Gedday all,
Yesterday I was speculating on the fact that at this time of year we should expect changes to occur in the character of cool changes crossing south eastern Oz:
Well... this morning I was looking over the past week's charts and noticed something interesting that occurred as an upper level long-wave drifted across southern Australia. I have put the relevant sequence of 500 hPa charts up on my web page, where you will see we had an upper trough off the westcoast of WA on the 24th. It moves across the Bight and is over Vic/Tassie on the 27th 00.
I have also put up a sequence
of surface thetae analyses (off the NMOC Coloured GASP charts site) for
the same sequence (unfortunately offset by 12 hours in each case).
The interesting aspect is that as the trough moves across, it forces ahead
of it at the surface a strong northwest - southeast oriented front across
Oz, in the thetae patterns. The thetae front is across the southern
tip of WA on the the 25 th. On the26 th it is east-west oriented across
the Bight, and a new/sharper one has formed stretching from southwest Queensland
down to Cape Otway (south east tip of the mainland).
On the 27th this latter
thetae front has become the main front and on the 28th (last night) it
has moved a little up the east coast.
The sequence is related to the fact that the normal tropical moisture source had moved down into southern WA with the monsoon depression earlier this week. However, earlier in the summer I was speculating on the fact (?) that the summer fronts show up almost exclusively in the theta fields and are not thetae fronts... Well... here is a difference as we start to drift in to winter: a nice thetae front.
Thetae at the surface being largely a measure of moiusture content: this is a front separating moister air (high thetae) from drier air (low thetae).
cheers
John McB
