There were a few interesting aspects of the front that went through Melbourne last Friday — Web-site: Maptool image of Victoria

a) Identification of the front in theta and theta-e space:

 Living in Melbourne over the years, I have built up a model in my head of summer-time fronts.  The frontal structure can be seen as parallel isolines in the low-level theta surface.  You shouldn’t see much frontal structure (in summer) in the theta-e surface because there are countering effects (warm/high-theta-e but dry/low-theta-e air ahead and cold/low-thetae but moist/high-thetae behind).  This is in fact what you see as the front goes through eastern Victoria on Friday evening.
:Web - theta chart,   theta-e chart for Friday evening.

This system didn’t actually come up from the south, but rather advected across almost at constant latitude from WA, and the same structure (good front in theta, poor front in thetae) can be seen a day and a half before back over the western Bight: Web - theta chart,  theta-e chart for Thursday morning.

However, look at what subsequently happens:   The front shot quickly across Victoria, and off into the Tasman amalgamating with the cut-off low that was already sitting there in the Tasman. At 2300 on the 11th (Saturday morning) there was still a good surface theta discontinuity across the east coast as the Front had shot up there in a southerly-buster fashion.  However, 24 hours later (theta chart 23Z 12th) there was very little sign of it.  But, by that stage it had transformed into the dry line or the quasi-permanent theta-e discontinuity across southern Queensland. (Theta-e chart: 23Z 12th).

The same process happened with the front a week earlier : that is, as the front moved into the Tasman, it lost all theta-frontal structure, but somehow trailed back and transformed into the theta-e front across southern Queensland.

This theta-e front across southern Queensland seems to be acting as a trigger for the diurnal, tropical, continental convection that has been going on for the past week.
 

B) Rapid movement: seems to have shot away from the parent low.

 The front was extremely rapidly moving, as can be seen from the map-tool loop as it crosses Victoria, and as can be seen from the sequence of 900 hPa vorticity charts as it crosses from central Bight to Tasman in 2 days:
900 vorticity charts: 00Z 10th
    00Z 11th
    00Z 12th

 It was also interesting that the parent higher latitude low did not move across, but rather branched off into a separate almost mesoscale cut-off that travelled through with the front.

Sequence of 500 hPa anals:
    00Z 10th  – while the front is across the central Bight
    00z 11th  - a separate cut-off trough passes into western South Australia
    00Z 12th - the cut-off mesolow travels across Tassie to amalgamate with the low in the Tasman.
 

 It  was all pretty dry dynamics with no rainfall to speak off.  However, it is fascinating that there was such a mobile and small scale feature travelled across southern Australia for no immediately obvious reason.
 

Tony Bannister

John,
at the end of your essay you comment

>  It  was all pretty dry dynamics with no rainfall to speak off.
> However, it is fascinating that there was such a mobile and small scale
> feature travelled across southern Australia for no immediately obvious
> reason.
>

One reason could be to cause operational forecasters heartache keeping up with
it.  It was an evil windchange with gusts up to 48kts associated with it in
Vic, as Rocky says here it was a killer change, we were lucky there were no major
fires happening.  Most of the models handled the big picture well, ie moving it
very quickly during the day through Vic and a very N/S orientation.  However
there was some devil in the detail.  We noticed as it moved across Vic during
the day that about 30-60 minutes before the main SW cold blast the wind would shift
from N to moderate WNW with a pressure rise but no temperature drop, then the
main SW kick with the strongest wind in the SW'ly about 1-2 hours after it
moved through.

It was a nice deep change as picked up on the Mt Gambier profiler.  I think
there was plenty of convergence and lifting going on around the windchange, the
atmosphere was just too dry ( from memory the GASP precip water anal was about
10 thingees in the airmass ahead of the change.)

Graham Mills
 

Strange to say, but fronts do evolve in time as they cross southern
Australia. Looking at John's images for last week's change I can see
the theta and the theta-e fronts approaching the southern SA
coastline. This is pretty common - for coastal frontogenesis/pre-
frontal troughogenesis to happen there needs to be a synoptic
situation in which there is a mid-latitude trough to the west or
southwest, so there's a theta-e front there.

Its almost impossible to do more than hypothesize (fantasize?) about
what may be happening between 12 or 24-hour snapshots - the
timescale of frontogenesis is a few hours. Multiple frontogenesis
events have occurred during this period. A modern forecaster looks
at hourly evolution of meso-LAPS output to try to understand these
processes.

Think of a front as an time-and space evolution of thermal and
kinamatic parameters - not as a thick black line with little knobs on.

Graham

John McBride

On Tue, 15 Jan 2002, Graham Mills wrote:

>  Multiple frontogenesis
> events have occurred during this period. A modern forecaster looks
> at hourly evolution of meso-LAPS output to try to understand these
> processes.
>

Yeh... I agree.... I am just learning my way around the internal web since
I/we started this discussion group.  Any help or guidance on good
diagnostics, or where to look for detailed structure as systems come
through will be appreciated.

As Noel Davidson pointed out a week or so ago, this discussion has
highlited how difficult it is to look at events a few days after they
have passed.... I believe the system that Metview works off is pretty
good; but that the LAPS analyses do not go into it... Is this correct?

There is also a nice diagnostic system called "Kenny" out there on the
AIFS system; but as far as I know we can't get at that either from our
desktops in Head Office.

Anyway, looking for to seeing your exposition of some of the details of
these fronts as they come through
 

John McBride: Further reply:

Just a follow-up to Graham (Guru Murrumbidgee)'s comments yesterday on the
importance of following the mesoscale evolution of events such as last
Friday's front.

I agree that a forecaster is most interested in the details, as seen in the
evolution of the one-hour time step Meso-LAPS fields.  In fact, stimulated by
Graham's remarks I have spent much of this morning scanning the web and
becoming familiar with the Meso-lAPS output.

Despite that, I still maintain an event like last Friday's is controlled by
the synoptic scale.  As shown on my web-page, the movement of the front across
south-eastern Australia was associated-with/accompanied-by a cut-off of a
meso-low at 500 hPa.  This meso cutoff and its passage across SE Oz were
forecast by GASP 96 to 120 hours in advance.  I have a hard copy of the GASP
prog from last Monday (7 January) sitting here on my desk, and I actually
printed it out and kept it, basically because of that meso-low evolution that
was forecast.
[GASP Prog from 12UTC 7 January:  Day 1,  Day 2,   Day 3, Day 4,  Day 5 ]

So... the detailed structure of developments of the front during the daytime,
the accelerations, the pre-frontal troughs etc are all part of the Meso-LAPS
scale processes.

However, there is a case for the argument that the umbrella-larger scale
behaviour determining whether there is a front there in the f irst place, and
how long it will take to cross Vic are governed by larger/synoptic scale
processes.