On 6/12/13 9:29 PM, Carl Tollander wrote:
Trailer parks create their own weather, if cities can.
In this case, I think the conceit was that the computer found the
*strong correlation* between tornado damage reports and trailer parks
(since that is where the threshold for damage is lower than conventional
homes).
On 6/12/13 9:19 PM, Steve Smith wrote:
/*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:friam-boun...@redfish.com] *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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