I was absent through much of December... hence my silence.

Anyway, we had a good summertime cool change yesterday.  This morning's news gave reports of a large number of fires (50 ?) of bushfires set off by lightning strikes as the front went through eastern Victoria.  Sydney papers reported extensive damage and unexpected sudden onset of winds as a "Southerly Buster" swept up the east coast.

It haven't actually got rolling yet this year.... but I'll put this note, plus a couple of charts on my web-page later this evening... In the meantime, does anyone have any more quantitative information on the two related events: the lightning-triggered fires and the buster.

Was there anything unusual or that we do not understand about this event?

Cheers

John McB
 

Meso laps 10 m: 00 UTC 7 Jan

 

Meso Laps 10m 12 UTC 7 January

 

Meso Laps 10m 00 UTC 8 January

Meso Laps 10m 12 UTC 8 January

Elly Spark

Dear John,

The southerly buster was not exactly unexpected. The lightning was caused by thunderstorms associated with the change.

We had:Gale warning for coastal waters; Severe Thunderstorm warning for Damaging winds.(Severe TS were with the
change) text of warning referred to damaging wind gusts both with storms and with change)
Airport warnings for sydney airport for severe gusts with the change.

I can't give you more details because I was not on yesterday. However, I don't believe there was anything that we don't understand. My impression that it was a good old fashioned southerly burster, with very hot conditions ahead of the change. WE don't see too many of these per annum though. Perhaps the most remarkable aspect is that the timing of the
front was forecast just about spot on last Saturday courtesy of GASP. I remember thinking our new PO1 put in too much detail for the Wednesday forecast wrt where the front would be in the morning and afternoon, because the timing is NEVER that precise 4 days out.  This time it was.

cheers,

John McBride

On Thu, 9 Jan 2003, elly spark wrote:

> I can't give you more details because I was not on yesterday. However, I
> don't believe there was anything that we don't understand. My impression
> that it was a good old fashioned southerly burster, with very hot
> conditions ahead of the change. WE don't see too many of these per annum
> though.

Hmm... we have approximately one cool change per week through the December - March period, i.e about 16 cool changes per summer; yet Elly tells me "we don't see too many of these Southerly Busters per annum"...... so tell me.... what is the difference between your normal summer front and the one that gives you a southerly buster??

Robert Moore

Most of those cool changes through Melbourne are shallow and seem to get slowed down  or modified by surface effects
( friction, warming, etc?). Deeper changes  seem to conform more to the gravity current theory where speed is related to the sq root of depth of layer and delta T across the front. In this case delta T was pretty huge and the depth of current was
maybe something like 3 to 5000 ft, as evidenced by Melbourne's Temp trace at 22Z that morning, where the layer of cooler SW air behind the change ( which was then near  Greencape) was 8000 ft. So my rule of thumb is if the Melb temp trace shows a good depth of cool air post frontal then the change is likely to blow your socks off in Sydney.

********************************************
Bob Moore
Sydney RFC