Monday, Apr. 18, 2016

Margo Price at SXSW 2016 NPR Music Front Row "About to Find Out" (0-3:00), "Four Years of Chances" (13:55-18:47),
"This Town Gets Around" (26:57-30:27)

Tornadoes

The United States has roughly 1000 tornadoes in an average year.  The US has more tornadoes than any other country in the world
(~ 1000/year).




A year's worth of tornado activity plotted on a world map.  Note the name at bottom left: T.T. Fujita, "Mr. Tornado."  The scale used to rate tornado strength and intensity is named after him. 

This is mostly just a consequence of geography.

 




Without any mountains in the way, cold dry air can move in the spring all the way from Canada to the Gulf Coast.  There it collides with warm moist air from the Gulf of Mexico to form strong cold fronts and thunderstorms.  There are some other meteorological conditions that come into play that make these storms capable of producing tornadoes.




This map (found on p. 161 in the ClassNotes) shows the average frequency of tornado occurrence in the US.  Tornadoes have been observed in every state (including Alaska), but tornadoes are most frequent in the Central Plains, a region referred to as "Tornado Alley" (highlighted in red, orange, and yellow above). 


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 on Wednesday) and have spinning winds of about 100 MPH or less.  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 a 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).

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.



At the present time about 75 people are killed on average every year in the United States by tornadoes.  This is about a factor of ten less than a century ago due to improved methods of detecting tornadoes and severe thunderstorms.  Modern day communications also make easier to warm people of dangerous weather situations.  Lightning and flash floods (floods are the most serious severe weather hazard) kill slightly more people than tornadoes.  Hurricanes kill fewer people on average than tornadoes. 

The increase in the number of tornadoes observed per year is probably more due to there being more people in locations that are able to observe and report a tornado rather than a true increase in tornado activity.
 

Here are some data for the past 5 years as a check on the statistics above (source of the 2015 information, you'll find similar Wikipedia articles for the other years)

Tornado statistics for past 5 year
Year
No. of confirmed tornadoes
No. of deaths
2015
1177
36
2014
893
47
2013
903
55
2012
939
69
2011
1700
553*
*  second largest death total in US history


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)


Here is some information about a November 2015 High Plains tornado outbreak.  November tornado outbreaks are fairly unusual.



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.