Tony Bannister

John McBride wrote:

>Ah... these things are so much more obvious after the event.
>
>Motivated by kevin Torey's analysis I extracted the relevant maptool
>images corresponding to Kevin's analysis times: 0000, 0600, 1400 and have
>put them on my webpage
>http://www.bom.gov.au/bmrc/clfor/cfstaff/jmb/03Dec08a.htm
>The 1400 UTC image, corresponding to the time of the event is particularly
>interesting.  You can see quite a strong almost-synoptic scale line of
>convergence across Melbourne, with ten knot  southerlies to the soutwest
>and twenty knot northerlies to the northeast.... easily sufficient to give
>the 200 hPa  uplift required to kick off the convection.

Is there anyway to quantify 10kt S'lies and 20kt N'lies equates to
easily 200hpa of lift?  This is an everyday forecast problem where the
forecaster is asking 'is that enough convergence to get ignition?'

cheers

Kevin Parkyn
Date: Tue, 09 Dec 2003 19:02:32 +1100
From: Kevin Parkyn <k.parkyn@BoM.GOV.AU>
To: Synoptic Discussion <synoptic_discussion@BoM.GOV.AU>
Subject: Re: [synoptic_discussion] [Fwd: good thunderstorm call last week]

Great to see so many people interested in the Melbourne event from all angles. I'll briefly comment by making a few observations from the operational met perspective.

To add to the story already provided by Pete Newham and others about  what transpired on the bench..............I received a call at home about 3am that morning from Ken Dickinson (senior met on duty that  night). I was already up - not sure whether it was because of the strobe like affect taking place in my bedroom or the hunger calls from my 6 week old daughter. Anyway, when the phone goes off during the night it is usually work, so I prepared myself. The conversation was pretty short really - Ken said "When can you get into work?" I replied  "In about 30 minutes". And that was about it! However, I nearly didn't make it to work as I had to drive through half a metre deep water for 100 metres  before breathing a sigh of relief (I live near Jells Park, which resides in Melbourne's eastern suburbs along Dandenong Creek).

The strong northerly low level wind is probably the main mechanism  behind why so much rain fell. The slow advection of the storm I believe to be really a function of the storm back building as a result of the  interaction between the cold pool generated by the storm, the trough and the strong northerly wind in the low levels. Once the storm had passed  Stuart observed the northerly inflow take an almost unbelievable right angle upwards trajectory into the back edge of the storm - hence the back building and the extreme water loading experienced.

When forecasting heavy rainfall over a broader area that is not necessarily convectively driven we often look for the presence of a strong low level jet in conjunction with all the other necessary  conditions. The isentropic motion associated with the northerly low level jet is often associated with heavy rainfall events in Victoria.  Although the northerly airflow in the case of the severe storm that affected Melbourne did not lift isentropically perhaps the conceptual  model is similar?

So what kicked the storm off in the first place and why just northwest  of Melbourne? The usual explanations have surfaced, but something Stuart Coombs brought to my attention is the possibility of a propagating  gravity wave generated by another storm. We see storms kicked off by such mechanism each spring/summer and usually it is a little hard to  depict in real time. In this instance, a monster of a storm had developed off the Otway Ranges in the evening (160 VIL on radar - it was assive). The gravity wave produced by this storm would place it near Melbourne about the same time we had lift off near Craigiburn as observed from radar.

Just a few thoughts from the bench to add to the mix. Hoping to get Rob  Webb to examine the radar sequence for further insight into the storm dynamics...............

Cheers.......Kevin
 

Kevin Tory
Hi there,

I've put together some hourly GMS images that Dave Pike retrieved for me, which shows some evidence of gravity wave
activity that may be linked with the storm development (see Kevin Parkyn's earlier message).  I'm no expert on gravity wave
generated thunderstorms so I wont comment on these images.

Cheers,

Kevin Tory.