What is the difference between a storm being tropical or extratropical?
Posted November 5, 2007
MIKE MOSS SAYS: Ronnie, There are significant differences in both the structure and the energy sources that power those two types of storms, although there are also storms that combine some characteristics of both types, typically known as Subtropical or Hybrid storms. 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 by 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.
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.