Geoscience Reference
In-Depth Information
Australian season ended with Cyclone Blanche on
26 May. The occurrence of these storms has effectively
meant that the cyclone season for Australia extends to
the end of May. Australian cyclones are also increasing
in frequency. The worst season to date occurred in
1989, which witnessed more than 14 cyclones in the
Coral Sea. Certainly, the number of cyclones affecting
central Pacific islands such as Fiji and Tonga has
increased significantly in the 1980s - coincidently with
more frequent ENSO events.
Several scales measure the intensity of tropical
cyclones. These are presented in Table 3.2. The most
comprehensive is the Dvorak Current Intensity, which
categorizes the magnitude of both tropical storms and
cyclones. The scale is related to central pressure and
differs between the Atlantic and the north-west Pacific
oceans. Scaled storms are more intense in the latter.
The schema utilizes satellite photography and is calcu-
lated using either cloud patterns discernible on visible
images or the difference between the temperature of
the warm eye and the surrounding cold cloud tops.
Unfortunately, the Dvorak scale does not increment at
regular intervals of pressure or wind speed. The most
familiar scale is the Saffir-Simpson scale developed in
the 1970s. The scale has five categories and applies only
to hurricanes. The scale has the advantage of being
related to property damage, so it can be used in a pre-
dictive capacity. A category 1 storm produces minimal
damage, while a category 5 storm equates with the most
destructive hurricanes witnessed in the United States.
These will deroof most buildings, flatten or remove
most vegetation, and destroy windows and doors. Note
that Hurricane Andrew - which struck the Florida
Peninsula in 1992 and is the second costliest hurricane
in United States history - has recently been reclassified
as a category 5 event. The Saffir-Simpson scale can also
be related to storm surge potential depending upon the
width of the
continental shelf
and how the storm tracks.
For example, a category 5 hurricane can produce storm
surge in excess of 5.5 m. This aspect is very convenient
for laypeople in flood-prone areas. A variation of this
scale is used in the southern hemisphere, mainly
Australia. The Australian scale includes higher wind
speeds than the Saffir-Simpson scale and is related to
maximum wind gust. Unfortunately, wind gauges in the
tropics of Australia are sparsely distributed so that it is
difficult to verify the scale. The accuracy and viability of
wind gauges is also questionable at wind speeds towards
the upper end of the scale.
Wor
ld cyclone disasters
(Cornell, 1976; Anthes, 1982; Whipple, 1982; Nalivkin,
1983; Holthouse, 1986)
Tropical cyclones are responsible for some of the
largest natural disasters on record. In the United
States, the deadliest hurricane struck Galveston Island,
Scales for measuring cyclone intensity (Dvorak, 1975; Simpson and Riehl, 1981; Australian Bureau of Meteorology, 2003).
Table 3.2
Mean sea
Mean sea
Dvorak
Australia-
level pressure
level pressure
Current
Mean wind
Southern
(hPa)
(hPa)
Intensity
speed
Saffir-Simpson
Wind speed
Storm surge
Hemisphere
Strongest gust
(km hr-
1
)
(km hr-
1
)
(km hr
-1
)
Atlantic
NW Pacific
Scale
Scale
height (m)
Scale
1
46.3
1.5
46.3
1009
1000
2
55.6
1005
997
2.5
64.8
1000
991
3
83.3
1
125
994
984
3.5
101.9
987
976
4
111.1
1
119-153
1.2-1.5
2
125-170
979
966
4.5
142.6
970
954
5
166.7
2
154-177
1.8-2.4
3
170-225
960
941
5.5
188.9
3
178-209
2.7-3.6
948
927
6
213.0
935
914
6.5
235.2
4
210-249
3.9-5.5
4
225-280
921
898
7
259.3
5
>249
>5.5
906
879
7.5
287.0
5
>280
890
858
8
314.8
