Geoscience Reference
In-Depth Information
A
B
Figure 11.9
A model of the areal (A) and vertical (B) structure of a hurricane. Cloud (stippled), streamlines, convective features and
path are shown.
Source
: From Musk (1988).
p. 114). The air rises when the pressure gradient can no
longer force it further inward. It is possible that the
cumulonimbus anvils play a vital role in the complex
link between the horizontal and vertical circulations
around the eye by redistributing angular momentum in
such a way as to set up a concentration of rotation near
the centre.
The supply of heat and moisture combined with low
frictional drag at the sea surface, the release of latent
heat through condensation and the removal of the air
aloft are essential conditions for the maintenance of
cyclone intensity. As soon as one of these ingredients
diminishes the storm decays. This can occur quite
rapidly if the track (determined by the general upper
tropospheric flow) takes the vortex over a cool sea
surface or over land. In the latter case, the increased
friction causes greater cross-isobar air motion, tem-
porarily increasing the convergence and ascent. At this
stage, increased vertical wind shear in thunderstorm
cells may generate tornadoes, especially in the northeast
quadrant of the storm (in the northern hemisphere).
However, the most important effect of a land track is
that cutting off of the moisture supply removes one of
the major sources of heat. Rapid decay also occurs when
cold air is drawn into the circulation or when the upper-
level divergence pattern moves away from the storm.
Hurricanes usually move at 16 to 24 km hr
-1
,
controlled primarily by the rate of movement of the
upper warm core. Commonly, they recurve poleward
around the western margins of the subtropical high-
pressure cells, entering the circulation of the westerlies,
where they die out or regenerate into extra-tropical
disturbances (see Figure 11.37).
Some of these systems retain an intense circulation
and the high winds and waves can still wreak havoc.
This is not uncommon along the Atlantic coast of
the United States and occasionally eastern Canada.
Similarly, in the western North Pacific, recurved
typhoons are a major element in the climate of Japan
(see D, this chapter) and may occur in any month. There
is an average frequency of twelve typhoons per year
over southern Japan and neighbouring sea areas.
To sum up: a tropical cyclone develops from an ini-
tial disturbance, which, under favourable environmental
