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 --
============================================================
FRIAM Applied Complexity Group listserv
Meets Fridays 9a-11:30 at cafe at St. John's College
to unsubscribe http://redfish.com/mailman/listinfo/friam_redfish.com
============================================================
FRIAM Applied Complexity Group listserv
Meets Fridays 9a-11:30 at cafe at St. John's College
to unsubscribe http://redfish.com/mailman/listinfo/friam_redfish.com
