Wednesday, November 20
Today, we will be using page
156, page 159a, page 159b, and page 160a from the Class
Notes Package.
Severe thunderstorms, wind shear, mesocyclones,
and wall clouds
Next we'll look at some of the conditions that can lead to
severe thunderstorm formation and some of the characteristics of
these storms. Severe thunderstorms last longer, grow bigger,
and become stronger than ordinary air mass thunderstorms.
They can also produce tornadoes.
Severe storms are more likely to form
when there is vertical wind shear (the picture above is on page
156 in the ClassNotes). Wind shear (Point 1)
is changing wind direction and/or wind speed with
distance. In the case shown above, the wind speed is
increasing with increasing altitude, this is vertical wind
shear.
A thunderstorm that forms in this kind of an environment will move (at perhaps the
average of the speeds at the top and bottom of the cloud) (Point 2). The
thunderstorm will move to the right more rapidly than the air
at the ground which is where the updraft begins. Rising
air that is situated at the front bottom edge of the
thunderstorm will find itself at the back edge of the storm
when it reaches the top of the cloud.
This produces a tilted updraft (Point 3). The downdraft
is situated at the back of the cloud. The updraft is
continually moving to the right and staying away from the
downdraft. The updraft and downdraft coexist and do not
"get in each others way." If you remember in air mass
thunderstorms, the downdraft gets in the way of the updraft
and leads to dissipation of the storm.
Sometimes
the tilted updraft will begin to rotate. A rotating
updraft is called a mesocyclone (Point
4). Meso
refers to medium size (thunderstorm size) and cyclone means
winds spinning around low pressure (tornadoes are sometimes
called cyclones). Low pressure in the core of the mesocyclone creates an inward pointing
pressure gradient force needed to keep the updraft winds
spinning in circular path.
The cloud that extends below the cloud
base and surrounds the mesocyclone
is called a wall cloud
(Point 5). The
largest and strongest tornadoes will generally come from the
wall cloud.
Note (Point 6)
that a tilted updraft also provides a way of keeping growing
hailstones inside the cloud. Hailstones get carried up
toward the top of the cloud where they begin to fall. But they then fall back into the strong core
of the updraft and get carried back up toward the top of the
cloud.
Here is a link to an exceptional time lapse video (http://www.mikeolbinski.com/timelapse/)
of a supercell thunderstorm in Texas from Mike Olbinski
again. Find the Supercell near Booker, Tx frame near the
bottom of the page. In time lapse you can see the rotation
of the wall cloud. If you watch closely you'll see another
interesting feature: moisture from air in the downdraft that
reaches the ground is drawn into the thunderstorm updraft
(starting at about 0:38 in the video up until the end of the first
segment of video). As the downdraft air moves upward cloud
begins to form.
It is worth trying to understand why the wall cloud surrounds the
mesocyclone and why it extends below the rest of the cloud.
Clouds form when air rises, expands, and cools as shown
above at left. The rising air expands because it is
moving into lower pressure surroundings at higher
altitude. Only when the air has risen high enough,
moved into low enough pressure, expanded and cooled enough
will a cloud form. Just for the purposes of
illustration we'll assume that once air has traveled from
the ground to 900 mb pressure higher up it will have
expanded and cooled enough for a cloud to form.
Air in the center of the rotating updraft has a little
lower pressure than the air surrounding it at the same
altitude. I've assumed that the pressure in the middle
of the mesocyclone at cloud base altitude is 890 mb. In
this part of the picture 900 mb pressure is found a little bit
closer to the ground. Thus air that rises into the
rotating updraft doesn't have to go as high before it
encounters 900 mb pressure and has expanded and cooled enough
to form a cloud.
A similar kind of thing happens in the formation of a tornado
cloud.
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 don't
usually 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 the direction and
speed of the wind inside a thunderstorm (and
sometimes in a 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
. 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.
Tornadoes
The
United States
has roughly
1000 tornadoes
in an average
year, more
than any other country in
the world.
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. A
comparable, more recent map can be found
at https://www.ncdc.noaa.gov/climate-information/extreme-events/us-tornado-climatology
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 create thunderstorms capable of
producing tornadoes.
Tornadoes have been observed in every state, but tornadoes
are most frequent in the Central Plains, a region
referred to as "Tornado Alley"
(highlighted in red, orange, and yellow above).
The figure above shows tornado deaths per
million people (from: https://blog.nssl.noaa.gov/nsslnews/2009/03/us-annual-tornado-death-tolls-1875-present/).
You can see a steady decline in fatalities beginning around
1925. The number of deaths appears to have leveled off
at roughly 0.2 deaths per million in the past decade or so
(note the y-axis is a logarithmic scale). This is
roughly a factor of 10 less than it was a century ago and is
due to much improved methods of detecting and sending out
warnings of tornadoes and severe thunderstorms.
The currently population of the US is about 325 million, so
0.2 deaths/million x 325 million is 65 deaths per year.
Here are some data for the past several years (a link to the
2018 data is included below). You can see that every so
often the United States experiences a particularly deadly
year. That was the case in 2011. An EF5
tornado struck Joplin, Missouri, on May 22 and killed 158
people (EF refers to the Enhanced Fujita Scale rating).
Tornado
statistics for past few years
|
Year
|
No. of confirmed
tornadoes
|
No. of deaths
|
2019
(ongoing)
|
1121
|
39
|
2018
|
1123
|
10 (record low)
|
2017
|
1418
|
35
|
2016
|
976
|
18
|
2015
|
1178
|
36
|
2014
|
928
|
47
|
2013
|
903
|
55
|
2012
|
939
|
69
|
2011
|
1697
|
553*
|
* second largest death
total in US history
You'll find a graphical display of the average annual tornado
frequency that extends back to 1995 at https://www.statista.com/statistics/203682/number-of-tornadoes-in-the-us-since-1995/
Tornado characteristics
Here are some basic tornado
characteristics (the figure above is also on page
159a)
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 later) 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. Overall 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.
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 158 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). You'll find a plot of the tornado paths from
the Washington Post here.
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.
|
A highway underpass is actually a very dangerous place to take
shelter from a tornado, here is a little more
information from the Ohio Committee for Severe Weather Awareness.
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. The following figure is on page
159b in the ClassNotes.
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
(without having to turn on my computer
and look up the Fujita scale online).
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.
At some point it became apparent that the Fujita (F) Scale was
probably overestimating the wind speeds in tornadoes. The
original scale has been replaced by the Enhanced Fujita (EF)
scale.
Here are simplified, easy to remember, versions of both scales.
Here is a comparison of the
actual scales
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). 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).
Here's
probably
the
best
photo comparison of the different levels of
tornado damage that I've been able to find. I
don't think I can embed the images in the lecture
notes without worrying about a copyright
violation.
Multiple
vortex
tornadoes
And finally, something that
was initially something of a puzzle to tornado
researchers.
|
|
|
One of the better examples that I've
seen of very different levels of damage in close
proximity. This is damage from an EF4 tornado that
hit Northwood ND on Aug. 26, 2007. (National Weather
Service photo, source
click on the Track Segments and Photos link)
|
Several
levels
of damage (EF1 to about EF3) are visible in the
photograph above. Here's
another
example with level F0 through F5
damage all found in a few block area.
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.
|
|
Sketch of multiple vortices in a large
tornado and the damage pattern they could leave on the
ground
|
An actual aerial survey of tornado
damage. This was an EF 4 tornado that hit Washington
Illinois on Nov. 17, 2013 (here is YouTube
video of the tornado and the damage left
behind). Photo by Zbigniew Bzdak for the Chicago
Tribune (source)
|
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. You can see what appear to be
multiple vortices in the first minute or so of the Hesston
KS
tornado video shown in Tuesday's class.
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 also probably larger than
shown here.
This ends our coverage of tornadoes.