370 Weather Systems
The transformation of a nonrotating mesoscale
convective system into a tropical cyclone requires
time for the converging low-level inflow to concen-
trate the ambient vorticity. A deep column in the
interior of the system also needs to be moistened,
eliminating the midtropospheric minimum in
e
,
thereby inhibiting the development of evapora-
tively cooled downdrafts. As the core finally begins
to warm in response to the heating and moistening
of the inflow air by the air–sea fluxes, pressure sur-
faces bulge upward in the upper troposphere,
inducing an unbalanced outward pressure gradient
force at that level. Upper tropospheric divergence,
in turn, induces outflow, which causes sea-level
pressure to drop. The radial sea-level pressure gra-
dient drives an inflow of boundary-layer air, which
acquires rotation due to the action of the Coriolis
force. The fluxes of latent and sensible heat from
the underlying ocean surface increase in response
to the increasing surface wind speed. Stronger
fluxes lead to stronger heating in the interior of the
storm, forcing stronger upper level divergence, and
so on.
Just where and when tropical cyclones develop
depends on many factors. By temporarily enhancing
the low-level convergence and vorticity andor can-
celling the planetary-scale vertical wind shear, a pre-
existing synoptic-scale disturbance (e.g., an easterly
wave that forms over sub-Saharan Africa and propa-
gates west-ward across the tropical North Atlantic)
can create an environment in which a cyclonically
rotating MCV can be transformed into a warm core
tropical cyclone. Tropical planetary-waves provide
week-long “windows of opportunity” for tropical
cyclogenesis, which correspond to intervals in which
the vertical wind shear over the region of inter-
est is suppressed. By exploiting these relationships
with the synoptic- and planetary-scale flow, weather
forecasters are often able to anticipate the develop-
ment of tropical cyclones well in advance of the
appearance of their embryonic mesoscale convective
systems.
Tropical cyclones require several days in order
to reach their peak intensity, at which time the dissi-
pation of kinetic energy in the belt of high winds
just outside the eyewall approximately equals the
rate of kinetic energy production in the low level,
cross-isobar inflow.
Steered by the mass-weighted, vertically averaged
flow, tropical cyclones typically tend to track west-
ward for a week or so and then recurve poleward
around the western flanks of the subtropical anticy-
clones. Typical rates of movement are on the order
of 5–10 m s
1
. As a storm drifts westward and pole-
ward it may vary in intensity from one day to
the next as it encounters different environmental
conditions and undergoes changes in its own inter-
nal structure. Some storms exhibit a pronounced
eyewall-replacement cycle in which the ring of con-
vection encircling the eyewall contracts with time
over the course of a few days and is eventually
replaced by an outer eyewall. Storms intensify dur-
ing the contraction stage and weaken during the
replacement stage when the decaying inner and
developing outer eyewall are in competition.
Eventually, tropical cyclones either drift into
higher latitudes where the sea surface temperatures
are too cool to sustain them or they encounter
land. Storms that make landfall are radically trans-
formed within a matter of hours. In the absence of
latent heat fluxes from the air–sea interface, the
extreme low pressure within the eye cannot be
sustained. As the depression in the pressure field
fills, the azimuthal circulation spins down under the
influence of friction. However, the extraordinarily
moist “core” of a remnant tropical cyclone may
retain its identity for up to a week, posing the risk
of flooding should it become stalled over one par-
ticular watershed for an extended period of time.
Some remnant tropical cyclones that drift poleward
toward the extratropical storm track take on char-
acteristics of extratropical cyclones and subse-
quently reintensify. Others become entrained into
existing extratropical cyclones, sometimes leading
to rapid deepening.
8.4.3 Storm Surges
The damage wrought by tropical cyclones tends to be
concentrated in coastal zones where the winds that
develop over the tropical oceans strike with full
force, sometimes in combination with even more
devastating storm surges from the sea. Storm surges
represent a superposition of several elements:
• A wind-driven onshore current. If the water
adjacent to the coast is shallow, the shoreward
flow may extend all the way to the bottom,
exerting a strong force on fixed objects that
stand in the way of the moving water.As water
is pushed against the shore by the wind stress,
sea level may rise by as much as several meters.
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