A bit of speculation on the final stages of Tropical Cyclone Beni in the South Pacific:

While at the WMO Tropical Cyclone meeting at Cairns in December, I heard a lot about extratropical transition, which is something I hadn't thought  about very much.

    I was scanning the daily satellite images last week and noticed Tropical cyclone Beni suddenly lose all cloud banding structure and fall apart within a couple of days, say between 29 January and 31 January (see relevant satellite images on my web-page: follow links to synoptic discussion, today's date)

    This is strange, I thought, how does a TC lose structure and intensity like that while still over the deep tropics (approximately 20 S) and not associated with recurvature?   Well.. the answer seems to be that the high pressure cell to its south became involved in an even higher latitude ridging, so that a blocking high formed with its associated split-jet pattern; thus effectively Beni became the low part of a high-low blocking dipole.

    To demonstrate this I have put on my webppage a sequence of 200 hPa GASP analyses for the period.  You can see that on the 29th and 30th January the Tropical Cyclone was located equatorward of any jetstream activity and close to a col/low-wind regime at upper tropospheric levels. On the 31 January, however, strong/intense high latitude ridging occurred such that the major jet stream had dipped to 50-60 S.  As always happens this causes a split-jet/block to form such that a weak "sub-tropical jet" started to form around the equatorward side of the Tropical cyclone.

My conceptual model (formed while sitting in the back of the room in Cairns) of the difference between a tropical cyclone and a mid-latitude/baroclinic cyclone is as follows:  a tropical cyclone has the structure of a barotropic cylindrical vortex extending from the surface to about 500-hPa; and then falls off rapidly above that level.  It also has an intense eye-wall inner core.  A mid-latitude system, on the other hand, is associated with instability mechanisms on the circumpolar westerly vortex; thus has the sub-tropical (or in some cases polar) jet stream as an integral part of the system on its equatorward side.

A consequence of the onset of blocking at the longitude of Beni is that it suddenly had a low-latitude component of a split jet forming above it, and it was effectively captured as the low part of the high-low blocking dipole. By the above McBride conceptual model, this means it has a jet stream around the equatorward flank and so has been transformed into a mid-latitude/baroclinic system.  Thus the rapid decay and loss of tropical structure observed in the two days was due to this extratropical transition.

An interesting thing then happened:  continuing the upper-level 200 hPa sequence for the next couple of days: on the 2-3 February a new split jet pattern formed upstream/to the west.  Thus the forming split over Beni collapsed and so Beni was no longer under the influence of a jetstream on its equatorward flank.  The process was so fast that the lower tropospheric vortex was still in contact with the underlying warm ocean; so re-formation or reintensification of the tropical cylone took place as it approached land.
 
 

Besides the upper-level sequence I have put on my page a couple of close-up satellite images for 3 February. 0900 UTC 3 Feb,   1500 UTC 3 Feb .
These show that Beni has reacquired a tropical cyclone appearance, as well as showing the new low-latitude upper level trough overland immediately to the west of the tropical system.

I captured these images at work yesterday.  While I was sitting in the kitchen last night typing this up, I could hear the news on the TV upstairs telling me ex-tropical cyclone Beni had made landfall and was bringing wide-spread rains to coastal Queensland.  It would be interesting to hear the experiences and views of the forecasters in Brisbane who would have been kept busy with this system over the past few days.

cheers

John McB

 Jeff Callaghan
John

 From 1200UTC 29 January 2003 to  31 January 2003, a 500hPa to 200hPa low developed just to the west of New Caledonia and Beni was moving beneath 40 knot vertical wind shear (CIMSS data ) to the east of this system at
1200UTC 30 January 2003. Beni was consequently weakening rapidly.

The low was part of an anticyclonic involution and ridging developed south  of Beni steering it westward. The ridging developed first at low levels while ridging was delayed at 500hPa south of Beni as a weakening trough  system passed to the south at 1200UTC  1 February 2003. This had the effect  of displacing the 500hPa centre of Beni to the southeast of the low level  circulation.

By 1200UTC 2 February 2003 a new anticyclonic involution developed over  eastern Australia culminating  in a new 500hPa low southwest of Beni. This southwest slope of the circulation resulted in a large area of warm air  advection on the eastern flank of the system,

We diagnosed the warm air advection from the EC 850hPa,700hPa and 500hPa  winds which of course were backing with height in the warm air advection  region. The convective blow up you show around 1200UTC 3 Feb 2003 was when the warm air advection area reached the strong low-level circulation of  Beni. There were 40 to 50 knot NW winds at 200hPa over the top of Beni,  which kept the convection away from the centre. Gannet Cay AWS (94379) was  in the convective region and recorded gales from 1300UTC 3 February 2003 to  0600UTC 4 February 2003 with a max 10 minute mean wind of 41 knots.

By 1200UTC 4 February 2003 there were 70 knot NW winds at 200hPa over the  system and the low level centre got further displaced from warm air  advection and gales. A quikscat image shows a low level centre at 0711UTC 4 February 2003 near 19S 151 E while a band of gales to  50 knots were  between 21S and 23S to the S and SE of the centre. (This gale area was also the area of strongest warm air advection from the 1200UTC 4 Feb 2003 EC  winds).

The isolated low level circulation then weakened however the warm air  advection region came on to the coast and produced very heavy rain

Jeff.

John McBride
Jeff,  Thanks for these insights.  I believe our  views are complementary rather than incompatible.

>  From 1200UTC 29 January 2003 to  31 January 2003, a 500hPa to 200hPa low
> developed just to the west of New Caledonia and Beni was moving beneath 40
> knot vertical wind shear (CIMSS data ) to the east of this system at
> 1200UTC 30 January 2003. Beni was consequently weakening rapidly.
>

The shear provided a mecahnism (shear is not a mechanism -- but you know what I mean?) by which the tropical cyclone structure was lost.  The reason for the development of the "new" low, I think was the split-jet blocking mechanism I described.

> The low was part of an anticyclonic involution and ridging developed south
> of Beni steering it westward. The ridging developed first at low levels
> while ridging was delayed at 500hPa south of Beni as a weakening trough
> system passed to the south at 1200UTC  1 February 2003. This had the effect
> of displacing the 500hPa centre of Beni to the southeast of the low level
> circulation.
>
I believe this is consistent with my blocking mechanism.
 

> By 1200UTC 2 February 2003 a new anticyclonic involution developed over
> eastern Australia culminating  in a new 500hPa low southwest of Beni. This
> southwest slope of the circulation resulted in a large area of warm air
> advection on the eastern flank of the system,
>

Yep..... In my terms, this meant a relaxing or loss of the upper-tropospheric low that was over Benni as the new block formed to the west.

> We diagnosed the warm air advection from the EC 850hPa,700hPa and 500hPa
> winds which of course were backing with height in the warm air advection
> region. The convective blow up you show around 1200UTC 3 Feb 2003 was when
> the warm air advection area reached the strong low-level circulation of
> Beni. There were 40 to 50 knot NW winds at 200hPa over the top of Beni,
> which kept the convection away from the centre. Gannet Cay AWS (94379) was
> in the convective region and recorded gales from 1300UTC 3 February 2003 to
> 0600UTC 4 February 2003 with a max 10 minute mean wind of 41 knots.
>

This is the mechanism by which the reintensification took place... though I believe it could only happen once Beni had lost its
extratropical/blocking-pair character.

cheers

 John Mcb