Lori Chappel

This may be of interest...

An intensifying low in the Arafura Sea several days ago moved inland  over the Top End of the Northern Territory last night and has stayed very  intense/ or continues to intensify with Darwin's winds below 14000ft currently in the range 40-60 knots.

The intense monsoon circulation is now located near (13S 134E), Darwin is about 400 km to the west. It is starting
to look a bit raggy on the satellite imagery now though.

regards

Lori

http://www.bom.gov.au/weather/nt/

Roger Edson

Hi,

No surprise here.  This is another case of Australia's famous 'Land-Foons'.  No other place in the world do I
see such consistency and maintenance of circulation (above the boundary layer) as in northern Australia...I could swear that some even seem to intensify as they move along this area.

Roger
=====
Roger Edson,Anteon Corp
University of Guam/WERI

John McBride

Following up on this system.. It has stayed there over Northern Australia for four days now, bearing all the large-scale structural characteristics of a tropical cyclone, but not having the intense inner-core structure.  The surface pressures remain at around 995 hPa.  It is a particularly beautiful system; so I have put a sequence of GMS enhanced images (one per day) and of 300-850 hPa Deep layer mean flow on my web-page (under 7 January entry).
 
 

As always the presence of these systems raise all sorts of fundamental questions...

*    Why do cyclonic systems over the tropical oceans remain above 990 hPa surface pressure unless they undergo transition to a typhoon-type state with an intense eye-wall type inner structure?

*    Why do we not get large scale cyclonic weather systems over the other tropical land masses (Africa, South America, IndoChina, India even?)..  The answer presumably is that we have no genesis mechanism.. baroclinic instability (at least large-scale wave-number six type baroclinic instability) does not operate away from jet streams and mid-latitude circumpolar baroclinic zone) and apparently whaever generates a TC (CISK, WISHE, whatever) does not operate over the tropical landmasses, with the exception of Northern Australia.

Another intersting aspect of these figures is that the structure nicely illustrates one of the major differences between a tropical cyclone and a baroclinic-mid-latituide weather system.  As you all know, mid-latitude systems are a result of baroclinic instabilirty and so they have a stabilising influenxce whereby they transport heat at the low levels and momentum at the upper levels meridionally... Thus they have a wave-type structure and are embedded in the baroclinic zone.  Tropical cyclones, on the other hand do not have this meridional transport property and thus are isolated systems.  Presumably this "extratropical transition process" we heard so much about in Cairns is related to (defined by?) when the cyclone loses its isolation and begins to perform a meridional transport process????... speculation on my part.... but it is all proveked by the splended isloated-ness (if there is is such a word) of the cuurent northern Oz monsoon deprerssion, as seen in the 300-850 hPa deep layer mean charts.

cheers

John McB