Take a look at the first 5 radar images below. They are from the EF3 tornadoes that struck the FOX19 viewing area 2 March 2012. The top image is from the Crittenden-Piner-Fiskburg-Morning View tornado, the second is from the
Peach Grove-Moscow-Hamersville tornado and the third and fourth are from the
Holton, IN tornado.
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| Crittenden-Piner-Fiskburg-Morning View Tornado Hook Echo and debris ball 4:26:57 PM Friday March 2, 2012 |
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| Peach Grove, KY-Moscow, OH-Hamersville, OH Tornado Hook Echo 4:37 PM Friday March 2, 22012 |
Notice how well defined the hooks are, indicating a rotating updraft and a mesocyclone in the top two images. Oh the other hand a hook appears to be absent for the Holton, In tornado. The fourth image is for approximately the same time but with the radar dish tilted up to the next higher angle. The hook echo is obvious but not as well defined as the hooks for the "Piner" and "Moscow" tornadoes.
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| Holton, IN Tornado KIND radar reflectivity, lowest scan with no apparent hook echo 3:52:50 PM EST March 2, 2012 |
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| Holton, IN Tornado KIND radar reflectivity, 2nd scan up scan with a small hook echo 3:52:50 PM EST March 2, 2012 |
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| Holton, IN Tornado KIND spectrum width, lowest scan, a well defined area of large spectrum width, i.e. a large variation of target direction and speed, indicating the EF3 Tornado, 3:52:50 PM EST March 2, 2012 |
With a "marginal" hook in the Holton super it would be nice to have additional evidence of a tornado and the fifth image provides it.
Spectrum width is a measure of the magnitude of the variation of movement of the targets. Lots of variation means high wind speeds, turbulence and big changes of wind direction and speed in a small space just like you find in a tornado.
The tornado stands out in the spectrum width image.
The lesson is that here 3 EF3s touched down. The evidence for a tornado was not obvious in the Holton, IN tornado with the lack of a visible or well defined hook. At a higher antenna elevation the hook is visible and using spectrum width the tornado can be easily seen. .
Now take a look at the Berlin, KY EF0 radar images. The black dot northwest of Brooksville is Berlin.
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Radar reflectivity of flanking line thunderstorms entering and moving through northwest Bracken Co. KY at
5:05:15 PM March 2, 2012. the black dot west of Brooksville is Berlin, KY and the approximate location
of the brief touchdown of an EF0 tornado. |
Not a hint of a hook echo and below where the red and green come together in a line a typical signature of outflow or the leading edge of a line of thunderstorms. In this case a flanking line off the super cell to the north.
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Same time as the image above but this is radial velocity. Green represents air flowing towards the KILN
Nexrad Radar to the north. Green is flowing into the thunderstorm. Red represents air flowing out of the
thunderstorm and heading away from the radar. The dark green patch around Berlin indicates higher wind speeds there. |
Notice the dark green around Berlin and indication of stronger winds. but green colors indicate winds heading towards the radar to the north or air flowing into the thunderstorm. Red indicates outflow from the line of thunderstorms. Neither the reflectivity or the velocity display have an easily detectable indicator of a tornado. Look below.
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Same as the two images above but this is the display of spectrum width or the range of variation of wind
speed and direction. Notice the black spot near Berlin indicating a great deal of wind variation and the leading edge tornado. |
The spectrum width display, similar to a display of turbulence but indicating variation in wind speed and direction along the line. Just north of Berlin a black spot indication too much variation to measure and the location of a small tornado.
The Berlin, KY tornado, an EF0, was a different type of tornado than the EF3s which were mesocyclone tornadoes.
Berlin, KY was a leading edge tornado. Leading edge tornadoes are difficult to detect because almost all leading edge vorticies are weak, small and are usually around for only a minute or two, maybe less. They form from air that gains spin outside the thunderstorm. The rotating air is pulled into the thunderstorm and tilted to nearly vertical and can cause a small tornado.
There is mounting evidence from Doppler on Wheels investigations that many large tornadoes may be caused by continual importation of rotation which is tilted to the vertical and stretched by the updraft. Stretching a rotating "tube" of air has the same effect as an ice skater pulling his or her arms towards the body, the rotation is concentrated in a smaller area and the rotation rate increases. When the tube is stretched the diameter shrinks and the rotation rate increases.
Take a look at the images I have produced to explain the importation of rotation:
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| Development of horizontal rotation from vertical wind shear |
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Rotation caused by vertical wind shear increases as air streams towards the thunderstorm through the
environment with large vertical shear values.
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| Large view of rotation being imported into a thunderstorm. Rotation can be imported over a large area. |
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An additional source of rotation involves the cold pool that develops under a
thunderstorm. As air is lifted up and over the cold pool horizontal rotation is
converted to rotation around a vertical axis.
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When the rotating tubes of air are tilted to the vertical by the thunderstorm updraft and stretched vertically they can spin up enough to cause a leading edge tornado.
Leading edge tornadoes only live a very short time and the radar signatures are weak so they are much harder to warn for than a mesocyclone tornado.
These rotation elements can become an significant source to maintain large tornadoes as the super cell travels across the landscape.
