I read it on iciweb or nmfireinfo or something. Not at all surprised
if it was much higher.
On 6/12/13 9:13 PM, Steve Smith wrote:
Carl-
Is 20,000 an official number?
My estimates of height were closer to 40000 from the relative scale of
the Sangres (~5000 above local elevation) and the top of the cloud
(about 8x as seen driving back from ABQ from the top of La Bajada).
- Steve
Uneducated question.
I there any relationship between the heat burst question and wildfire
weather dynamics? Yesterday there was a 20000 foot smoke monster
above one of the local fires. We just see the outside of that.
C.
On 6/12/13 1:31 PM, Nicholas Thompson wrote:
Dear Fans of Elevated Mixing Layers,
I have several questions about the account of the heat burst
(reproduced below). But first, let's develop a bench language
between us that will help us avoid confusion. Let's call air that
is warmer than average for its altitude "Warm" and air that is
colder than average for its altitude, "Cold." And let's call
"Moist", air that has a high content of water vapor and
distinguish it from air that is accompanied by lots of liquid or
frozen water which we will call, "water burdened.". The capital
letters in each case will remind us that Warm air may not be cozy
and Dry air may have a lot of moisture with it. Thus, air can have
a temperature many degrees below zero and still be Warm and can be
Dry, even though it is mixed with many tons of water.
Ok, so now for the problems:
/A heat burst is caused when a shower or thunderstorm weakens over a
layer of dry air. As the last of the precipitation from the
weakening shower or thunderstorm falls through the layer of dry air
*NST*//*è*//*Note that the explanation as written does not make use
of the fact that this falling precipitation will impart downward
momentum to any air if falls through. *//*ç*//*NST*//, the
precipitation begins evaporating thus causing the air to cool. /
/As this air cools it will become more dense,/
/*NST*//*è*//*Hold on, here. Evaporation will also cause the air to
become *//*less*//*dense because it is becoming more Moist. I am
not sure how trade off between these two variables works. I would
love to see a table with temp on the x axis, water vapor on the y
axis and density on the z axis. In fact, I would like to see a
family of such tables for different levels of the atmosphere.
*//*ç*//*NST*////eventually more dense when compared to the
surrounding warmer air and as a result, begins descending to the
surface at a high rate of speed. Eventually, all of the
precipitation within the descending air evaporates. *NST*//*è*//*So,
now we have a Cool, Moist falling airmass. This sort of thing
happens all the time in thunderstorms and is called a "downburst".
*//*ç*//*NST*////At this point the air is completely dry
*NST*//*è*//*No. Wrong. The most that can be said is that all the
water in it has evaporated. This does not make it Dry. In fact, it
makes it Moist. *//*ç*//*NST*////and because no more evaporation can
occur, the air can no longer cool. The air however continues to
descend toward the surface due to the momentum it has already
acquired. As dry air descends through the atmosphere, compression
due to increasing atmospheric pressure causes the air to warm.
*NST*//*è*//*Well, I suppose. But we still have Moist air, don't
we? As it descends, it's relative humidity will fall, but the
amount of water vapor in the packet will not decrease because the
packet is falling. *//*ç*//*NST*////It is important to note that the
density of this air is now going to begin decreasing because of the
increasing temperature. However, because the descending air already
has a great deal of momentum carrying it to the surface, the
increase in temperature and resultant decrease in density does
little to slow the descending air. So, the dry air continues to
descend, all the while warming more and more due to the
aforementioned compressional heating. Eventually, this descending
air reaches the surface and the momentum, which was moving downward
towards the surface, is now moving horizontally along the surface in
all different directions, thus resulting in a strong wind! In
addition, the intrusion of the very warm and very dry airmass from
aloft, will cause the temperature at the surface to increase very
quickly, and the dewpoint at the surface to decrease very quickly.
Acquiring all the needed ingredients for a heat burst can be
difficult, thus making the development and observance of a heat
burst rare.*NST*//*è*//*We all know there was an elevated mixing
level (layer of very Warm, Dry air) over running Moister Cooler air
moving up from the Gulf. If we could find a way to get that layer
down to the surface, then we would have explained the heatburst. The
only think I can think of is that the falling mass of ice and water
and the mass of falling air it took with it actually drives the EML
through to the surface, but does not itself reach the ground. Ugh.
More skyhooks. One feature of this explanation that puzzles me is
the fact that the heat burst lasts as long as it does. A typical
down burst last for a few minutes at most. Why does this warm air
which (ex hypothesi) is less dense than the air it has penetrated
not "bounce".*/
/**/
/*Also, I am wondering if a falling mass of ice and water can reach
the ground but set up a downward momentum in the column over it that
will continue to drag air down to the surface for some time after
the moisture is out of the picture. */
/**/
/*These heat bursts seem a lot like Chinooks. A Chinook is also an
exceptionally hot and dry wind. They occur when a Cool Wet airmass
is driven over a high mountain range. The increase in altitude of
the air squeezes out all the moisture and when the airmass comes
down the other side of the mountain range it is hot and dry. What
we need to explain heat bursts perhaps is to discover something out
there on the flatlands to perform the function of the mountain range. */
/**/
Nick
*From:*Friam [mailto:[email protected]] *On Behalf Of *Roger
Critchlow
*Sent:* Tuesday, June 11, 2013 2:26 PM
*To:* The Friday Morning Applied Complexity Coffee Group
*Subject:* [FRIAM] Atmospheric mechanics and thermodynamics
I was highly amused to read the description of how a heat burst
happens here:
http://www.washingtonpost.com/blogs/capital-weather-gang/wp/2013/06/11/stunning-late-night-heat-burst-in-nebraska-99-degrees-at-5-am/
because it invokes the momentum of an atmospheric packet, something
that I don't think any of our weather discussions has ever brought
into our explanations.
Also note how the explanation proceeds as a logical-causal fait
accompli, there is no physics or math involved in the explanation,
just a narration of a sequence of physical causes.
-- rec --
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Meets Fridays 9a-11:30 at cafe at St. John's College
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Meets Fridays 9a-11:30 at cafe at St. John's College
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