TC Group,

    I have been assigned the task of writing a rapporteur report on
"Formation definitions" for an upcoming Tropical cyclone conference. The
latest events in the western Pacific play nicely into the problems of my
topic.  The large monsoon depression in the western north Pacific now has
a min SLP near 1000 mb, a huge circulation with winds to near gale force
on the south side, and all numerical guidance going for intensification.

    Historically, these monsoon depression type cyclones have not been
considered to be "tropical cyclones".  There are currently no TC
advisories on this system or its southern twin.  The irony of all this is
that if a small shear pattern type TC formed that could receive a Dvorak T
number indicating 30 or 35 kts (even if only over an area the size of
Rhode Island -- my own very small hometown state), it would be blessed
with TC advisories. Yet, a monster monsoon depressions with wind gusts of
over 40 kts at several Micronesian islands, and rough seas for a thousand
miles gets nothing.  As I wrote yesterday, my plane almost got thrown
sideways off the runway at Kosrae from westerly gusts to gale force.
    Question:  Would you consider this to be a TC now, or if not, why?,
and, when does it become a TC?  I am searching for some consensus on this
for my rapporteur report to the WMO conference.

    Regards,  Mark Lander

Chris Fogarty

Something similar in the Atlantic would be a sub-tropical depression.  The circulation at
first is often very large - almost like a gyre with convection oriented with the
streamlines around it in the mid-levels.  There may be gales at large radii...but we do
not designate these as strictly TCs because the system lacks that definitive warm-core at
that stage.  A warm core owing to convection near/over the surface vortex and a smaller
radius of 35-kts winds roughly co-located with the convection might be a nice way to tell
your sub-tropical depression (or monsoon depression?) is worthy of TC status.
 
With the advent of satellite data, I think we're on our way to formulating an objective
scheme for tropical cyclogenesis (from initially large depressions) and vice versa...the
transition from tropical to extratropical.
 
I think the key factors are the mid-trop warm temp anomaly (e.g. AMSU) and radius of
maximum winds (e.g. QuikSCAT) to help decide classification.
 
Chris
-----------------------------------------------------------------------
Chris Fogarty
Research Meteorologist
Dalhouse University / Maritimes Weather Center

Jon Gill

Hi Mark,

I think we've all been around this issue for long enough to know that there is no 'correct'
answer to this that fits all circumstances. For some it is a TC on the basis of a definition that stipulates 'of tropical
origin' and 'attendant gales associated with the low'. For others, it is not a TC because it does not possess 'gales in all quadrants' or perhaps it fails to have a 'warm-core structure' or the winds are not 'in a tight belt close to the system centre'.

etc

The way to address this problem I feel is to state, a priori, what the PURPOSE of the definition is, then pose the definition, and then adjudge accordingly.

So, for example...

Circumstance 1:
Let us say the purpose is the protection of life and property from gale-force (or stronger) winds associated with tropical low pressure systems. Then we construct an appropriate, relatively non-restrictive definition, e.g. "A TC is a low pressure system of tropical origin with associated winds of at least gale force". On this basis, the system you are looking at is a TC.
-or-

Circumstance 2:
Let us say the primary purpose is to monitor TC climate change and you wish to use an internally consistent database that contains only those systems that possess the stringent scientific/meteorological attributes that have
traditionally been ascribed to TCs, i.e. tropical origin, warm core, cental convection, gales in all quadrants, etc. On this basis,
the system you are looking is not a TC.

The first *might* be labelled an operational, user-focussed definition. The second *might* be labelled a traditional, scientific
definition. Of course, they are not always mutually exclusive - in fact, all systems that fit the second definition will fit the
first.

There is no single definition that suits everyone, because not everyone has the same goals.
Which is why there is the interminable debate. By stating the purpose of the defintion (i.e. the outcome you are trying to achieve), then you can focus the debate and get concensus within each of the definitions that you construct.

For your report, I suggest posing a few useful definitions, and then go from there. You could use the current case as an example that does not fit all (equally valid for their purpose) definitions. Flowing from this would then be a discussion of the problems that arise when a one-size-fits-all definition is imposed e.g. community not warned of gales,
corruption of historical TC databases,
etc

Cheers...Jon

James Franklin

Mark,

For the record, here is TPC's operational definition of a tropical cyclone (from the Glossary on our web page), and for comparison, subtropical cyclone:

Tropical Cyclone:  A warm-core non-frontal synoptic-scale cyclone, originating over tropical or subtropical waters, with organized deep convection and a closed surface wind circulation about a well-defined center. Once formed, a tropical cyclone is maintained by the extraction of heat energy from the ocean at high temperature and heat export at the low
temperatures of the upper troposphere. In this they differ from extratropical cyclones, which derive their energy from horizontal temperature contrasts in the atmosphere (baroclinic effects).

Subtropical cyclone:  A non-frontal low pressure system that has characteristics of both tropical and extratropical cyclones.
The most common type is an upper-level cold low with circulation extending to the surface layer and maximum sustained winds generally occurring at a radius of about 100 miles or more from the center. In comparison to tropical
cyclones, such systems have a relatively broad zone of maximum winds that is located farther from the center, and typically have a less symmetric wind field and distribution of convection.
A second type of subtropical cyclone is a mesoscale low originating in or near a frontolyzing zone of horizontal wind shear, with radius of maximum sustained winds generally less than 30 miles. The entire circulation may initially have a diameter of less than 100 miles. These generally short-lived systems may be either cold core or warm core.

It's hard for me to tell from the image, but your system appears to lack what we would consider to be organized deep convection, and probably lacks a "well-defined" circulation center and warm core as well (I confess ignorance
of the thermal structure of monsoon depressions).

Something with gales that is not a TC in the Atlantic would not get "nothing", but would be warned on in High Seas products.  TC advisories would be reserved for TCs.  Naturally, these definitions may be bent when we feel it is appropriate to do so.
 

Regards,

James
-----------------------------------------------------------
James L. Franklin

Hurricane Specialist
National Hurricane Center

Alipate Waqaicelua
Mark,

Bula!

The WMO Regional Association V Tropical Cyclone Committee for the SW Pacific
approved in its meeting held in Manila, Philippines, in May 2002, that
a TROPICAL CYCLONE is defined as A non-frontal cyclone of synoptic scale
developing over tropical waters and having a definite organised wind
circulation with a maximum 10-minute average wind speeds of 34 knots (63 km per hour) or
greater near the centre.

The criteria for naming is thus :
A tropical depression will be named as a tropical cyclone whenever
observations and/or Dvorak intensity analysis indicate the presence of gale force winds
near the center that are likely to continue.

Best Regards,

Alipate Waqaicelua
RSMC Nadi
Fiji

Mark Lander
Date: Tue, 8 Oct 2002 11:26:51 +1000 (GMT+1000)
From: Mark A. Lander
Subject: [Tropical-storms] Is this a TC yet?
--

TC Group,

    Yesterday's monsoon depression in the western North Pacific continues to strengthen.  It has not yet been deemed to be a TC by any warning agency, yet the winds at Kosrae (6N 162E) have now increased to sustained gales (see attached pic), and minimum sea level pressure at Pohnpei (7N 158E) is less than 1002 mb.

    I have been tasked to write a report on "formation definitions" to the upcoming IWTC workshop, and I am very interested in any opinions as to when this tropical cyclonic vortex becomes a "tropical cyclone'.  What does it take?  What threshold of structure must it acquire?  It already  has gales, so it would have to be an instant tropical storm if warned on.
Or are the gales in the SE quadrant too far from the center?  How close to the center do they have to be?
    To those in other basins, I would like to add that many of the  typhoons of the western North Pacific begin life this way (i.e., as monsoon depressions at low latitude that slowly consolidate).  Perhaps TC formation criteria has been Atlanto-centrized.  If TC formation criteria had been derived historically from the western North Pacific, the monsoon
depression would be regarded as a common incipient stage to the development of mature TCs, and the advisory threshold would include this stage.

    Regards,  Mark Lander

 Noel Davidson
G'day Folks,

Thought it would be interesting to look at the vertical structure of the MD that Mark has been
discussing. Attached is a N-S section of zonal wind averaged over a 5deg longitude span thru' the centre. This is from
a 12 hour forecast by our tropical prediction system (37km horiz resolution) valid 0000UTC 8 October.

Interesting points are :
The cyclone pair on either side of the equator - Is the SHEM circulation also a TC? I'm sure the ER people will be getting
excited if they are not already.
The depth of the circulations.
The easterly max wind in the boundary layer for both circulations.
The TUTT-type structure to the north of the MD.
The jet to the south of the SHEM MD.

    -----------   Really quite beautiful.

I am not really an advocate of circulations being called "tropical cyclones" when they don't possess an inner core
cloud structure. But I am slowly coming around. Could there be an inner wind max in there, in the mostly
clear area and maybe slightly above the surface?

Noel Davidson