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10 How do tropical cyclones form?


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

10 How do tropical cyclones form?

To undergo tropical cyclogenesis, there are several favorable
precursor environmental conditions that must be in place (Gray 1968,

1. Warm ocean waters (of at least 26.5 C [80 F]) throughout a
sufficient depth (unknown how deep, but at least on the order of
50 m [150 ft]). Warm waters are necessary to fuel the heat
engine of the tropical cyclone.

2. An atmosphere which cools fast enough with height such that it
is potentially unstable to moist convection. It is the thunderstorm
activity which allows the heat stored in the ocean waters to be
liberated for the tropical cyclone development.

3. Relatively moist layers near the mid-troposphere (5 km [3 mi]).
Dry mid levels are not conducive for allowing the continuing
development of widespread thunderstorm activity.

4. A minimum distance of at least 500 km [300 mi] from the equator.
For tropical cyclogenesis to occur, there is a requirement for
non-negligible amounts of the Coriolis force to provide for near
gradient wind balance to occur. Without the Coriolis force, the
low pressure of the disturbance cannot be maintained.

5. A pre-existing near-surface disturbance with sufficient vorticity
and convergence. Tropical cyclones cannot be generated spontaneously.
To develop, they require a weakly organized system with sizable spin
and low level inflow.

6. Low values (less than about 10 m/s [20 mph]) of vertical wind
shear between the surface and the upper troposphere. Vertical wind
shear is the magnitude of wind change with height. Large values of
vertical wind shear disrupt the incipient tropical cyclone and can
prevent genesis, or, if a tropical cyclone has already formed, large
vertical shear can weaken or destroy the tropical cyclone by
interfering with the organization of deep convection around the
cyclone center.

Having these conditions met is necessary, but not sufficient
as many disturbances that appear to have favorable conditions do
not develop. Recent work (Velasco and Fritsch 1987, Chen and
Frank 1993, Emanuel 1993) has identified that large thunderstorm
systems (called mesoscale convective complexes [MCC]) often produce
an inertially stable, warm core vortex in the trailing altostratus
decks of the MCC. These mesovortices have a horizontal scale of
approximately 100 to 200 km [75 to 150 mi], are strongest in
the mid-troposphere (5 km [3 mi]) and have no appreciable signature
at the surface. Zehr (1992) hypothesizes that genesis of the
tropical cyclones occurs in two stages: stage 1 occurs when the
MCC produces a mesoscale vortex and stage 2 occurs when a second
blow up of convection at the mesoscale vortex initiates the
intensification process of lowering central pressure and increasing
swirling winds.


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