29 January 2006:

Monsoon Depression or "Landphoon" over northern central Australia
Posted to the International Tropical Storms list 29 January

Gedday,

There is a major field experiment currently taking place, based in Darwin northern Australia: It is called TWPICE (Tropical Warm Pool International Cloud Experiment), as described at http://www.bom.gov.au/bmrc/wefor/research/twpice.htm

Over the past week we have seen a vortex develop over us that has since moved southwards over the continent. On my web-page at http://www.bom.gov.au/bmrc/clfor/cfstaff/jmb/00_2006_28Jan_a.htm
I have placed this email along with some figures.  There has also been a recent post on it by Lori Chappel.

The current structure is that of a classical "land-phoon".  By that we mean it has the large scale structure of a tropical cyclone, but without the inner core, eye wall and spatially-restricted zone of very high wind-speeds around the eye. 

The  mean sea level analysis for yesterday morning showed it has a central pressure of 994 hPa. 




The reasons for saying it has a tropical cyclone- type structure are basically that it is approximately vertically stacked, is restricted to the troposphere, and has an upper level warm core, centred at about 300 hPa.  The low-level circulation is shown on the 850 hPa  TXLAPS analysis (also for yesterday).  This shows the vortex at approximately 17S, 136 E with a strong

 

westerly monsoon current equatorward and trade easterlies poleward; thus, by definition it is located within the monsoon trough.

A north-south vertical cross-section through the centre of the system reveals a deep cyclonic flow with both the westerlies (equatorward) and the easterlies (Poleward) extending with very little shear from the surface up to 400 hPa.  Above that level the circulation quickly dies off,  meaning by thermal wind the system has a warm core located at around 300 hPa.





Another important aspect of the similarity in structure to a tropical cyclone is that the associated convection form in rotating bands, similar in appearance to the outer bands of a tropical cyclone.

This is shown on a MODIS image for 0130 UTC on 27 January




and also on the radar.  On my page, I have placed two loops  from the Tennant Creek radar: the first from yesterday morning (2250 27 Jan to 0220 28 Jan UTC) which shows both the banded structure and the rotation.  The second from this morning (2310 - 2330 UTC 28 January)   shows an eye-type structure.

I have also placed a loop of the national radar composite extending over 12 hours during the afternoon and evening of the 27th January.  This also shows the rotation clearly.  An interesting feature of this is that it reveals the MCC-type convective blow-up in the Gulf of Carpentaria  was actually part of the outer-cloud band propagation of the Land-phoon.

The existence of these tropical-cyclone type systems over land poses a number of questions for understanding tropical cyclone dynamics and monsoon depression dynamics.  They have been discussed over the years on this email list (tropical storms).  I think the first person to write about them in the literature was me (McBride, J.L. and T.D. Keenan, 1982: Climatology of tropical cyclone genesis in the Australian region. J.Climatol.,  2, 13-33;  McBride, J.L., 1987: The Australian summer monsoon.  Monsoon Meteorology, C.P. Chang and T.N. Krishnamurti, Eds., Oxford University Press, p.203-231.)  Since I am in Darwin, I don't have copies of either publication here; but from memory the second of these contained a photo of the radar screen at Alice Springs airport showing an eye-type structure in the cloud circulation.

As for the interesting questions:

    What is the development mechanism and is it the same as for tropical cyclones.?  It could still be an air-sea interaction on the large scale as much of the circulation is still over the sea during the developemnt phase.
 
Why is the strong monsoon current on the equatorward side so "dry"/devoid of convection.  How can you have deep monsoon westerly flow that is so dry?

What is the mechanism for intensification over land? (see sequence of MSL charts below)

How does it maintain itself as a rainfall depression for so long when not over the ocean?

Fascinating stuff

Regards

John McBride


Sequence of Mean Sea Level charts: