It
would be nice to get this discussion up and active again.... so here's
one:
I have a "personal" chart wall in the corridor outside my office where I hang the current charts and progs. I was standing there looking at it this morning. Guo Ming Wang wandered past and posed this question to me, in his words, "about Australian meteorology".
Guo Ming wants to know why in this country the severe weather or the damaging thunderstorms occur mainly in Sydney and Perth.
One may quibble and say, its not just Sydney-Perth and the Sydney observation could be extended all the way up the east coast to Brisbane and slightly further north..... Still... it's a good question: any takers??
John McB
Andrew Tupper
Surely
some of this is a question of perception - where the greatest
population
densities are, and where they whinge the loudest?
Andrew Tupper
John McBride
I think I heard/read somewhere that insurance premiums covering storm damage are greater in the Sydney-Brisbane area than elsewhere. Is this true? Does anyone have any info on this?
John
McB
On Tue, 23 Sep 2003,
Andrew Tupper wrote:
> Surely some of
this is a question of perception - where the greatest
> population densities
are, and where they whinge the loudest?
Jeff
Callaghan
John
here
is a quick and dirty list of notable severe TS in the Brisbane area-
Note
in particular the Nov 1973 and Jan 1985 storms.
but there are other areas of Southeast Qld which may be worse e.g. the Esk/ Toogoolawah, see below:-
February
9th, 1990:
A
severe thunderstorm downed five 40m high electricity transmission towers
2km south of Harlin. The towers were built to withstand wind speeds of
up to 56 metres per second (200 km/h) but were blown over.
March
18th, 1998:
Another
five high voltage transmission towers were blown over by a severe thunderstorm
near Harlin.
February
5th, 2002:
The
Esk area was struck by a severe hailstorm. Many houses reported hail damage,
and at least one house on the Gatton-Esk road had every window smashed
by hail. Hundreds of large trees were also brought down, blocking
the
road.
February
7th, 2002:
A
supercell thunderstorm hit the Watts Bridge Memorial Airfield near Toogoolawah.
One hangar was completely destroyed and several others sustained major
damage.
Mt Isa maybe another hotspot considering it has had 19 severe TS in the last 10 years and its inhabited area is not much bigger than a large city suburb
Jeff
Harald Richter
Yes, John, I'll bite as you knew I would,
> Guo Ming wants
to know why in this country the severe weather or the
> damaging thunderstorms
occur mainly in Sydney and Perth.
If
answers were principal components, the following would probably explain
90+ % of the variability: reporting bias.
The
number of recorded thunderstorms is a function of the actually occuring
number of storms and the reported fraction thereof. In Australia
I'd venture a guess that the T'storm climo spatial pattern is dominated
by the second part.
> One may quibble
and say, its not just Sydney-Perth and the Sydney
> observation could
be extended all the way up the east coast to Brisbane
> and slightly further
north..... Still... it's a good question: any
> takers??
IMO
Sydney happens to sit in the southern fringes of an area with higher lightning
counts. I have a NASA lightning climatology hanging in my office
which is based on optical stroke counts from space. I'd expect this
technique to be far less biased than ground-based reports. The NASA
climo shows an increase in Australian lightning frequency from
S
to N modulated by a general decrease away from the coastline. The NASA
lightning maximum is near Wyndham (WA).
A lightning
climo is not a particularly good indicator of thunderstorm severity, though.
My _anecdotal_ impression on where the Australian SVR probability is highest
points towards a triangle Sydney - Longreach - Rockhampton - Sydney.
The
northern cutoff is explained by the time-mean equatorward extent of decent
shear, the western and southern cutoff by the time-mean inland/poleward
penetration of rich boundary layer moisture. Needless to say, SVR
occurs outside this triangle, but I suspect less often on a multi-year
time scale.
Harald
John McBride
The lightning climatology referred to by Harald is at http://thunder.msfc.nasa.gov/otd/otdglobal.html
I have reproduced the actual image below:
Its
fairly interesting. I doesn't show any Sydney-Brisbane preference
at all; rather the high occurrences occur around the tropical/northern
coastline. There is a maximum on the northwest between Broome and
Darwin; and the high occurrences extend much further south along the east
coast than along the west..... all interesting... and thought provoking.
Another
interesting feature is the zone of high occurrence extending southeastwards
from the east coasts of the three southern hemisphere continents.
A propos the original question on occurrences of storm damage: of course a storm requires more than being highly electrical to inflict damage. My impression is the really damaging ones are those that are well-organized into travelling squall-lines. The occurrence of these is not necessarily closely related to the frequency of occurrence of lightning, low level vertical shear (as Harald noted) being one other necessary condition.
cheers
John McB
Robin Hicks
Not only is the population
density a factor in measuring the severity of damaging severe thunderstorms,
but insurance figures are largely affected by the spatial risk potential.
For example, a relatively minor thunderstorm might damage a valuable crop,
or a swathe of hail might damage cars in a car yard etc, while a major
storm might cause minimal insurance damage over a remoter area. The Bulahdelah
Tornado (c 1970?) is thought to have been one of the highest F rating
tornadoes in Australia,
judged by its damage to forestry, but since it was over a largely
uninhabited area, it does
not rate a mention in the list below.
See
http://www.idro.com.au/disaster_list/default.asp
for a listing of natural
disaster events in Australia with insurance estimates of damage. Note that
the Sydney Hailstorm damage is an order of magnitude higher than any storm
anywhere else, again because of the greater density of infrastructure.
The Thunder Days map that
I worked on some years ago, now at http://www.bom.gov.au/climate/averages/climatology/thunder/tdays.jpg
confirms from ground based
observations the highest frequency over the NW of the Top End and the Kimberleys.
The local maxima over northeastern NSW are probably the result of the favourable
combination of vertical structure and topography
As for comparing insurance premiums, it would be necessary to ensure that Nett premiums are compared, since a significant component of what retail policyholders pay are in fact State based taxes and levies that would vary from state to state.
Robin Hicks
John
McBride
On Tue, 23 Sep 2003, Robin
Hicks wrote:
> See
> http://www.idro.com.au/disaster_list/default.asp
> for a listing
of natural disaster events in Australia with insurance
> estimates of damage.....
An interesting list, thanks for this.
Am I missing something???? But, why doesn't the Sydney-Hobart yacht race event: Dec 98-January 99 appear on this list of disasters as ranked by insurance payout costs?
John McB
Robin
Hicks
Hi John,
The small print at the head
of the list defines an entry in the list as follows
MAJOR DISASTERS SINCE JUNE
1967 - REVISED TO AUGUST 2003
(Including only events likely to cost $10 million or more and events declared a disaster by an appropriate Government Authority irrespective of extent of loss sustained.)
My guess is that because it's damage was to vessels at sea (marine insurance) rather than over land, it wasn't declared a disaster by an appropriate government authority
Robin Hicks
SRDM
National Climate Centre
