From
John McBride
From
jmb@BoM.GOV.AU Fri Dec 7 14:27:08 2001
To start the ball rolling, I draw attention to the current band of
stratiform cloud across
the continent and the rainfall we have had in the
bight over recent days (see
attached rain_20011206.gif).
The source of this
can be identified nicely
as an intrusion of moisture from the tropics as can
be seen in the evolution
of the theta-e fields over recent days, from Jim
Fraser's GASP diagnostics
page (go to the NMOC homepage
http://vortex.ho.bom.gov.au:80/,
and click on the link to GASP charts
(colour).
Attached I include the theta-e evolution from 11Z on the 3rd
(0112031100.gif) to last
night's chart (0112061100.gif) Surface theta-e
variations are largely dominated
by variations in moisture as a change of 1
gm/kg mixing ratio is equivalent
to 2.5 degrees change in temperature. Thus
we can use these charts
to see moisture changes and moisture advection.
0112031100.gif
0112041100.gif
0112051100.gif
0112061100.gif
Looking at the start of the sequence, there is an initial intrusion of
tropical moist air (in orange)
across northwest WA (0112031100.gif)
On the
following chart (0112041100.gif)
this funnels down into a large scale
advection of the moisture
(orange) to the Bight, which remained there through
the rest of the sequence.
As I said earlier. If you have interest in starting a discussion
group
along these lines, please
let me know, and I'll set one up.
Regards
John McBride
BMRC
From jmb@BoM.GOV.AU Tue Dec 11 11:29:24 2001
b) Current rain situation over northern/central Oz
Nevertheless to keep things
going, I draw attention once again to the
current rain situation over
Northern and Central Australia. The daily
totals are relatively small
(generally only 10 mm over the large areas);
but they have been falling
for four or five days now; so there must be a
reasonable accumulations
of more than 50 mm over an area of the order of
10-15 degrees lat by 10-15
degrees long.
This is a pretty innocuous
looking synoptic pattern to have produced so
much rain. Possibly
climate researchers who go back and classify these
things will count this as
a north-west cloud band; but its not very
impressive on the satellite
image and there is not much curvature in the
500 hPa trough. My
mate Mills who is both old enough to remember Sutcliffe
and young enough to have
heard of Q-vectors tells me that it is probably
associated with a left jet
entrance. This may be right as there is a nice
jet core over the upper
trough in the Tasman Sea.
Anyway, the point of all
this is the following: Say (for arguments sake)
you were a climate researcher
and you wanted to be able to associate major
Australian rain events
with some sort of synoptic system (a cold front, a
monsoon depression, a northwest
cloud band, a cut-off low... etc); what
would you say about this
one?
Cheers
John McB
From
W.Wright@BoM.GOV.AU Tue Dec 11 12:26:44 2001
Subject:
RE: Synoptic Discussion: Include in Objaids list/ current rain si
tuation
Date:
Tue, 11 Dec 2001 12:20:51 +1100
Hi John,
RE current situation:
is this the same one that was operating last week? - I
note that, up to 9am yesterday,
Giles (in interior WA) had had 260mm for the
week - just short of their
average ANNUAL rainfall total. I have not been
exercising my mind much
on the synoptics of the interior Australian rainfall
belt, but for what it's
worth, I would not call it a Northwest Cloudband.
For one thing, much of the
rainfall has been convective, whereas one of the
defining characteristics
of cloudbands is that they are produced by slow,
steady uplift.
Cheers
Bill Wright
Date:
Tue, 11 Dec 2001 13:43:57 +1100
From:
Milton Speer <mss@BoM.GOV.AU>
Subject: Re: Synoptic Discussion:
Include in Objaids list/ current rain situation
John and all,
NT has been a bit too far
north of where I have been focusing but I agree about
the jet stream. Focusing
on NSW it's been one of the most active Spring/early
Summer periods for upper
jet maxima affecting the area for a long time. More
important than the rainfall
totals down here have been the number of wind
events from both non-severe
and severe storms. Frontal systems have been more
frequent (about 3 or 4 days)
than usual (about 5 to 7) with strong low level
temperature gradients, and
lower than normal SLP in the fronts/troughs
extending SE from central
Australia. There has been some reasonable, though
patchy inland Spring rainfall
from the NW cloud bands ahead of the fronts in
NSW but the most striking
feature, I think, in this region has been the strong
upper winds with regularly
occurring right jet exits together with the
well-developed low level
troughs extending SE from central Australia to low
pressure sytems crossing
in the 40's and 50's lats.
That's a NSW perspective
anyway.
Season's salutations,
Milton
From:
Andrew Tupper <andrewt@BoM.GOV.AU>
Subject:
Re: Synoptic Discussion: Include in Objaids list/ current rain
situation
John,
Central Australia; call it
what you will, it was wet! Middle level winds
have been northwesterly
at Giles for pretty much all of December, with
persistent troughing over
WA. There was certainly a left hand upper jet
entrance over the region,
and - importantly - there was moist air at the
surface so more rain made
it to the ground than is often the case. If
anybody's got some pertinent
comments about this event, we'd be interested.
People in Alice Springs are
complaining about the rain over the past couple
of years; we're starting
to get media interviews asking us if the climate
has changed.
Andrew Tupper
Senior Met, RFC, Darwin
From:
Peter Bate <P.Bate@BoM.GOV.AU>
Subject:
Re: Synoptic Discussion: Include in Objaids list/ current rain
situation
Andrew, John, etc,
Last night on the news I
heard Mark Stafford-Smith of Primary Industries (I
think) in Alice Springs
put the wet conditions in central Aust down to the
Inter-decadal Pacific Oscillation
(IPO) - he reckons there's a correlation.
I haven't looked at this
though.
Peter
From: John McBride <jmb@BoM.GOV.AU>
> Last night on the news
I heard Mark Stafford-Smith of Primary Industries (I
> think) in Alice Springs
put the wet conditions in central Aust down to the
> Inter-decadal Pacific
Oscillation (IPO) - he reckons there's a correlation.
> I haven't looked at this
though.
>
Crikey Moses........ If its
not the Antarctic Circumploar Wave, its the
IPO..... I thought these
guys were supposed to be scientists!!!!
John Mcb
From: John McBride <jmb@BoM.GOV.AU>
Sorry about the knee-jerk reaction on the IPO.
There are a couple of problems
with attributing the recent central
Australian rainfall to the
IPO. First is the
fact that it is very difficult
to nowcast where you are in the IPO cycle.
All the IPO indices use
a low-pass (or band-pass) filter with a 13-year
cut-off. The types
of filters vary; but even the most crude ones must use
at least 13 points of the
time series; thus at best you can apply it as
off 6 1/2 years ago.
Thus you may be able to tell where you stood in the
phases of the IPO 6 1/2
years ago; but its nigh impossible to tell where
current conditions stand
in the IPO (Scott Power is currently doing some
research on diagnosing/nowcasting
the IPO)
The second problem is that
these longer time scale phenomena look
impressive in correlations
and composites of filitered time series (where
the annual, seasonal and
shorter than 13 years periodicity contributions
to the variance have been
removed). Their contributions to the
total variance of
rainfall tend to be very small, howver, which goes
against attributing rainfall
over a 3-6 month period to one of these
longer time-scale phenomena
Thirdly, some of us at BMRC
have severe reservations as to the extent to
which the IPO is actually
indepenedent from ENSO (something which no doubt
we can debate with Scott
about, now that he has returned to BMRC).
Cheers
John McB
From: "Neville Nicholls" <N.Nicholls@BoM.GOV.AU>
Colleagues:
Further to John's comments
regarding Central Australian rainfall
and the IPO, I have attached
a figure showing the 12-month
rainfalls for the last 50
years (the 12 month periods end in
November; the last year
is December 2000 to November 2001). The
time-series was produced
by our new system for rapid diagnosis of
Australian climate variations.
The time-series shows that
the last two years have been very wet,
but it doesn't suggest that
this is part of any inter-decadal
fluctuation, let alone an
oscillation. They just look like a couple of
very wet years. We would
need a few more wet years, before we
wanted to attribute them
to an inter-decadal time-scale.
Cheers,
Neville
From: "Neville Nicholls" <N.Nicholls@BoM.GOV.AU>
Subject: Re: the IPO
At the risk of extending
this discussion, and despite the figure I
sent around a day or so
ago, here is another time series of rainfall
in central Australia.
New-fig: This area is slightly west
of the earlier one, and
this time I show only Sept-Nov
rainfall, and as anomalies. This time
there is a nice multi-decadal
oscillation. Of course there are not
many data from this region,
so I think we have to be a bit
suspicious, but it looks
interesting.
Neville
Neville Nicholls
