Re: [PLUG] Waaaaay Off Topic: Thunderstorm Movement

["Lee H. Marzke" <lee@marzke.net>]

Rich,  that is an excellent description and overview spoken like a real weather junky.

What caused you to have so much interest ?

I'm a commercial/instrument pilot and instructor in Airplanes and also a Glider instructor so weather conditions
are of life/death importance to me and a hobby in Gliders.

see comments below.  <long>

----- Original Message -----
> From: "Rich Freeman" <r-plug@thefreemanclan.net>
> To: "Philadelphia Linux User's Group Discussion List" <plug@lists.phillylinux.org>
> Sent: Monday, July 17, 2017 5:03:53 PM
> Subject: Re: [PLUG] Waaaaay Off Topic: Thunderstorm Movement

> On Mon, Jul 17, 2017 at 4:27 PM, Casey Bralla <MailList@nerdworld.org> wrote:
>>
>> When a thunderstorm moves across the landscape, is it like a wave moving on
>> the ocean, or a inner tuber floating on a river?  In other words, is it the
>> same moving air mass dumping rain as it moves, or is a storm an atmospheric
>> wave phenomenon that moves into new air all the time?
>>
> 
> I believe it is both.  I'm not an expert but it is an area of interest for me.
> 
> The entire air mass is certainly moving along the surface of the
> earth.  Aloft winds are faster than surface winds and generally are a
> few dozen miles per hour.

Thunderstorms often develop along frontal boundaries that are moving,  and also
'pop-up' without frontal activity - called air-mass thunderstorms as often
seen in Florida every afternoon.   In one case the lifting action is the front,  in the
other it's just the butterfly effect.

Fronts can be sharply defined as in cold-fronts.  These fronts push the warmer air in
front up a very steep slope, and often cause a narrow band
of squall-line thunderstorms out in advance of the actual front at the surface.
Much like a drop of water rolling across a glass surface,  the leading edge of the
front is rounded and steep and easily pushes the warm air out of the way.

I'd say sharply defined, fast moving cold fronts behave somewhat similar to waves,
but slow moving warm fronts not so much.

With warm fronts,  the warm air is moving into the cold, but it results in a very narrow
wedge sliding into the cold.  Like a narrow Axe blade of warm air slowly sliding and  lifting
the cold air above it. That produces a very wide front, with widespread embedded thunderstorms, low
visibility and constant rain over very large areas.  Embedded means that the thunderstorms
are hidden by the surrounding clouds and bad weather so much that you can't see them to avoid
flying into them unless your above them in the stratosphere.


<long diversion>
Unexpected flight next to invisible weak frontal wave in a glider in late 1980's

I once witnessed a very-very weak frontal wave close-up that was 5000 ft high, and a line of nice fair-weather
cumulus clouds right in front of it.  I happened to be piloting L-23 Sailplane back and forth
and climbing in the region a just few hundred feet in front (upwind) of the line of clouds. 

You could actually see the clouds forming / climbing in patches / wisps from 2000 ft above
ground (AGL) up to the tops around 5500 AGL  .  Near the top you could see wisps of clouds flow
over the top of the front/wave and be left behind.  I flew back and fourth just beside these clouds
watching and discussing this with my passenger, assuming this was some kind of weird
frontal wave action.   The clouds were literally moving straight up this invisible wall, and falling
over the top. 

I didn't realize at the time that over the ~1H duration of slowly climbing from 2000 ft to 5000 ft in front of these clouds we had drifted 
over the ground ( e.g. the clouds were moving at 5 knots over the ground,  not stationary as we first thought).  The problem was the airport
was now on the other side of this very narrow band of clouds and since I couldn't see the airport,  I didn't realize how
far we had drifted downwind.

Flying gliders downwind of the airport is generally avoided since it's harder to guess if you high enough to get back home.
We just noticed we were getting close to Worcester MA, which means we quite far downwind of
our departure at Stirling MA, and we needed to get through the narrow band of cumulus to get home. 

The line of clouds had a few holes and was only 500ft wide at 5000 ft, so I found a hole and punched through the 500ft
to the upwind side of the line and found our airport at least 5 miles ( upwind ) of us. 

This was a sudden  O-sh** moment as we both ( passenger was also a glider pilot )  instantly realized we were much further
downwind than we realized or intended.

Our 2-seat glider was was a LET Blanik L-23  with 28:1 glide ratio, and we were at 5000 ft, so our quick guestimated
glide ability was 15 miles no wind, but going into the wind could be far worse.  Are we high enough enough to glide 5 miles upwind?
My passenger ( also a glider pilot ) and I started calculating silently, and looking for landable fields just in case.
We sweated it for a few minutes.   We quickly calculated the MacCready speed-to-fly into a guessed 5 knots of wind ( 49K plus 2.5Knots )
which give us the best glide angle into the wind, and then flew that speed for penetration.

Soon however, the destination airport was getting lower and lower on the plexiglass windscreen, ( by fractions
of a degree only a glider pilot would notice )  so we knew we were going to get back with altitude to spare.

</long diversion>


> 
> However, within that air mass there is also evolution, mostly in the
> vertical direction.  Warm, humid air from the surface rises creating
> large vertical clouds.  As the air rises it cools, but due to the high
> heat capacity of water the moist air from the surface cools faster
> than the air surrounding it (that is, as it is lifted due to being
> hotter and less dense it stays hotter than the surrounding air which
> is lower in humidity).  That keeps the rising air warmer than the
> surrounding air and allows it to continue to rise until the water has
> condensed out into plumes tens of thousands of feet high.

Adiabatic cooling is how a 'parcel' of air cools when lifted compared to the changing temperature
of the surrounding air.  The adiabatic dry lapse rate is  2C / 1000 ft. in a generic atmosphere.

<diversion>
The weather service routinely launches rapidly rising balloons that have radiosonde packages
that radio back the temperature and altitude readings as they rise.  This is plotted
as the Actual lapse rate on a Skew-T Log-P diagram.   Glider pilots use these to
predict expected max height of lift by taking the expected surface high temperature for the day
and lifting a surface parcel of air at that temperature along the adiabatic lapse rate on the chart until it
intersects the actual aloft temperatures reported from the morning soundings.

Using this, Early in the  day it can be guessed if the thermals that day are good enough for a race, or a cross country flight.

</diversion>


As the parcel climbs and cools, it may eventually may run of of heat to climb further, or as
Rich mentions if it hits the dew point and water condenses, the latent heat of vaporization is
given off which begins a positive feedback loop.  The means with enough water in the air
the positive feedback  may continue and the air parcel may rise to the stratosphere.

So this leads to the items necessary for a thunderstorm to form without a front.
1. Very Moist air providing energy from condensation.
2. Something to give the morning parcel of air a kick upward to get it started ( like air flowing over a hill )
   or a hot section of ground causing a thermal.
3. Unstable lapse rate ( meaning the parcel stays warmer than the surrounding air until it starts to condense )

This happens almost every day in Florida.

> 
> You also get interactions when large air masses collide.  That
> involves both movement across the surface of the earth (bringing the
> air masses together), and then evolution within the area as they
> interact.

And you haven't even mentions Derechos  which have gained notoriety lately.  See the 2012 Derecho:
http://www.spc.noaa.gov/misc/AbtDerechos/casepages/jun292012page.htm
 
These are certainly wave-like movements of bands of thunderstorms over extremely long distances.

> 
> And you can also get weather that results from the interaction of air
> with land it passes over, such as when an air mass is forced up over a
> mountain.

Orographic lifting

Upslope Fog, as air flows up mountain, it cools and condenses, and forms fog.

Air flowing down a mountain and warming from compression ( Chinook winds )


> 
> And I believe that the air movements that are created within a storm
> can also draw in surrounding air.

The rush of air currents in front of a squall line are a result of all this air being sucked up into
the young thunderstorm.  Soon however,  rain begins ( the mature stage ) and the falling rain
causes down-currents,  micro-bursts, etc.  and the new cold air from aloft hitting the surface
and spreading out in all directions.   These downdrafts and/or micro-bursts can happen before
rain hits the surface as much of the initial rain is falling into dry air and it evaporates
before getting to the ground.  Evaporating precip is called 'virga' and is often visible on the
horizon on clear days with dry air and scattered showers. 

If virga produces a well defined downdraft or micro-burst it can be very bad for  jet on final approach
to a runway,  and  pilots are given alerts when this is suspected.

The problem with Jet engines (as opposed to piston ) is it takes very long time ( perhaps 10sec ) to go
from flight-idle thrust on a jet engine to full power. If the airplane is on final,  and hits a downdraft
you can drop those 1500 ft to the ground sooner than you can spool up the engine from idle to full power.

The procedure to reduce this threat is a constant angle descent at fairly high power - called a
'stabilized' approach where engines are run at high power setting wile on approach by adding extra
flaps,  lowering gear, etc.   You will often here the engines increase power on short 5 mile final
as the pilot lower gear and more flaps and slats to allow extra power (without gaining speed) and descending.

The higher-power means that full power can be achieved much faster than possible from flight-idle power, and
escaping from a sudden downdraft or micro-burst is more likely.



> 
> Somebody else will likely have a better explanation, and I suspect the
> more dominant factors vary by type of storm/etc.

Well I certainly had more diversions , not sure I said it any better.


> 
> --
> Rich
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-- 
"Between subtle shading and the absence of light lies the nuance of iqlusion..." - Kryptos 

Lee Marzke, lee@marzke.net http://marzke.net/lee/ 
IT Consultant, VMware, VCenter, SAN storage, infrastructure, SW CM
___________________________________________________________________________
Philadelphia Linux Users Group         --        http://www.phillylinux.org
Announcements - http://lists.phillylinux.org/mailman/listinfo/plug-announce
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