Aug. 10, 2004
How are tropical cyclones different from tornadoes?
A: While both tropical cyclones and tornadoes are atmospheric
vortices, they have little in common. Tornadoes have diameters
on the scale of hundreds of meters (feet) and are produced from
a single convective storm (i.e. a thunderstorm). A tropical cyclone,
however, has a diameter on the scale of hundreds of kilometers
(miles) 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.
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 (air pressure).
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.