Thu., Nov. 19 notes

Thursday Nov. 19, 2015

I had originally planned on playing some Tour de France bicycle racing video in honor of this weekend's El Tour de Tucson. But most of the good coverage with my favorite pair of commentators has been removed from YouTube because of copyright violations. So I settled on some old songs from Dire Straits: Dire Straits "Down to the Waterline", "Sultans of Swing", "Walk of Life", "Brothers in Arms".

The Forces and Winds Optional Assignment has been graded and was returned today. Almost everyone in the class received full credit (0.4 extra credit pts). You'll find answers to all the questions here.

A student in the 9:30 class pointed out that some of the links on the class webpage weren't working. The problem has been fixed I believe.

Note also there is a new 1S1P/Optional Assignment available. The topic is El Niño, a weather phenomenon that might have a big effect on our winter weather this year. You can earn 1S1P pts (for a written report), extra credit points (you need to download and answer some questions) , or both 1S1P and extra credit pts. The assignment is due the Tuesday after Thanksgiving (Dec. 1) though if you could turn it in before Thanksgiving I'd be grateful as that will give me a chance to get a head start on the grading.

This will most likely be the last 1S1P topic of the semester. There are still some students that have very few 1S1P pts. You can always turn in late work, I'll decide later what to do with it. If it looks like the late work could have a big effect on your overall grade I'll usually give you half credit for a late, well written, 1S1P report.

We spent the entire class on tornadoes
Tornado characteristics

Here are some basic tornado characteristics (the figure above is also on p. 161)

1. About 2/3rds (maybe 3/4) of tornadoes are F0 or F1 tornadoes (this is referring to the Fujita Scale, which we'll learn more about further down in today's notes) and have spinning winds of about 100 MPH or less. That surprises many people, they think tornado winds are faster than that. Microburst winds can also reach 100 MPH. Microbursts are much more common in Tucson in the summer than tornadoes and can inflict the same level of damage.

2. A very strong inwardly directed pressure gradient force is needed to keep winds spinning in such a small diameter circular path. The pressure in the center core of a tornado can be 100 mb less than the pressure in the air outside the tornado. This is a very large pressure difference in such a short distance. The PGF is much stronger than the Coriolis Force (CF) and the CF can be neglected.

The same pressure drop can be found in strong hurricanes but it takes place over a much larger distance. The PGF isn't as strong and the CF does play a role.

3. Because the Coriolis force doesn't play a role, tornadoes can spin clockwise or counterclockwise, though counterclockwise rotation is more common. This might be because larger scale motions in the cloud (where the CF is important, might determine the direction of spin in a tornado). I did a little bit of research after class and it appears that most tornadoes in the southern hemisphere also spin in a counterclockwise direction just as in the northern hemisphere. Here's a link to an article that addresses this question.

4, 5, 6. Tornadoes usually last only a few minutes, leave a path on the ground that is a few miles long, and move at a few 10s of MPH. There are exceptions, we'll look at one shortly.

7, 8. Most tornadoes move from the SW toward the NE. This is because tornado-producing thunderstorms are often found just ahead of a cold front where winds often blow from the SW. Most tornadoes have diameters of 10s to a few 100s of yards but tornadoes with diameters over a mile have been observed. Tornado diameter can also be much larger near the base of the thunderstorm than it is near the ground.

9, 10. Tornadoes are most frequent in the Spring. The strongest tornadoes also occur at that time of year. You don't need to remember the specific months. Tornadoes are most common in the late afternoon when the atmosphere is most unstable. There is a secondary tornado season in the fall.

The 1925 Tri State Tornado

This figure traces out the path of the 1925 "Tri-State Tornado" . The tornado path (note the SW to NE orientation) was 219 miles long, the tornado lasted about 3.5 hours and killed 695 people. The tornado was traveling over 60 MPH over much of its path. It is still today the deadliest single tornado ever in the United States (you'll find a compilation of tornado records here). The Joplin Missouri tornado (May 22, 2011) killed 162 people making it the deadliest since 1947 and the 7th deadliest tornado in US history.

Tornado outbreaks

Tornadoes often occur in "outbreaks." The paths of 148 tornadoes during the April 3-4, 1974 "Jumbo Tornado Outbreak" are shown above. Note the first tornadoes were located in the upper left corner of the map and all of the tornado paths are oriented from SW to NE.

The April 25-28, 2011 outbreak is now apparently the largest tornado outbreak in US history (358 tornadoes, 346 people killed)

There was a small tornado outbreak earlier this week associated with the strong cold front that passed through Tucson. Tornado outbreaks in November it seems are pretty rare. Here is some more information about this November High Plains tornado outbreak.

As we learn more about tornadoes I'm hoping you'll look at tornado videos with a more critical eye than you would have otherwise. So we took a moment, at this point, to have a look at some tornadoes caught on video. If you click on the links below you'll see the same or a similar video that I found online. The videos shown in class were from a tape called "Tornado Video Classics".

The numbers in the left column identified the tornado on the tape. The next column shows the Fujita Scale rating (the scale runs from F0 (weakest) to F5 (strongest). The locations and date are shown next. The last column has comments and things to look for when watching the video segment.

Video ID	Fujita Scale rating	Location	Date	Comments
54a	F3	Grand Isle 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. The online video is longer than the one shown in class and has some good closeup video. See especially the last couple of minutes of the video
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. The driver of the van was killed!
47	F2	Minneapolis MN	Jul. 18, 1986	Tornado cloud appears and disappears. The online video compares features seen in this tornado with one created in a laboratory.

The online Kansas turnpike video also has a warning that a highway underpass is actually a very dangerous place to take shelter from a tornado. Here is some additional information from the Norman OK office of the National Weather Service. Slide 6 lists some of the reasons why underpasses are so dangerous.

Supercell thunderstorms

In the next video you'll see
1. Some additional footage of the Andover KS tornado (the one that tore through the parking lot and the one that caught up the people driving on an interstate highway and forced them to seek shelter under a bridge).

2. Pictures of new and distant supercell thunderstorms and wall clouds.

3. A computer simulation of the growth and development of a supercell thunderstorm.

But first we need to learn a little bit about supercell thunderstorms (p. 159)

Here is a relatively simple drawing showing some of the key features on a supercell thunderstorm (found on p. 163 in the ClassNotes). In a supercell the rotating updraft (shown in red above) is strong enough to penetrate a little way into the stratosphere. This produces the overshooting top or dome feature above. A wall cloud and a tornado are shown at the bottom of the mesocyclone. In an ordinary thunderstorm the updraft is unable to penetrate into the very stable air in the stratosphere and the upward moving air just flattens out and forms an anvil. The flanking line is a line of new cells trying to form alongside the supercell thunderstorm (similar to convergence between prexisting winds and thunderstorm downdraft winds that can lead to new storm development alongside a dissipating air mass thunderstorm).

Here is a more realistic drawing of a supercell thunderstorm. A typical air mass thunderstorm (purple) has been drawn in so that you can appreciated how much larger supercell thunderstorms can be.

I haven't been able to find the video that I showed in class online. But the intent of the video was just to illustrate the complex air motions inside supercell thunderstorms. Here's an alternate video that also shows development of a tornado. The computer animations that people are able to produce are sometimes very amazing.

Weather radar and "hook echoes"

Thunderstorms with rotating updrafts and supercell thunderstorms often have a distinctive radar signature called a hook echo. This is one of the ways that scientists are now able to better detect and warn of tornadic thunderstorms

We haven't discussed weather radar in this class. In some ways a radar image of a thunderstorm is like an X-ray photograph of a human body.

An X-ray image of a person doesn't usually show the entire body, often just the bones and skeleton inside.

The radio signals emitted by radar pass through the cloud itself but are reflected by the much larger precipitation particles. The radar keeps track of how long it takes for the emitted signal to travel out to the cloud, be reflected, and return to the radar antenna. The radar can use this to determine the distance to the storm. It also knows the direction to the storm and can locate the storm on a map. The intensity of the reflected signal (the echo) is often color coded. Red means an intense reflected signal and lots of large precipitation particles. The edge of the cloud isn't normally seen on the radar signal. The amount and intensity of the precipitation is sometimes used in Tucson during the summer to issue a severe thunderstorm warning.

A Doppler radar (something we didn't discuss in class) can detects small shifts in the frequency of the reflected radar signal caused by precipitation moving toward or away from the radar antenna. This can be used to determine wind speeds inside the tornado.

Below is an actual radar image with a prominent hook echo. The hook is evidence of large scale rotation inside a thunderstorm and means the thunderstorm is capable of, and may already be, producing tornadoes.

This is the radar image of a thunderstorm that produced a very strong tornado that hit Oklahoma City in May 1999
( http://www.spc.noaa.gov/faq/tornado/radscel.htm ). The hook echo is visible near the lower left hand corner of the picture. Winds in the tornado may have exceeded 300 MPH. You can read more about this tornado here. And here is some storm chase video of the tornado.

Tornado life cycle
Hopefully the next time you see a tornado either in person or on video you'll be able to say whether it is early or late in its life cycle and whether it appears to be a stronger or weaker than average tornado.

Tornadoes begin in and descend from a thunderstorm. You would usually 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 (see figure below). Once the air cools enough (to the dew point temperature) a cloud will form.

Tornadoes can go from Stage 2 to Stage 3 (this is what the strongest tornadoes do) or directly from stage 2 to stage 4 or 5. Note a strong tornado is usually vertical and thick as shown in Stage 3. "Wedge tornadoes" actually appear wider than they are tall.

Here is video of the Laverne Oklahoma tornado that was shown in class and that shows the initial dust swirl stage up to the mature stage very well.

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 (though still a dangerous) tornado.

A tornado cloud forms is mostly the same way that ordinary clouds do. In an ordinary cloud (left figure above) rising air moves into lower pressure surroundings and expands. Expansion cools the air. If the air expands and cools enough (to the dew point) a cloud forms. In a tornado air moves horizontally into lower pressure at the core of the tornado. The air expands and cools just like rising air does. If the air cools enough a cloud appears.

Tornado intensity and the Fujita Scale

It is very hard to actually measure the speed of the rotating winds in a tornado. Researchers usually survey the damage caused by the tornado and assign a Fujita Scale rating. The original scale, introduced in 1971 by Tetsuya (Ted) Fujita. A simplified, easy to remember version is shown below. A very basic and grossly oversimplified idea of the damage that each level can produce is included. This is simple enough that I can remember it and can use it to estimate tornado intensity when I see damage on the television news.

The fact that the interior walls in a home as the last to go in a tornado means this is probably the best location to seek shelter from a tornado if a better location (such as an underground storm cellar) is not available.

The Fujita (F) Scale probably overestimated the wind speeds in tornadoes.
It has been replaced by the Enhanced Fujita (EF) scale.

Here are simplified, easy to remember, versions of both

Here is a comparison of the actual scales (I didn't show this figure in class)

There's also a much more detailed set of guidelines for determining the EF scale rating from a survey of tornado. Different objects and structures react differently when subjected to tornado (or microburst) strength winds.

The EF scale has 28 "damage indicators" that can be examined to determine tornado intensity. You can think of these as being different types of structures or objects that could be damaged by lightning.. Examples include:

Damage Indicator	Description
2	1 or 2 family residential home
3	Mobile home (single wide)
10	Strip mall
13	Automobile showroom
22	Service station canopy
26	Free standing light pole
27	Tree (softwood)

Then for each indicator is a standardized list of "degrees of damage" that an investigator can look at to estimate the intensity of the tornado. For a 1 or 2 family home for example

degree of damage	description	approximate wind speed (MPH)
1	visible damage	65
2	loss of roof covering material	80
3	broken glass in doors & windows	95
4	lifting of roof deck, loss of more than 20% of roof material, collapse of chimney, garage doors collapse inward, destruction of porch roof or carport	100
5	house slides off foundation	120
6	large sections of roof removed, most walls still standing	120
7	exterior walls collapse (top story)	130
8	most interior walls collapse (top story)	150
9	most walls in bottom floor collapse except small interior rooms	150
10	total destruction of entire building	170

You'll find the entire set of damage indicators and lists of degrees of damage here.

Here's some recent video of damage being caused by a tornado as it happened (caught on surveillance video). The link wasn't working when I tried to show it in class. It shows a house roof being lifted intact off a house. If you listen to the news commentators, there was someone in a pickup truck in the street that survived the tornado. The tornado struck West Liberty, Kentucky on March 2, 2012.

Here are photographs of some actual tornado damage and the EF Scale rating that was assigned to each

EF2 Damage roof is gone, but all walls still standing	EF4 Damage only the strong reinforced concrete basement walls (part of the wall was below ground) are left standing. It doesn't look like there would have been anywhere in this building that would have provided protection from a tornado this strong.	EF5 Damage complete destruction of the structure

At this point we watched the last of the tornado video tapes. It showed a tornado that occurred in Pampa, Texas. Here is a pretty similar video that I found online. It's missing the commentary that was on the video shown in class. Near the end of the segment, video photography showed several vehicles (pick up trucks and a van) that had been lifted 100 feet or so off the ground and were being thrown around at 80 or 90 MPH by the tornado winds (the large dark objects seen between about 5:40 and 6:10 on the video). Winds speeds of about 250 MPH were estimated from the video photography (though the wind speeds were measured above the ground and might not have extended all the way to the ground).

Multiple vortex tornadoes
And finally, something that was initially something of a puzzle to tornado researchers.

Several levels of damage (EF1 to about EF3) are visible in the photograph above. It was puzzling initially how some homes could be nearly destroyed while a home nearby or in between was left with only light damage. One possible explanation is shown below.

Some big strong tornadoes may have smaller more intense "suction vortices" that spin around the center of the tornado (they would be hard to see because of all the dust in the tornado cloud. Tornado researchers have actually seen the damage pattern shown above scratched into the ground by the multiple vortices in a strong tornado.

The sketch above shows a tornado located SW of a neighborhood. As the tornado sweeps through the neighborhood, the suction vortex will rotate around the core of the tornado.

The homes marked in red would be damaged severely. The others would receive less damage. Just one suction vortex was used here, there are usually several. But the tornado diameter is probably larger than shown here.