sub tropical / extra tropical

[Ed Schoenborn]

What is the difference between storms that are sub tropical and those that are extra tropical? Ed

 

[John Massura]

Sub-tropical storms are storms that have both tropical, and non-tropical characteristics. For instance, the storm might have convection more typical of a tropical cyclone, but be cold core (relatively cold air aloft, as opposed to warm air of normal tropical cyclones). They are relatively common in October/November in the North Atlantic (maybe 1 every other year, I'm guessing). They are also fairly common in the eastern North Pacific, north and east of Hawaii, although they are almost never officially warned on. They are also rarely encountered in the Mediterranean Sea, and possibly other ocean basins (though I can't recall seeing any in other places, except for the hurricane in the South Atlantic in March 2004, which started off subtropical). I'm sure others would have more detailed information.

Extratropical cyclones are storms that have transitioned into regular mid-latitude cyclones, with a frontal system, and also get their energy from temperature differential (i.e. jet stream), as opposed to latent heating with normal tropical cyclones.

 

[Jack Beven]

What is the difference between storms that are sub tropical and those that are extra tropical? Ed

I’ll take a shot at answering Ed’s question.

First, some background. There are three major sources of energy for synoptic-scale atmospheric cyclones: 1. Baroclinic energy from temperature gradients and vertical wind shear, 2. Diabatic energy, usually released as latent heat by convection, and 3. Barotropic energy from horizontal wind shear. The first powers the familiar mid-latitude storm systems we chase across the Plains, the second powers tropical cyclones, and the third powers the monsoon cyclones of the eastern hemisphere. These energy sources give the cyclone their characteristic structures – the tight wind fields and warm core of a tropical cyclone or the broader wind fields and cold core of a mid-latitude frontal low (more on this later).

Now, these energy sources are not mutually exclusive – more than one can be working in a cyclone at any given time. This can cause some issues with identifying the type of cyclone you might be dealing with operationally – with the associated issues on how they are warned on.

I don’t remember the first paper that explicitly stated that a tropical cyclone could merge with a baroclinic zone to lose its tropical characteristics – to become extratropical. It may have been C. H. Pierce’s paper on the 1938 New England hurricane in the Monthly Weather Review. However, the recognition of the process of extratropical transition meant that cyclone types and energetics were not fixed – they could change during the life of the cyclone.

A complication came when forecasters in the 1960’s (notably Paul Hebert) saw cyclones that seemed to simultaneously have characteristics of both tropical and non-tropical cyclones – deriving some energy from air-mass temperature contrast and some energy from organized convection. Some of these cyclones would evolve into full-blown tropical cyclones, while others would retain the mixed characteristics through their life. He noted that these systems often had a non-tropical origin – forming along frontal systems or under upper-level lows and troughs. He dubbed these "subtropical cyclones" in a paper published in the Mariners Weather Log in 1973, and would later develop a satellite-based intensity scheme for estimating their intensity.

Another complication came from studies of extratropical bombs – rapidly intensifying non-tropical cyclones (i. e. the Project ERICA ultrabomb talked about at http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=304&tstamp=200602). The studies revealed that these systems could develop a warm core, which was though to be the signature of a tropical cyclone. The extratropical warm core, though, forms from a seclusion process – a contortion of the associated frontal systems that traps a pocket of warm air near the cyclone center. This contrasts with the diabatic nature of the tropical cyclone warm core.

I was very interested in these in-between cyclones even before I started working at the National Hurricane Center, and once there I got to observe several very interesting systems in real time. These included: 1. The Mediterranean "Hurricane" of January 1995, which looked like a hurricane in a most non-tropical environment, 2. The hybrid coastal storm of late December 1994, which developed organized convection but may have never lost frontal characteristics, and 3. A strange system in the Gulf of Mexico in late September and early October 1994, which had the thermal structure of a tropical cyclone and the wind distribution of an extratropical low. The latter system is still the strangest cyclone I’ve studied, and after 14 years I’m still not sure what made it tick.

After struggling with these systems and others, I figured out that there was a need to revise how cyclones were classified. So, to replace the ‘tropical-subtropical-extratropical’ characterization, I came up with this two-dimensional classification diagram, where the axes where the core temperature and the frontal nature:

This was part of a talk I gave at the tropical meteorology conference in Ft. Collins, Colorado in 1997. A couple of attendees – Jenni Evans (professor at Penn State) and her student Bob Hart (now professor at Florida State) heard it, and they decided to develop a methodology to formalize cyclone classification. This eventually became the Cyclone Phase Space (http://moe.met.fsu.edu/cyclonephase/), which used the analyzed and forecast fields from numerical weather prediction models to determine the current and future state of the cyclone. The CPS has become a very important tool in National Hurricane Center operations, helping us to analyze the shades of gray in the cyclone types. It helps to determine the potential and timing for extratropical transition, tropical transition (a term coined by Lance Bosart and colleagues), and the formation of subtropical cyclones.

There is another tool used to help evaluate the cyclone thermal structure – the Advanced Microwave Sounder Unit (AMSU) on the NOAA polar-orbiting satellites (http://amsu.ssec.wisc.edu/). This instrument can depict the vertical and horizontal temperature distribution of the cyclones and aid evaluation of what phase they are in.

Now, the formal definitions Ed is asking for are available on our glossary page at http://www.nhc.noaa.gov/aboutgloss.shtml. However, these don’t do full justice to complexities of cyclone classifications, particularly for the hybrid systems. An example that highlights this is the recent cyclone near the coast of the Carolinas. It was certainly evolving toward a subtropical or tropical cyclone, with the formation of central convection and possibly an inner wind core. However, the abundant surface observations suggest that the frontal structures didn’t dissipate before landfall. If that’s correct, it would put this cyclone in the class I call "frontal hybrid" – drawing energy from both diabatic and baroclinic sources, but too frontal for the National Hurricane Center to start advisories. Our definitions specify non-frontal cyclones.

I’m sorry this is so long, but I hope it helps Ed and others understand how complex cyclone classification can be. Nature doesn’t feel a need to fit our classification schemes - either cyclone or supercell – and the associated operational decisions are often far from clear cut.

Jack Beven
National Hurricane Center