Classification of Tropical vs Subtropical Cyclones

[cycloneye]

GARY PADGETT'S
MONTHLY GLOBAL TROPICAL CYCLONE SUMMARY



CLASSIFICATION OF TROPICAL VS SUBTROPICAL CYCLONES


James Franklin of TPC/NHC pointed out the obvious but sometimes
troublesome fact that there are no well-defined boundaries between
cyclone types. With regard to energy sources, cyclones come in a
continuous spectrum, but forecasters (at least at NHC) have to
pigeonhole them into three categories: tropical, subtropical, or
non-tropical (extratropical). In deciding whether a particular storm
is tropical or subtropical, James indicates that he would first look
at the structure of the wind field. If the highest winds appear to
be the result of central core convection, then it would be tropical.
If they result from synoptic-scale gradients or forcing, then the
system would be subtropical. Thermal structure is also important,
but the data to definitively answer that question is often not
available. NOTE: It should be pointed out that, while in some
circles the terms subtropical and hybrid are often used synonymously,
this tends to not often be the case at NHC. Since its inception in
public warning terminology in 1972, the term "subtropical storm" has
become increasingly restricted in its application to hybrid-type
marine cyclones. In other words, a subtropical cyclone is a hybrid
between classical extratropical and tropical cyclones, but not all
hybrids are considered subtropical storms.

Jack Beven, also of NHC, states that he considers three main
characteristics in deciding whether or not a given system is tropical
or subtropical: satellite appearance, amount and behaviour of central
convection, and any available information on how frontal a system is.
He admits that all three are rather subjective quantities. Chris
Landsea of AOML/HRD feels that a tropical cyclone should be called such
when there is convection near or over the center of the system (within
about one degree of latitude/longitude), it is warm core in the lower
troposphere, is non-frontal, and has a relatively small radius of
maximum winds (smaller than about 2.5 degrees of latitude/longitude).

David Roth of HPC feels that for classification as a tropical
cyclone, a system should have no large dry slots, no cold fronts or
stratus clouds, and should have deep central convection. In David's
opinion, the (usually) small cyclones sometimes seen in the Atlantic
(and also often in the Mozambique Channel) which may contain eye
features but have shallow convection and shallow warm cores with
cold cores aloft should be classed as subtropical rather than as
tropical cyclones.

Commenting also on the topic of classification of tropical cyclones,
Rich Henning, a meteorologist at Eglin AFB and a member of the
Hurricane Hunters squadron, suggested that perhaps there should be a
sliding scale based on the latitude of the system and the time of year.
Rich writes, "For example, for a system at a high latitude in November,
there had better be a burst of deep convection at or near the center
of the vortex that creates some evidence of a warm core and the
establishment of a tighter pressure gradient near the center that can
be traced to the convective event, i.e., that can be distinguished from
the larger-scale mid-latitude cyclone gradient in which it may be
embedded. For lower latitudes and/or when formation is from July to
October, this may not be as strictly enforced, especially when cyclo-
genesis occurs over water that is warmer than or equal to 26.5 C. For
cooler water temperatures, I am always skeptical about a system in the
absence of deep, persistent convection at or near the vortex core."

Another perspective on the subject of classification of subtropical
cyclones is the issue of entering them into the official tropical
cyclone database (Best Tracks file). Rich Henning feels that
subtropical systems should be included in the Best Tracks file so that
it will contain a complete tropical cyclone climatology for research
purposes. It is generally admitted that in the Atlantic basin (and
likely in other basins also) before the satellite era, many
subtropical/hybrid systems were treated as tropical cyclones. This is
the primary reason that in the early 1970s, while not being named
operationally, subtropical storms were added to the Atlantic database
so as not to skew the climatology of tropical cyclones in years
following the introduction of the subtropical concept. David Roth
also feels that subtropical storms and subtropical portions of named
tropical cyclone tracks should be added to the Best Tracks file.



Interesting articule that I found while searching the internet about how the NHC folks decide to classifie systems as pure tropical or subtropical.Right now there is debate about the strong gale center in the North Atlantic that some people say it is at least subtropical.This may answer some of the questions that many have about how the NHC do the classification of systems.

Any comments are welcomed about this interesting theme.

[Aslkahuna]

Well, the recent Nor'easter did have a very large dry slot, a (strong) cold front, and a stratus cloud field associated with it and although it did have central convection, it didn't look all that deep to me.

Steve