Thursday Nov. 16, 2006

Today was the first of two 1S1P Assignment #3 due dates. 

The Quiz #4 Study Guide is now available online.  There will likely be some modifications made in the next week or so.

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We spent the entire period today on tornadoes.

Tornado life cycle (don't worry about learning the names of the various stages).  Tornadoes begin in and descend from a thunderstorm.  You might see a funnel cloud dropping from the base of the thunderstorm.  Spinning winds will probably be present between the cloud and ground before the tornado cloud becomes visible.  The spinning winds can stir up dust at ground level.  The spinning winds might also be strong enough at this point to produce some minor damage.

In Stage 2, moist air moves horizontally toward the low pressure in the core of the tornado.  This sideways moving air will expand and cool just as rising air does.  Once the air cools enough (to the dew point temperature) a cloud will form.  The tornado is colored green above just to reinforce the fact that it is a true cloud and isn't just composed of dust  (dust may mix with the cloud and turn the tornado brown)

Tornadoes can go from Stage 2 to Stage 3 (this is what the strongest tornadoes do) or directly from stage 2 to stage 5.  Note a strong tornado is usually vertical and thick as shown in Stage 3.

The thunderstorm and the top of the tornado will move faster than the surface winds and the bottom of the tornado.  This will tilt and stretch the tornado.  The rope like appearance in Stage 5 is usually a sign of a weakening tornado.



Tornadic thunderstorms have rotating updrafts called mesocyclones (cyclone refers to winds spinning around low pressure, meso means medium size scale).  Air moving into toward the low pressure core of the mesocyclone will expand and cool. The cloud that extends below the cloud base and surrounds the mesocyclone is called a wall cloud.  The largest and strongest tornadoes will generally come from the wall cloud.

At this point we looked at the first of four video segments.  The video showed the early stages in the life cycle of a tornado in Luverne, Oklahoma.  The tornado was initially 200 to 300 yards wide but grew to about 1/4 of a mile in diameter.  The tornado was initially almost stationary then began to move toward the NE at about 10 MPH.  This tornado was eventually given an F3 rating (on the Fujita Scale)


Before viewing the next video tape we looked at some of the characteristic features of supercell thunderstorms.

Supercells are first of all much larger than ordinary air mass thunderstorms (see comparison in top left figure above).  In ordinary thunderstorms the updraft is unable to penetrate into the very stable air in the stratosphere.  The upward moving air just flattens out and forms an anvil.   In a supercell the updraft is strong enough to penetrate into the stratosphere a little ways. This produces the overshooting top or dome feature above.  Walls clouds are shown at the bottoms of both of the sketches above at left.  The flanking line is a line of new cells trying to form alongside the supercell thunderstorm.

A photograph of a distant supercell thunderstorm was shown in the next video tape.  A computer simulation of the air motions inside a supercell thunderstorm was also shown.




A radar picture of a supercell thunderstorm will often have a characteristic hook shape.  The hook is caused by spinning motions inside the thunderstorm  An example of a hook echo is shown in the figure above at right.  The orange shaded area is the thunderstorm updraft, the mesoscylone.  Blue shaded areas shown where precipitation falls out of the cloud.  The flanking line of new cells is forming along a gust front produced when cold downdraft air from the thunderstorm collides with prexisting winds.  Weak tornadoes can sometimes form along the gust front.  The largest and strongest tornadoes come from the mesocylone and wall cloud.

The lower figure shows some actual hook echoes observed on radar.  An untrained observer could well miss the hook echo feature on these images.

Before going onto the next video it would be worthwhile to learn more about the Fujita Scale that is used to rate tornado strength or severity.  The Fujita scale is used to rate tornado strength and severity.  The Fujita Scale runs from F0 (weakest) to F5 (strongest, though there are a very few tornadoes with winds over 300 MPH that have been given an F6 rating).

The scale below is an "easy to remember" version (you can compare this with the scale in Table 10.2 (p. 281) in the textbook).  About 2/3 rds of all tornadoes are F0 or F1 tornadoes and have spinning winds of 100 MPH or less.  Only a few percent of tornadoes develop into F4 or F5 tornadoes, but those account for about 2/3 of all tornado deaths (slightly less than 100 people are killed every year by tornadoes in the US).

winds < 100 MPH

F0
F1	roof damage, mobile home tipped over

microburst winds can cause this degree of damage


winds 100 to 200 MPH

F2	roof gone, outside walls still standing
F3	outside walls gone, inside walls intact

winds 200 to 300 MPH

F4	home destroyed, debris nearby
F5	home destroyed, debris carried away

We next looked a series of tornadoes caught on video.  The tornadoes and comments are given in the table below.

54a	F3	Grand Island, NE	Mar. 13, 1990	tornado cloud is pretty thick and vertical
61f	F3	McConnell AFB KS	Apr. 26, 1991	this is about as close to a tornado as you're ever likely to get.  Try to judge the diameter of the tornado cloud.  What direction are the tornado winds spinning?
52	F5	Hesston KS	Mar. 13, 1990	Watch closely you may see a tree or two uprooted by the tornado winds
51	F3	North Platte NE	Jun. 25, 1989	Trees uprooted and buildings lifted by the tornado winds
65	F1	Brainard MN	Jul. 5, 1991	It's a good thing this was only an F1 tornado
57	F2	Darlington IN	Jun. 1, 1990	Tornado cloud without much dust
62b	F2	Kansas Turnpike	Apr. 26, 1991	It's sometimes hard to run away from a tornado.  Watch closely you'll see a van blown off the road and rolled by the tornado.
47	F2	Minneapolis MN	Jul. 18, 1986	Tornado cloud appears and disappears.

Some photographs of damage produced by tornadoes of different strengths. (figures similar to these were shown in class)
middle left: F1 (roof damage)
bottom left: F2 (roof is gone, walls are still standing)
top right: F3 (exterior walls are down, interior walls still standing)
middle right: F4 (complete destruction, debris is nearby)
bottom right: F5 (complete destruction, most of the debris has been carried away)

(source: T. Theodore Fujita: His Contribution to Tornado Knowledge through Damage Documentation and the Fujita Scale, Bull. Amer. Meterological Soc., vol. 82, pps 63-72, 2001.)

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Tornado researchers were sometimes surprised by the very different degrees of damage that were sometimes found in close proximity.  One house might be severely damaged by a tornado while the house next only received light damage.  Scientists eventually began to understand that some large and strong tornadoes may contain several smaller and more intense suction vortices.  They are sometimes hard to see because of all the dust and debris in the main tornado cloud.  The suction vortices leave unusual markings on the ground that were sometimes observed in aerial surveys of tornado damage.

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The last video showed a strong tornado that occurred in Pampa, Texas.  The winds in this tornado were strong enough to lift pickup trucks off the ground and throw them out and away from the tornado at 80 to 90 MPH.  Winds speeds were estimated to be about 250 MPH in the bottom portion of the tornado.

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The following information on laboratory and computer simulations of tornadoes wasn't covered in class.

Air motions inside tornadoes are complex and difficult (dangerous) to study.  Researchers resort to laboratory simulations and computer models. 

Winds thought to be found in a weak tornado.  Friction probably causes the closed circulation near the bottom center of the tornado.

Winds in a somewhat stronger tornado.


Vortex breakdown.  Downward moving air is found in the core of this tornado.  This tornado would also have a larger diameter than the weaker tornadoes above.


Vortex breakdown has reached the ground.  This may lead to the formation of multiple vortices.