From: John McBride
To: synoptic_discussion
Subject: Earth has not anything to show more fair
 

For those of us sitting over here in Head Office, the development over WA and then over the
Bight over the past couple of days has been quite beautiful:

One would have to describe it as a text-book example of cut-off low development followed by
baroclinic-leaf/northwest cloudband development. You can tell the weather is extraordinary
when even such a non-synoptic person as Neville-ELNino-Nicholls comes into my office to talk
about weather.

I have grabbed the loop for the past two days and placed it on my web-page

satloop

In the first part of the sequence there is large area of open-cellular cloudiness depicting a
wide-spread field of cold air. Within the cold air mass you see a comma cloud development
move northward as it develops rotation and grows in size until it finally lands on Perth as a
deep active mass of heavy rain and convection.

While this is happening the layer/upslide cloud east of the broad upper level trough streams
into the Bight.  It comes across in two separate pulses. When the second pulse crosses the
southern coastline half way across South Australia it thickens and deepens (presumably
downwards) into a major baroclinic leaf structure. Beautiful!!!!!

Date: Fri, 19 Aug 2005 12:14:53 +1000
From: Wasyl Drosdowsky
To: 'John McBride'
Subject: RE: Earth has not anything to show more fair

John,

I created the attached plot (Hovmoller of 500hPa anoms for August along 45S)
on the CDC site.

note my clumsy black line in lower left corner showing downstream
amplification from trough at about 20E on about 14th,  to ridge at 70E on 15th to trough
at 120E on 16th (Tue) and on to ridge about 160E on 17th.
System over WA reached its peak about Wed 17th.

Note new amplification of trough near 30E on 18th. trough over Australia is
moving slowly east, and should be at about 150E by Sunday. All progs
(GASP/LAPS/EC) have an amplifying trough over us on Sunday night (21st),
which is about the right timing for another downstream event.

Wasyl.

John McBride

> From: Wasyl Drosdowsky
>
> I created the attached plot (Hovmoller of 500hPa anoms for August along 45S)
> on the CDC site...... etc.

Hmmm... does anyone out there know anything much about
energy-dispersion/Rossby-group-velocity-propagation?

You don't have to be a Humboldt scholar to be able to continue the line along once the energy
dispersion begins.  But what starts it off in the first place?  what are the upstream
conditions that set off a wave-train.... any clues?

Have a good weekend

John McB
 

John McBride
22 August

This is further to our discussion last week of the cut-off development in cold air off WA.
Wasyl (of "not a Humboldt Fellow" fame) searched the MODIS site and found a nice image of the PVA-max (or cyclonic vorticity advection maximum) in the cold air.  It is so nice I have put it on my webpage at
    http://www.bom.gov.au/bmrc/clfor/cfstaff/jmb/05Aug18a.html
 

Modis Image Bands 1-4-3 (True colour) 17 August 2005  0245 UTC   2 km resolution


Higher resolution (500m) image (Warning: 3.17 mb)

The picture is actually terrific.  For a start have a look at the open cellular convection, the pattern we always interpet as what we call "cold air", and over the ocean designate as south-westerly to southerly gradient level flow.  The convective structure of the cold air pattern is well-delineated, as are the open cloud free polygons, surrounded by the deep convective lines.

Even more fascinating is the PVA max itself.  From this picture you can see very clearly that it is a massive organisation of deep cellular convection.  The convection is organised as a field of deep cells travelling along together in the southerly flow.  The organisation of the cells into a discrete unit does not involve any physical linking through a joint upper level anvil, as there is no mesoscale anvil.  The cloud in the core or eye of the PVA structure is entirely deep convection.

The band immediately to the west also seems to be entirely convective, with still no anvil as such but a huge amount of cirrus blow-off in an eastward direction.  To me, this picture is amazing.  I guess we have always known about it, since the early days of satellites.... still, who understands it?

Whats more do the models produce an area of deep convection in the location of the PVA max like this.... Millsie, Lawrie Rikus?
 

Moving on, I have placed a couple more Modis pictures on my page from the same day.
The first is the image immediately south of the cold air field (i.e immediately south of the first image).  It is interesting in a couple of ways:  a) it gives another view of the cold air; b)  you can see the high latitude "polar" low, with a nice tropical cyclone type "eye".

Modis Image Bands 1-4-3 (True colour) 17 August 2005  0250 UTC   2 km resolution


 

The next image, also for the same day is further east over the Bight.  There I have lines up two Modis images to show a  picture of a weak-ish cold front over the Bight, with the front leading down to the parent low.

Modis Image Bands 1-4-3 (True colour) 17 August 2005
Two pictures:  0515 UTC (southernmost image) 0520 UTC northern image

For cross-references purposes: closest image from above loop: 

Cheers

John McB