Hurricanes: Their Nature and Impacts on Society - Climate Science ...

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TROPICAL CYCLONES ON PLANET EARTH
remain concentrated. When large vertical wind shear is present, the thunderstorms
associated with the surface circulation are blown downstream, leaving
the low-level center isolated from the thunderstorms (Aberson <strong>and</strong> DeMaria
1994). Figure 3.12 illustrates how vertical shear influences the development of
thunderstorms associated with a tropical low pressure system.
This process of low-level wind convergence <strong>and</strong> thunderstorm development
in the middle <strong>and</strong> upper troposphere, plus upper-level wind divergence, has
been described as a "thermal heat engine". A pioneering study of this coupling
between the lower <strong>and</strong> upper troposphere is given in Riehl (1948). A thermal
heat engine can be described as a fire in a fireplace, in which the warm air
from the burning wood rises up a chimney, with low-level fresh air feeding the
fire. In a developing tropical system, thunderstorm formation is analogous to
the wood in the fire. The thunderstorm produces heat in the process of
converting water vapor to liquid water <strong>and</strong> ice as the air rises. Technically the
conversions (<strong>and</strong> release of heat) from these processes are called condensation
(for water vapor to liquid) <strong>and</strong> deposition (for water vapor to ice). The heat
from this phase change is referred to as latent heating. Pressure will continue
to fall at the surface in the center as long as the divergence aloft is greater
than the convergence at low levels. Generally, the air which diverges aloft
sinks at the periphery of the cluster of thunderstorms, with some of the air
recycled back into the storm. If a significant quantity of the air is recycled,
then the pressure cannot drop very far (Merrill 1985; Petrosyants <strong>and</strong>
Semenov 1995). In addition, the subsiding air warms <strong>and</strong> dries the region
surrounding the cloud system, providing an atmosphere which is less conducive
to thunderstorms. This is why developing tropical systems are usually
surrounded by a clear, cloudless area.
At this point the developing system appears as an unorganized cluster of
thunderstorms with only a weak surface wind circulation. It is called a tropical
disturbance. If conditions for development remain favorable, including low
vertical wind shear, warm ocean surface, <strong>and</strong> sufficient moisture throughout
the troposphere, they may further develop into a tropical depression when
the sustained surface winds, circulating around the center, strengthen up to
no more than 39 miles per hour (18 meters per second) (see Emanuel (1988)
for an overview of this process of development). At this point, hurricane
forecasters number the depression in order to identify it for further
observation. The winds in a tropical system result from air seeking to flow
from regions of relatively high pressure to regions of relatively low pressure.
Because of the circulation of the system, due to the rotation of the Earth, the
winds spiral in, giving the developing storm its characteristic spiral
appearance.
Meanwhile, as the storm is developing, it continues to move over the open
ocean. In the Atlantic Ocean Basin, a typical track for the developing storm is
east to west from off the coast of Mrica toward the West Indies <strong>and</strong> North
American continent.