
expand.

I have also tried Circular AC-00-6A, and did not find it.

I would also note that note of these sources is likley to answer the
questions of color mapping on a GDL-49/Garmin 430.

-Sami

wrote:


"O. Sami Saydjari" wrote:


As I was coming into Grand Rapids (KGRR) yesterday, the controller
announced that there was a "level 5" return (or perhaps she said "cell")
on final to runway 8, so I was vectored to 35 where there were mostly
"just level 4" stuff. So, forgive my ignorance, but where are these
levels defined? I assume they have to do with what sort of radar return
they get off a cloud and therefore has to do with rain intensity. Does
a level 5 return imply that a thunderstorm is likely to be generating it?

Also, I have a GDL-49 Satellite Data Link Transceiver linked to my
Garmin 430 display. I understand (and now have seen) that these things
just are not adequate to locate big cells. Rather, they show large
areas of showers. Are these based on satellite imagery, or from
aggregated ground station data or what?

Also, how to the radar levels (1 thru 6?) relate to the colors shown on
a GDL-49 display on a Garmin 430? Does red equal level 5 or level 6? I
think only have red, yellow and green...so it seems there are not
enough colors to show 6 levels.

-Sami, N2057M, Piper Turbo Arrow III


You might try checking the AIM, for starters. Then, perhaps, the IHB.

"O. Sami Saydjari" wrote in message
...
In googling around a bit, I found the following:

------------------------------------------------------------------------

At: http://www.weatherzone.com.au/misc/g...y.jsp?letter=V

"Video Integrator and Processor, which contours radar reflectivity (in
dBZ) into six VIP levels:
• VIP 1 (Level 1, 18-30 dBZ) - Light precipitation
• VIP 2 (Level 2, 30-38 dBZ) - Light to moderate rain.
• VIP 3 (Level 3, 38-44 dBZ) - Moderate to heavy rain.
• VIP 4 (Level 4, 44-50 dBZ) - Heavy rain
• VIP 5 (Level 5, 50-57 dBZ) - Very heavy rain; hail possible.
• VIP 6 (Level 6, 57 dBZ) - Very heavy rain and hail; large hail
possible."

---------------------------------------------------------------------------
At:
http://www.garmin.com/manuals/773_Ga...deAddendum.pdf

Ok, now, from the Nextrad intensity table on page 13 of the Garmin
"400/500 Series garmin Options Displays", I learned:

GREEN means 5-30 dBZ
YELLOW means 30-55 dBZ
RED means 55-75 dBZ

So...
Green is VIP Level 1
Yellow is VIP Levels 2,3,4
Red is VIP levels 5 and 6


Don't draw this conclusion unless you know exactly what the standards Garmin
is using and what the radar system is using and whether they are compatible
with the VIP levels that you are using. Nexrad seems to use different
scales depending of what mode it is in and additionally, there are at least
2 VIP scales (maybe more), one that has six levels and one that has 15
levels. It also seem to produce different dbz readings than an airborn
radar. There was a major airline accident in the SE (Georgia I think) where
an airliner flew into a 50+ dbz echo (on an airborn radar) and the airliner
was virtually destroyed by hail (the engines where broken up internally. I
doubt that this is what Garmin is displaying as a borderline yellow/red
echo. There is a good chance that you will not survive flying into a VIP 4
storm.

Mike
MU-2

"Jim Burns" wrote in message
...
Sami, you've hit upon the answer to your question, and one thing to keep
in
mind is that radar don't know squat about whether the rain is from a
thunderstorm or not, all it sees is the "reflection" of the precip, the
higher the number, the higher the bounce back. Granted, a thunderstorm
that
is full of rain and hail will definately produce a higher return.
Additional information about the current conditions vs. forecast
conditions
combined with front locations, movement, direction, sigmets, and airmets
should provide you with a more complete picture of what your Data Link is
showing you.

Over the past couple weeks, we've had two totally different low pressure
systems move through Wisconsin. The first contained a leading edge of
thunderstorms, high winds and heavy rain. The last contained high winds,
heavy rain, but no thunderstorms. The echoes for both storms were level
2's
and 3's. The difference between the two storms was the amount of lifting
action available. The first storm occurred during a period of relatively
hot and unstable conditions. The second occurred during cool and more
stable conditions, it was much less violent but the radar returns were the
same.

Jim Burns

Yes, the real issue is that it takes vertical motion to get the droplets to
coalesce together to procduce larger drops with higher reflectivity. If you
have little moisture, it takes more vertical motion to get the same radar
echo, hence the rule of thumb that the storm intensity for a given
reflectivity goes up with decreasing availible moisture. Radar is about
probablilities-a particular echo intensity has a particular probablility of
producing a given level of turbulence, hail ect.

Mike
MU-2

"O. Sami Saydjari" wrote in message
...


Newps wrote:



O. Sami Saydjari wrote:

As I was coming into Grand Rapids (KGRR) yesterday, the controller
announced that there was a "level 5" return (or perhaps she said "cell")
on final to runway 8, so I was vectored to 35 where there were mostly
"just level 4" stuff.


That's pretty funny because even the airlines ain't flying thru a level
four return on final.


That is pretty odd. I am sure she said level 4. The rain seemed only
moderate to me and visibility seemed like it was 2-3 miles. Perhaps I got
lucky. How sure are you that airlines do not land in level 4? If so, it
seems that they would advise a light aircraft to divert or hold.

The approach controller may have been using the 15 level VIP scale where
level 4 is not severe. If you fly into the type of level 4 that Newps is
talking about, you might (probably) won't come the other side. The airport
would probably shut down with a "real" level 5 thunderstorm in the near
vicinity

Mike
MU-2

The approach controller may have been using the 15 level VIP scale where
level 4 is not severe. If you fly into the type of level 4 that Newps
is talking about, you might (probably) won't come the other side. The
airport would probably shut down with a "real" level 5 thunderstorm in
the near vicinity


What question would one ask to ascertain if it's "four out of 15" or
"four out of six"?

Jose
--
Get high on gasoline: fly an airplane.
for Email, make the obvious change in the address.

"Jose" wrote in message
...

The approach controller may have been using the 15 level VIP scale where
level 4 is not severe. If you fly into the type of level 4 that Newps is
talking about, you might (probably) won't come the other side. The
airport would probably shut down with a "real" level 5 thunderstorm in the
near vicinity


What question would one ask to ascertain if it's "four out of 15" or "four
out of six"?

Jose
--
Get high on gasoline: fly an airplane.
for Email, make the obvious change in the address.


He may not know know the answer but you could always ask: "Has anybody
successfully come out the other side?" :-)

As a practical matter he is going to start covering his rear on tape if it
looks like anyone is venturing into a level four (out of six) thunderstorm.
"Say intentions", "how many souls on board", "do you have airborn radar" are
all clues not to go in.

A level four is considered a "strong" thunderstorm, one step below "severe"
thunderstorms with large hail, very high winds and tornados.

Mike
MU-2