Geoscience Reference
In-Depth Information
over the western sections of the Atlantic, Pacific
and Indian Oceans, where the subtropical high
pressure cells do not cause subsidence and stability
and the upper flow is divergent. About twice per
season in the western equatorial Pacific, tropical
cyclones form almost simultaneously in each
hemisphere near 5
diameter. Research indicates that energy can be
transferred from the cumulus-scale to the large-
scale storm circulation through the organization
of the clouds into spiral bands (see
Figure 11.9
and
Plate 11.2
), although the nature of the process is
still being investigated. There is ample evidence to
show that hurricanes form from pre-existing
disturbances, but while many of these distur-
bances develop as closed low pressure cells, few
attain full hurricane intensity. The key to this
problem is high-level outflow (
Figure 11.10
). This
does not require an upper tropospheric anti-
cyclone but may occur on the eastern limb of
an upper trough in the westerlies. This outflow
in turn allows for the development of very low
pressure and high wind speeds near the surface. A
latitude and along the same
longitude. The cloud and wind patterns in these
cyclone 'twins' are roughly symmetrical with
respect to the equator.
The role of convection cells in generating a
massive release of latent heat to provide energy for
the storm was proposed in early theories of
hurricane development. However, their scale was
thought to be too small for them to account for the
growth of a storm hundreds of kilometers in
°
(A)
Annular zone
Trailing
convective
clouds
Surface
streamlines
200mb
streamlines
0
300
km
(B)
km
TROP
OPAU
SE
X
Cirrus canopy
Cirrus outflow
Y
15
TROPOSPHERE
10
Annular
Annular zone
Trade Wind
cumulus
zone
(sinking)
(sinking)
5
Outer convective band
Deep cumulonimbus
Outer convective band
0
0
300
Eye
Trade Wind
cumulus
Shallow
cumulus
Deep cumulus, cumulonimbus
and high-level cirrus
km
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). By permission of Cambridge University Press.
