Geoscience Reference
In-Depth Information
three million insurance claims were filed, bank-
rupting several insurance companies. The storms cost
$US12 and $US6 billion damage, respectively, of
which less than 50 per cent was insured. Destructive
as these storms were, they had a theoretical return
period of 5-10 years and were matched by similar
magnitude events on at least eight other occasions,
including the 16 October 1987 windstorm that
devastated southern England. The 1:100 year storm
event in Europe is currently estimated to cost
$US100 billion. Intense mid-latitude storms still
plague Europe today as much as they did at any point
over the past millennium.
lows gain latent heat from warm ocean currents, which
tend to form on the western sides of oceans. For some
regions, where easterly moving low-pressure systems
pass over a mountainous coastline and then out to sea
over this warm poleward flowing current, intense
cyclonic depressions can develop with structural
features and intensities similar to tropical cyclones.
This development is shown schematically in Fig-
ure 3.15. These regions include the east coasts of
Japan, characterized by the Japanese Alps and the
Kuroshio Current; the United States, characterized by
the Appalachians and the Gulf Stream; and south-
eastern Australia, characterized by the Great Dividing
Range and the East Australian Current. Lows may also
intensify off the east coast of South Africa, dominated
by the Drakensburg Range and the Agulhas Current.
The lows are not associated with any frontal structure,
but tend to develop under, or downstream of, a cold-
core low or depression that has formed in the upper
atmosphere. The depressions are associated on the
surface with a strong mobile polar high that may stall
or block over the adjacent ocean. The initial sign that a
low is developing is a poleward dip in surface isobars
on the eastward side of these highs.
East
-coast lows or 'bombs'
Formation
(Sanders & Gyakum, 1980; Holland et al., 1988)
East-coast lows are storm depressions that develop,
without attendant frontal systems, over warm bodies
of water - usually off the east coasts of continents
between 25 and 40° latitude. The development of such
lows exhibits the forcing upon atmospheric instability
by both topography and sea surface temperatures. The
b
weak inland low
e
d
d
east-coast low
c
f
cold
inshore
water
c
warm offshore
water
a
a) Strong onshore temperature gradient
b) Upper atmospheric low pressure forms
c) Strong surface air flow
d) Strong orographic uplift
e) Explosive development of low
f ) Intense surface low forms
14
isotherm
˚
C
Schematic development of an east-coast low.
Fig. 3.15
