Subject: Thursday am --- Urgent -- look at current Mornington Island Rapic

Hello all,
I arrived at work late today (9.40) and just turned on the computer --

Everyone: --Drop everything, bring up Rapic, and look at the Mornington Island radar
screen..... an absolute ripper of a squall line. I guess it is what we
use to call a North Australia Cloud Line (or when Holland used to rave
about it, a PLCL)......

radar image 1    radar image 2  radar image 3
satpic 1   satpic 2   satpic 3

The "expert" on these beasts is Wasyl Drosdowsky who has written a couple
 of papers on them (see for example Mon. Wea. Rev., June 1989).  From our
 limited experience with them in AMEX Phase I, they depend on low level
 easterly flow for their existence.  Given that,this one should continue
 through Weipa; but should die round about the time it gets to Darwin.

More later.
On other matters
a) I hear people haven't been posting becauyse the list of names is too
long... I'll go looking for Lena today to see how to set up an alias.
b) Can someone out there tell me how I go about saving an animated
gif/radar-loop

cheers

John McB
 

From J.Kepert@BoM.GOV.AU Thu Dec 13 09:59:09 2001
To: John McBride

John
The loops don't seem to be animated gifs, but rather a bunch of stills that are
looped by what I guess is a clump of java. You can save them individually, then
turn them into an animated gif or an avi using "Gif construction set pro" which
is shareware and has so far worked for me. This way you can generate loops of the
whole event, not just the last hour.
 

More meteorologically, there seems to be a small echo on the leading edge of the
main line, which is propagating southwards. Is this (a) common, (b) an illusion,
or (c) a mental artifact on my part, resulting from yesterday's Christmas party?

If (a), then what is the cause?

Jeff

From Harald Richter
From hxr Thu Dec 13 12:52:36 2001
Subject: aus-wx: MCC over S Gulf of Carpentaria
To: aussie-weather@world.std.com (Aussie Weather)
Date: Thu, 13 Dec 2001 12:52:27 +1100 (EDT)

Folks,

A large MCC over the S Gulf of Carpentaria is present in the
23:32 UTC GMS-5 VIS image. The anvil is nearly round and measures
several hundred kilometres in diameter!!

Some of the specs are:
(1) Lightning: The system is very lightning active with hundreds of strikes
over a timespan of minutes. The lightning display at

http://www.weatherzone.com.au/latest/Aus_Lightning.jpg
shows (presumably) massive aliasing along a straight line between
a GPATS sensor and the actual lightning location (unless you
believe in a perfectly linear, cloud-free squall line extending from the
Gulf to Bourke)!

(2) Moisture: The boundary-layer airmass over the Gulf has been very
good for a number of weeks now. Surface dewpoints are generally
in the mid-20s (C). A region of dry surface air extends all
the way up to the Gulf from the S (for the first time in weeks, IIRC).

(3) Temperature: I speculate that the dry intrusion into the Gulf country is
a vital ingredient for the occurence of the MCC. The 0000 UTC surface
temperature shows mid-30s C at Mount Isa. To the N a
pronounced moisture/temperature boundary delineates the
cooler and moister Gulf airmass. The deep boundary layer to the S,
along with a cross-boundary ageostropic secondary circulation
(forced by a cross-boundary theta_v gradient?) might have led to the
MCC initiation.

(4) Mid-level flow: A weak anticyclone is apparent over the S Gulf, overlaying
the N extends of the "Queensland Trough" on the surface.
The winds aloft (= 500 hPa) are weak (< 20 kts), as one
would expect for a classic non-severe rainy electric MCC...

(5) Sounding: YBMA (Mt Isa) 23Z sounding shows steep lapse rates up to 500 hPa, and
a large T/Td spread up to 700 hPa. Putting a 30/25 surface
parcel from the S Gulf under the sounding gives me more
CAPE than I dare mention in public ...

My scientific conclusion regarding the event: Wow!
Harald
 

From R.Stringer@BoM.GOV.AU Thu Dec 13 13:40:45 2001

Date: Thu, 13 Dec 2001 13:40:16 +1100
To: Jeff Kepert <J.Kepert@bom.gov.au>
Subject: Re: Thursday am --- Urgent -- look at current Mornington
Island Rapic

Hi Jeff et al,
My favourite name for these things was the "Oenpelli Express" - evocative
of the impact they sometimes have on Darwin. That name seemed to fade away,
maybe because greater satellite coverage (temporal) and radar coverage
(Gove) showed that most of the really strong long lived events were
generated from Cape York (and the locally generated events could come from
anywhere in the convective field over the Top End depending on the wind
structure).

I remember giving a lunch time talk presenting 3 case studies (mostly just
the satellite sequence). The first two case were classics - deep daytime
convection over Cape York picked up by the easterlies then propogating
across the Gulf, whacking Gove and later Darwin, with some weakening and
re-intensifying along the way, then out across the Timor sea to land on
East Timor. Total life maybe 24 to 36 hours. The third started out in a
similar fashion but in the western Gulf decided to head south-southwest as
a weakening ripple across the Barkly then Alice Springs district, giving a
wind SPECI at Tenant Creek. I can't remember having any explanation for
that behaviour but thought it was interesting to note the departure from
the standard model!

Cheers, Russell
 

Date: Thu, 13 Dec 2001 14:27:08 +1100
From: Mark Williams <mark.williams@BoM.GOV.AU>
To: John McBride <jmb@bom.gov.au>
Subject: Re: Thursday am --- Urgent -- look at current Mornington Island Rapic

John
If you want a gif viewer there is one attached. Just double click and it will
install. You will get 30 days out of it without registration. The software is
licenced to Monash which should be OK with us. I don't have the rego number with me
but can provide if you want to use the software in the long run.

Re the event - I've seen these before. I've seen HUGE ones right over the top end -
cover the whole lot. If yo want my 2 bobs worth they tend to form under a ridge and
trough in the westerlies has got someting to do with it. Go to
http://vortex.ho.BoM.GOV.AU:80/cgi-bin/dfcgi/difacs.cgi?0316. There is a short wave
trough over WA which appears to be mobile. Perhaps this is precipitating some upper
divergence, though it may be to be too far west??

Also there is something else going on that is very interesting. Go to
http://vortex.ho.BoM.GOV.AU:80/cgi-bin/dfcgi/difacs.cgi?1374 and there is an absolute
classic example of a major heat source right on the equator. Then if you go back to
http://vortex.ho.BoM.GOV.AU:80/cgi-bin/dfcgi/difacs.cgi?0316 you have upper
equatorial easterlies and if you go to
http://vortex.ho.BoM.GOV.AU:80/cgi-bin/dfcgi/difacs.cgi?0304 you have low level
equatorial westeries. Other Gill type features are not so clear. I'm watching it.

Mark
 

From: Geoff Crane <g.crane@BoM.GOV.AU>
To: John McBride <jmb@bom.gov.au>
Subject: Re: Thursday am --- Urgent -- look at current Mornington Island Rapic

G'Day John,
The trouble with getting old is that you remember things from too far back.
The term "Oenpelli Express" was in use during my time in Darwin (68-76).
But it had nothing to do with anything out of the Gulf.
It was used to describe the afternoon thunderstorms that formed on the
western Arnhem escarpment in the Oenpelli area, then rolled west across the
Marakai Plain to reach Darwin any time from early evening to early am, but
typically around 9pm to 11pm.

This morning radar imagery from Mornington Island seemed to cease about
half an hour before the squall line would have reached it. Maybe there was
a wind squall well ahead of the cloud line. We are not sure whether it is a
radar fault or a comm fault but the effect is likely to be the same - no
more pix until we can get a tech out there, hopefully tomorrow.

Incidentally, I tried to reply to all the addressees but my eMail system
didn't like that. I'm sure it will be easier when we get a group address.
Cheers, Geoff.