We have an interesting current situation over Norther Australia, whereby a monsoon depression has developed over the far north of the continent.  This has many of the characteristics of a tropical cyclone.

On my web-page (link -below – click on synoptic discussion; then on today’s date), I have placed the current TLAPS broad scale analyses for 850 hPa, 500 and for 250 hPa as well as a current enhanced satellite image.

Comparing the three analyses, you can see 25-40 kts cyclonic winds at 850 and 20 to 40 kts at 500 hPa: This is the classical Tropical cyclone structure, with very little shear (and so very little thermal structure) through this lower half of the troposphere.  Then when we look at the 250 hPa flow, the system has more or less disappeared, meaning a warm core between 500 and 250 hPa – Once again the classical large scale structure of a Tropical Cyclone.

So.. Why is this interesting?  Well.. It presents a case that a large component of tropical cyclone development lies in the formation of the synoptic scale upper tropospheric warm core; and that this part of the process is not dependent on being located over tropical oceans; but can actually happen over land.

The Darwin Office of the Bureau has put out a cyclone advice on this system stating there is a possibility  it will  undergo the transition to a tropical cyclone if the centre passes over the seas. (see threat_map on my web-page.

Cheers

 John McB

From: "Lance F. Bosart" <bosart@atmos.albany.edu>

Hi John,

        Agree that the structure weakens between 500-250 hPa. However, I
would want to do an independent check with raob/aircraft/satellite winds to
check out the nature of a possible equatorward extension of the 250 hPa
trough over interior Australia toward the Gulf. I would also want to check
out the possibility of a weak jet-entrance region in the deformation region
northeast of where the trough appears to "disappear".

                                                        Lance

John McBride
Lance, Thanks for this... I have been meaning to write to thank you for
your feedback on the cold-front/upper cut-off systems of a week or so
back.... but, you know how busy we get in the office these days.

I'll have to think about what you are saying... but, I'm not sure I get
the point.  Even if the system is partly forced by jet entrance type
dyndmaics, it is still a tropical-type system to the extent that it still
must be upper-troposhere warm-cored.

Regards

John mcB

Lance Bosart
Hi John,

        Agree. I'm looking for ways to create synoptic scale ascent to
bring the larger scale to quasi saturation so as to minimize the
debilitating effects of unsaturated downdrafts. If you have a bit of a
jet-entrance region present then the associated deformation region can help
to develop a weak outlfow channel to higher latitudes while nurturing the
convection association with the low- and mid-level circulation center. This
process should help to enhance the developing warm core.

        Bed time here on the east coast of N. America.

                                                        Lance
 
 

From: Brian Kabat <brian.kabat@ssec.wisc.edu>
John,

I have activated the UW-CIMSS AMSU Tropical Cyclone algorithm over the
suspect area out of pure curiosity.  It does indeed appear there may be
some upper tropospheric warming over northern Australia.  Ch. 7 indicated a
+0.34 C anomaly already at 12/04 UTC.  A subsequent pass caught the area
out on the limb.  You can find the accompanying imagery at:
http://amsu.ssec.wisc.edu/shaust33.html

Regards,
Brian Kabat
 

From: Kerry Emanuel <emanuel@texmex.mit.edu>

Hello John: During TEXMEX  we surveyed a number of cloud clusters with
strong cyclonic flow from the upper troposphere down to around 900 mb,
though the boundary layer was still anticyclonic. These were precursors to
TCs. We concluded from the evolution of these systems as well as from their
thermodynamics that the initial cyclonic flow was driven mostly by
evaporation of rain from stratified anvil systems. Do you think the system
you refer to may represent the same class of disturbances? The relevant
TEXMEX reference follows.  Regards,  Kerry

ftp://texmex.mit.edu/pub/emanuel/PAPERS/bisteremanuel97.pdf

John McBride

Hey Kerry,

Thanks for the response.  My intitial reaction is that I doubt it, simply
because the cyclonicity we see in situations like the one over the Gulf
is apaprently very so involved with the large scale monsoon flow, and with
the partial u/partial y vorticity on the polar side of the broad monson
low-level current.

However, I have an open mind on these things... and its worth
investigationg.  I am out of the office all next week; but I am cc-ing
this to Noel Davidson.  When I return in a couple of weeks time, perhaps
we can run a test by artificilly imposing a mid-lower troposhere cooling
term under the rain areas on one of the largae scale meso-LAPS model runs.

cheers

John McBride

Kerry Emanuel

John:  In the TEXMEX cases, the MCCs invariably formed within easterly wave
troughs, though it was never clear to us what role they played, or whether
or to what degree their vorticity was essential in the mesocyclone
development. I hope to go back to Mexico someday with a more comprehensive
observing system.  Cheers, Kerry

Noel Davidson
Hello Kerry (and John),

The processes you described in the 1997 MWR paper on the genesis of Guillermo,
are rather similar to some things written in :

Davidson, N.E., 1995: Vorticity budget for AMEX. Part I: Diagnostics.
Mon.Wea.Rev. 123, 1620-1635.
Davidson, N.E., 1995: Vorticity budget for AMEX. Part II: Simulations
of monsoon onset, midtropospheric cyclones and tropical cyclone behavior.
Mon.Wea.Rev. 123, 1636-   .

In there, particularly Part II, it is described how it seems possible to spin up the midlevel circulation via upper level heating, and then have the vortex develop downwards via a parameterised vertical transport of
momentum (associated with the rain shower). When the vortex makes touch down,
the increased surface fluxes act to enhance the low level spin up.
This scenario is/was very similar to that observed.
There is no requirement for large low level convective heating.
In my opinion, most operational forecast models cannot, or do not, represent these processes,
and so genesis forecasts can be rather poor. I believe that even though they (models),
at times, may have favourable environmental conditions, they generally cannot build
the vortex correctly (mid level spin up and then downwards development).
An experiment similar to that suggested by John, is described in there as well.
The result was that with low level cooling the low levels didn't spin up,
even though one might expect that with a strong vertical gradient in
subgrid heating, there should indeed be some (spin up). It may well have adversely
affected PBL structure and fluxes.  Anyway we could easily
re-visit this problem - it might be fun. The apparent requirement for environmental
cyclonic vorticity still remains somewhat of a mystery. {My wild speculation is that
there is a need for a weak low level perturbation, to which an upper level disturbance can
couple. But I think the current major issue (for genesis) is/are the moist processes.}

Because of the severity of the TC genesis problem around the Australian coast,
we will be specifically targeting this problem over the next 3 years.
~ 6 storms (and numerous TDs) per year develop near to the coast - the challenge is to forecast
genesis,
track and intensity before knowing that the storm has actually formed (it can potentially affect
communities within 12 hours) - quite a forecast problem.
Initially we hope to assemble comprehensive data sets (conventional obs, scat and satellite data,
surface obs, imagery, and rainfall estimates) and to assimilate this data using coupled
convection/prognostic cloud schemes (not done yet). In this way we hope to represent
the upper level stratiform cloud decks and associated processes a little better, and
understand the whole genesis problem somewhat more.
Maybe we could also use TEXMEX data?

I've said too much. Cheers
Noel Davidson

John McBride:

I have added another satellite image to my web-page, showing a larger area.  This is to point out the organisation of the convection north of Australia into an elongated "rain-band" of scale about 2,500 km length by about 300 km across.  Looking back at the 850 hPa flow,  (also on web-page) there is a low level jet lying along this band.

These elongated bands of cumulinimus and MCC-type systems are quite common in the region immediately south of New Guinea; and various people over the years have speculated that the geometry may be related to a chanelling of the low-level westerly monsoon current by the New Guinea highlands.

Back in the days I worked with Keenan and Holland we used to refer to them as Wilkie_Neal cloudbands, after an old Bureau Tech Note wroitten by Ray Wilkie and Bruce Neal.  During the AMEX-EMEX experiemnt of January 1987, we carried out a number of flights into these bands.  They are described in the literature by Mapes and Houze:   Quartely Journal, 1992: paper I, p.927-963.  I just now went up the library and had a very quick perusal of that paper:  The cloud band seems to be the focus of EMEX flights 8, 9 and 10.  From my very quick perusal, I gain the impression the governing winds are at 500 hPa; and that the low level winds are thought to be the result of downward transport of mid-level air by downdrafts.
This doesn't seem consistent with the much better defined jet at 850 than at 500 on the current Darwin analyses, nor  with my general synoptic experience... but perhaps I perused the paper too quickly.  I'll read it more slowly over coming days, and get back to you, if I have anything to say.

cheers

John McBride

Brian Mapes

From: Brian Mapes <bem@cdc.noaa.gov>

John McBride wrote:

> convection north of Australia into an elongated "rain-band" of scale about
> 2,500 km length by about 300 km across.

Wow, that is one wet and windy looking slab of atmosphere!
http://www.bom.gov.au/bmrc/clfor/cfstaff/jmb/02Feb13/oz_satpic.gif

> in the literature by Mapes and Houze:   Quartely Journal, 1992:
> paper I, p.927-963.  I just now went up the library and had a very quick
> perusal of that paper:  The cloud band seems to be the focus of EMEX
> flights 8, 9 and 10.  From my very quick perusal, I gain the impression
> the governing winds are at 500 hPa; and that the low level winds are
> thought to be the result of downward transport of mid-level air by
> downdrafts.  This doesn't seem consistent with the much better defined jet
> at 850 than at 500 on the current Darwin analyses, nor  with
> my general synoptic experience... but perhaps I perused the paper too
> quickly.  I'll read it more slowly over coming days, and get back to you,
> if I have anything to say.

EMEX 8,9,10 convective storms were aligned along a westerly flow channel
like this, but not so beefy and multitudinous looking on satellite!

They moved north, apparently (from dropsondes) feeding on air with
a more northerly component below about 850 hPa (Ekman/friction layer).
Occasionally a little spur of convection would protrude north of the
main
line, snag the westerlies and race downwind, laying down a new cold pool
(seen by boundary-layer aircraft). Then a new main line of convection
would develop on the north edge of this pool, and the old line would
fizzle and turn to stratiform precipitation.

I didn't mean to imply "governing winds" at 500 hPa.
There was just a nice case of pressure-enhanced downdraft momentum
transport in one aircraft transect, so I included it in the paper.

The aircraft wind maps in that paper are at ~5 km altitude because
that was the flight altitude, for radar reasons.
I think of the whole surface-550hPa or so layer being accelerated
together by the deep heating (plus Coriolis of course). The divergence
and vorticity tended to trail off quickly thru 500-400 hPa, and winds
were nibbled away & Ekman turned below about 850 hPa, so I liked
700 hPa as a core (if not necessarily "governing"!) monsoon wind level
(at least for 1987 Australia).

-Brian
************************
Brian Mapes     bem@cdc.noaa.gov
NOAA-CIRES Climate Diagnostics Center

From: "Zehr, Ray" <Zehr@CIRA.colostate.edu>
I have a question and a comment.

so does the "formation of the synoptic scale upper tropospheric warm
core"  require deep convection and/or convectively generated MCS type
stratiform precipitation.... or can in happen without that ??

....this same type thing happens over continental Africa, when there's a
low shear environment, however often there's not... so you get squall
lines and strong easterly waves de-coupled from the low levels....
RZ