There's a lot more corn area than there are trailer park area. Also
still not convinced interacting microclimates aren't relevant. The
corn thing could indeed produce showers, maybe. But it's nightime so
maybe it did. Nobody said anything about the humidity going up or down
as the temperature blasted.
On 6/12/13 9:38 PM, Steve Smith wrote:
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:[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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