Posted May 15, 2007
Updated May 16, 2007
No, of course not. The image you see of last week's "hybrid" storm off the east coast, the first named storm in the Atlantic for 2007, is of Subtropical Storm Andrea. However, if it had formed in 1972, it may well have been publicly designated as a "neutercane," terminology for storms that show characteristics of both extratropical and tropical cyclones. Now they are referred to officially as Subtropical Storms and more informally as hybrids. Most people have long fogotten them, but the National Hurricane Center actually issued advisories on Neutercanes Bravo (which later became fully tropical and evolved into Hurricane Betty) and Charlie that year, at a time when hurricances were named from the standard list of tropical cyclone names and neutercanes were to be named by military phonetic alphabet. Apparently, the term neutercane proved very unpopular, in a hurry, and was dropped from use the following year. From that time until 2002, subtropical storms went unamed unless they progressed to a fully tropical structure and picked up a name as a tropical storm or hurricane. Beginning in 2002, however, the National Hurricane Center began applying names from the seasonal list of tropical cyclone names to subtropical systems as well. Therefore, now that we've had Andrea this year, the next storm with some tropical characteristics, whether it is classed as tropical or subtropical, will be named Barry, and so on.
What exactly is a hybrid, or subtropical storm? The simple answer is a storm that combines some of the defining features of the storm types we usually classify as extratropical or tropical. Briefly, an extratropical storm, also called a "midlatitude" or sometimes "baroclinic" low pressure system, derives it's kinetic energy from the potential energy associated with strong horizontal temperature contrasts within the atmosphere. In practice, this means that significant extratropical lows are usually located along a frontal zone. In our general vicinity, this usually involves notably colder and often less humid air to the north and west of the low center, and vice versa to the south and southeast. These lows are characterized by wind fields that feature the greatest speeds at a significant altitude (up near jet stream level), involve significant vertical wind shear, and they are "cold core" through a great depth of the atmosphere, meaning that the temperature near the center of circulation is lower than at similar altitudes as you move away from the system. Finally, the strongest surface winds associated with mature midlatitude systems are often either located at a sizable distance from the center or spread over a large area surrounding the low.
By contrast, classically "tropical" systems have a distinct life cycle, organizational structure and energy source. They tend to form in areas with little or no horizontal temperature gradient and light vertical wind shear, and they draw their kinetic energy principally from buoyancy associated with the release of latent heat provided the convective condensation of water vapor that evaporates from warm (usually around 80 degrees or higher) water surfaces. Once they have become organized, they are marked by strong winds that are most intense near the surface within a fairly tight band around 10-50 miles from the center (within the "eyewall") and feature a "warm core" in which temperatures near the center of the storm are warmer than surrounding areas at the same height.
Subtropical storms combine some characteristics of both extremes to a significant degree. As with many natural phenomena, there is some artificiality to the way we classify them, as many storms at some point in their life cycle may have some of these characteristics on a continuum that may not fit so neatly into one of three specific "boxes" for classification. Already, there are sub-types of extratropical systems and likewise "type a" and "type b" subtropicals, and so on. In general, though, subtropical systems have weakened horizontal temperature contrasts, but enough to still contribute to the energy of the system, and also begin to develop convection near the inner portion of the storm due to moving across warm surface waters. They may have large and fairly intense surface wind fields, and they begin to develop a "warm core" in the lower atmosphere, while often remaining "cold core" at higher altitudes. This "cold over warm" center is partly responsible for the idea in the early 70s that the system was becoming "neutral," and hence the neutercane.
If a subtropical storm detaches completely from strong upper level flow and its associated wind shear, and the system moves over sufficiently warm surface waters for a long enough, it may transition completely to tropical cyclone status. On the other hand, if it is approached by a new upper level disturbance or merges with an advancing frontal system, it may transition back to fully extratropical nature. In the case of Andrea, it came fairly close to making a tropical transition, but water temperatures beneath the storm were not quite warm enough relative to temperatures aloft, and then later in its life cycle it did merge with a cold front that crossed through our state during the weekend, at which point it became more extratropical again and sped off into the Atlantic.
Historically, around three to four subtropical cyclones develop in the North Atlantic each year, and their formation peaks around May and again during later September and October. In many instances they have gone unnamed, and/or simply been considered intense "Nor'easters," but it seems likely we will see more of them as named storms now that the Hurricane Center has begun naming them in the same manner as tropical systems.
This is a very brief overview of the subtropical storms. Those of you interested in more details can find them in several good references online. Here are some suggestions:
http://www.aoml.noaa.gov/hrd/tcfaq/A7.html (this page has a nice graphic comparing extratropical versus tropical storm structure)
http://www.hpc.ncep.noaa.gov/research/roth/Subpreprint.pdf (this is a brief, informative paper called "A Fifty Year History of Subtropical Cyclones")