Re; smoke monster, it's like seeing inside a tornado. We don't know.
OTOH, if it was moist, maybe there wouldn't have been a fire?
We may be conflating questions, here. Was originally just trying to see
if one or the other model was any simpler.
On 6/12/13 9:32 PM, Nicholas Thompson wrote:
What I don't understand is why all that rising moist air doesn't
produce a shower.
Nick
*From:*Friam [mailto:[email protected]] *On Behalf Of *Carl
Tollander
*Sent:* Wednesday, June 12, 2013 10:10 PM
*To:* [email protected]
*Subject:* Re: [FRIAM] Atmospheric mechanics and thermodynamics
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 *NSTèNote 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. çNST*, the precipitation
begins evaporating thus causing the air to cool. /
/As this air cools it will become more dense,/
/*NSTè 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.
çNST* 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. *NSTè So, now we have a Cool, Moist
falling airmass. This sort of thing happens all the time in
thunderstorms and is called a "downburst". çNST* At this point
the air is completely dry *NSTèNo. 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. çNST* 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. *NSTè 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. çNST* 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.*NSTè 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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