/*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.
*/
Corn.
Reminds me vaguely of the punchline in a (Bruce Sterling?) short story
about a US Weather Service computer churning on the question of how to
redirect the path of tornados away from populated areas.
It's answer was to set up large mobile home parks in otherwise empty
areas to draw the tornados to them.
- Steve
Out by Grand Island, everything is planted in corn, due to corn
prices. Supported largely by irrigation from wells (the Platte River
being a mile wide and inch deep, except when its not). Right now,
miles and miles and miles of dried up corn husks, because of the
drought and the Ogallala Aquifer not being what it once was. If corn
future prices hit a certain level, it might become reasonable
economically to turn on the pump anyway and you can end up with a very
sudden humidity spike over a large area that was hours or minutes
before, very very warm and dry. When do you do that? After sunset,
to minimize loss due to evaporation, since water is at a premium.
Or not. Speculative. Checkable. Not quite sure what the model would
be then....
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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