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07 How are tropical cyclones different from tornadoes?




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This article is from the Storms FAQ, by Chris Landsea landsea@aoml.noaa.gov with numerous contributions by others.

07 How are tropical cyclones different from tornadoes?

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 or 20 kt) 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.

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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