Robb Webb

Date: Mon, 23 Sep 2002 12:18:20 +1000
From: Rob Webb
Subject: [synoptic_discussion] RE: Voice from Greater Indonesia.

 Hi all

I thought I'd throw in a couple of comments in addition to Elly's re. the
very strong winds over the Illawarra district last week

This is something I've noticed many times in this particular area and I'm
very keen to go back and look at these to get a feel for what actually is
going on. There is an intense downslope wind that affects the area, often
well before other low elevation sites become well mixed and in fact stronger
than the vast majority of our elevated stations. I think I could find 5 or
so occasions over the last couple of years where the Wollongong AWS has had
50+ knot gusts in such situations and commonly it begins not long after
sunrise. This latest one involved a 69 knot gust late at night
It would be nice to get a better handle on it. As it was, we could add value
the day before for the SES by saying watch out for the N Illawarra area.

I think I'll go back and play around with some sondes from some of these
cases to get a feel for the position of any inversions. Wind direction has a
big part to play also.

Is there anyone out there who is witnessing these events regularly or at
least studied them closely? There are one or two events from NSW that have
been looked at; our obs network is a little better these days so it might be
worth another look.

Bye for now

Rob
NSW Severe Weather

John McBride
Robb, This is interesting..... The best way forward is probably for you to
let us all know whenever it happens, so we can get a bit of a feel for
both the climatology and the dynamics.  When I suspect something
interesting is going to happen, I usually go into Kamal Puri's web site
with the .05 LAPS fields and download the current forecasts for post
analysis.  BMRC internal page/Model Development Group/ Experimental
results/ Operational NMC Regionbal Forecasts/LAPS .05 degree NSW
Region.

These high resolution 3-hrly snapshots of pressure, wind, temperature,
dewpoint can tell you a lot about the underlying physics.

cheers

John McB

Elly Spark
John,

My impression is that it happens everytime there are gale force or close to
gale force westerlies over the Southern Tablelands. (note, westerlies, not
southwesterlies or northwesterlies). Typically the AWS that seem worst
affected are Montague Island, Nowra and Wollongong (Its at Albion Park), and
since it has recently been installed, Kiama. I should think there would be
quite afew examples of the phenomenon during last years Christmas/New Year
Bushfires, when we similar gale force westerly winds.

I remember Montague Island with near 45 knot mean winds (don't remember
date, but should be able to get it from the ADAM database easily enough) for
a number of hours when winds on the Southern Tablelands were less than that,
and in an environment where the surrounding South Coast stations were less
than 20 knots. In the same situation Wollongong was also high but not as
high as this.

Wollongong AWS has an additional complication in that it is situated at the
foot of  a  W-E valley coming down from the great divide. It goes from about
Bowral to Robertson  to Albion Park.   So frequently it gets winds higher
than the surroundings in the morning, which I think ccan can be explained by
a downslope katabatic. During the day there may also be a funnelling effect
down the same valley.

So I think there could be combinations of mountain waves/downslope katabatic
winds and funnelling.
Tania Poliakova in our office has recently developed a mountain wave
calculator to assist with the forecasting of turbulence. I don't know if it
estimates possible surface wind speed as well as turbulence, or if it could
be addapted to do so?

Elly

Robert Moore

Given the frequency of gales at Wollongong airport (Albion Park) AWS and
similar winds at Kiama and Bellambi and also Nowra I think most houses
that could blow away have blown away - hence not sure that SES needs too
much advance notice of a big westerly! As far as down slope winds go
its more like down cliff winds around there, or maybe its more to do
with being at mouth of the valleys that run back into the cliffs -
Macquarie Pass, Jamberoo Pass and the Shoalhaven mouth. I've noticed the
winds are usually pretty consistent in direction (westerly) and mean
speed (30-40 knots or so) so maybe this discounts rotors or mountain
waves touching down, which I assume would be more irregular.

********************************************
Bob Moore
Sydney RFC

Stephen Lellyet
Hi all,

I wouldn't be too quick to discount hydraulic jump activity, especially if
high leeside winds at surface levels are coincident only with flow
more-or-less perpendicular to the ridge line of the escarpment (as Elly has
pointed out).

It would be interesting to note on how many occasions the strong surface
flow is coincident with obvious mountain wave activity, noting the local
conditions underwhich that occurs (keeping in mind that you can have
mountain waves without any hydraulic jump present).  I understand also that
in some specialised conditions, flow through horizontal constrictions, such
as gullies, can produce hydraulic jump behaviour without mountain waves - it
would be interesting to investigate if any of the gullies along the
escarpment are producing this type of behaviour as Bob suspects, though I'm
not sure our observation network has enough resolution to distinguish such
effects.

I agree with John McBride that running 0.05LAPS may well be very
instructive - could 0.05 LAPS be run for a series of dates when high surface
winds were recorded in the area?

Cheers,
Steve.

Elly Spark

I agree with Bob that the sccenario below can explain a lot of the strong
winds at Albion Park ,Kiama. and Nowra.  However, what about Montague
island?. (but, come to think of it, not Merimbula or Bega or Bateman's Bay)

Elly