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Subject: L1) How are tropical cyclones different from tornadoes?
Contributed by Chris Landsea
While both tropical cyclones and tornadoes are atmospheric
vortices, they have little in common.
Tornadoes have diameters on the scale of 100s of meters and
are produced from a single convective storm (i.e. a thunderstorm
or cumulonimbus). A tropical cyclone, however, has a diameter
on the scale of 100s of *kilometers* and is comprised of
several to dozens of convective storms. Additionally, while
tornadoes require substantial vertical shear of the horizontal
winds (i.e. change of wind speed and/or direction with height)
to provide ideal conditions for tornado genesis, tropical
cyclones require very low values (less than 10 m/s [20 kt,
23 mph]) of tropospheric vertical shear in order to form and grow.
These vertical shear values are indicative of the horizontal
temperature fields for each phenomenon: tornadoes are produced
in regions of large temperature gradient, while tropical
cyclones are generated in regions of near zero horizontal
temperature gradient. Tornadoes are primarily an over-land
phenomena as solar heating of the land surface usually
contributes toward the development of the thunderstorm that
spawns the vortex (though over-water tornadoes have
occurred). In contrast, tropical cyclones are purely an
oceanic phenomena - they die out over-land due to a loss of a
moisture source. Lastly, tropical cyclones have a lifetime that
is measured in days, while tornadoes typically last on the scale
of minutes. For more information on tornadoes you can go
to the Storm
Prediction Center's FAQ maintained by Roger Edwards.
An interesting side note is that tropical cyclones at
landfall often provide the conditions necessary for tornado
formation. As the tropical cyclone makes landfall and begins
decaying, the winds at the surface die off quicker than the
winds at, say, 850 mb. This sets up a fairly strong vertical
wind shear that allows for the development of tornadoes, especially
on the tropical cyclone's right side (with respect to the
forward motion of the tropical cyclone). For the southern
hemisphere, this would be a concern on the tropical cyclone's
left side - due to the reverse spin of southern hemisphere
storms. (Novlan and Gray 1974)
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