A)
The severe thunderstorms over Southern Queensland on Saturday night(22
Dec).
Damage on the goldcoast has been estimated at being of the order of 5 million dollars associated with this overnight thunderstorm system. Hopefully someone from the Brisbane office would like to comment on these. I was looking at the charts on Saturday daytime and was interested in the cold-front that had passed through Melbourne during the daytime on Thursday 20/12 (see Melbourne wind and temp plot for Thursday).
Looking at the sequence of 850 hPa vorticity plots off the TLAPS web site, On the Friday morning (21st 00UTC) the front has moved through SE Oz and you can see a line of cyclonic vorticity (blue colour) reaching up across eastern Australia. Moving ahead, on Saturday morning (22 00UTC) the parent low has moved across into the Tasman to about 160E, but still the front (cyclonic vorticity line) is hanging back across the Australian continent through southern Queensland and into the northern Territory, leaving a zone of low pressure and cyclonic surface activity and so a region for diurnal thunderstorm activity.
It
was presumably thunderstorms in this trailing frontal zone that brought
about all the damage on Saturday night.
I have often wondered about the mechanisms for this occurring: When a summertime front crosses southeastern Australia and moves off into the Tasman, why does it leave a trailing frontal zone hanging back across the continent. It was in trying to understand this phenomenon that I originally asked Jim Fraser to set up the daily maps of surface theta on the NOC site.
If
anyone has any insights on this, derived either from forecasting experience
or from dynamics, please let us know.
Melbourne: maptool: last Thursday showing the front coming through.
850 vorticity: 00GMt
21/12
00GMT 22/12
B) The monsoon trough:
Looking at today's 850 hPa chart from TLAPS, you can see a good monsoon trough extending across the eastern half of the chart: from about (0, 135) to about (15, 180). However, there is no monsoon across Darwin and north of WA. Why?
Well.. Part of the answer may be in the upper levels. A monsoon trough requires higher latitude easterlies and lower latitude westerlies at 850 and the opposite at 200 hPa. Look at the 200-850 shear chart for today from the TLAPS site: You have a nice monsoon trough structure of easterly zonal shear at low latitudes and westerly zonal shear at higher latitudes right across the eastern half of the chart. However north of WA, we have a huge low-latitude cyclone in the shear pattern, the direct opposite of what you want for monsoons and deep convection. How did that get there? Well...look at the IPV 350 chart for last night, and you'll see a large yellow area up there of cyclonic IPV... this can be traced back to having sheared off from the higher latitude "Polar" region of cyclonic IPV.
The effect I am claiming is the direct opposite of what I claimed a week ago whereby a sheared higher latitude IPV zone was triggering convection in a Coral Sea tropical low?
Maybe, when the system shears off completely (like the current one) it inhibits tropical convection; whereas as it can enhance it (through upslide type mechanisms) when it is still connected to a higher latitude trough. Who knows??
Today's 850 flow
Today's 200-850 shear
Last night's 350 IPV
If
anyone out there has any other insights on the dryness of the Oz monsoon,
please let us know. I suppose an obvious explanation is the MJO, which
has us in a strong dry/subsident phase according to the anomaly charts
on Matt Wheeler's website.
Cheers
John
McB