Scientists are actively trying to prove or disprove a number of tornadogenesis hypotheses. It is complicated science that draws on information from observations, theory, and mathematical and physical models. These are some basic ideas (basic to a scientist, that is) about the processes that might cause tornadoes to form from supercells:
Dynamic Pipe Effect
Development of a tornado begins when
horizontal winds coming together from different directions are strong 3-4km
above the ground and weak or absent near the ground. The result is that rotation
first increases aloft. The young tornado will build downward by something
called the dynamic pipe effect (DPE): air can not enter through the sides
of this belt of rotating air, but can pass through its ends like a pipe.
The partial vacuum created within the pipe draws weakly rotating air up into
the pipe's lower end. This causes the air to spin faster and eventually become
part of the pipe. New sections on the rotating pipe form at lower and lower
altitudes through this same process until the pipe (tornado) is in contact
with the ground.
Another type of tornado development occurs when converging horizontal winds have the same windspeed through all levels in the thunderstorm. Rotation increases all at once and spans several kilometers along the vertical pipe. The tornado, in this case, forms nearly independent from how high it is above the ground, and develops very rapidly from the ground, up.
Rear Flank Downdraft (RFD)
The Rear Flank Downdraft (RFD)
may play a role in tornadogenesis. The RFD is a region of dry air pushed
towards the ground by the thunderstorm on the backside of, and wrapping around
a rotating updraft, and eventually the tornado. It is often visible as a
clear slot wrapping around a wall cloud (a persistent lowering from a rain-free
base of the main thunderstorm). On radar, the presence of a hook or a small
feature hanging from the thunderstorm may indicate the presence of an RFD.
Scientists think the RFD may play a significant role in determining the development
of a tornado, how long it lasts, and how intense it is. Some scientists think
that the RFD, by wrapping around the low-level rotating updraft, forces the
rotation to concentrate and lower to the ground.
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