How Do Hurricanes Work?
With hurricane season underway and Arthur currently churning off the Southeast coast (as of July 2, 2014), now is the time to get prepared in the event you find yourself in the path of one of these storms. Although most of us understand what a hurricane is, specific details related as to how they develop are sometimes not as clear. Let's investigate the basics of hurricane formation and find out how these storms differ from typical mid-latitude systems.
Technically speaking, a tropical cyclone is defined as a warm-core, non-frontal system that originates over tropical waters, with organized deep convection and a closed surface wind circulation about a well-defined center. Once fully developed, a tropical cyclone maintains its strength by the extraction of heat energy from the ocean. This differs considerably from the majority of other disturbances commonly seen across the mid-latitudes, which derive their energy from temperature and moisture contrasts across the atmosphere.
There are two main schools of thought as to how tropical disturbances develop into organized systems such as tropical storms or hurricanes. These are outlined below.
The first theory of tropical development is known as CISK (aka Convective Instability of the Second Kind). In this formation process, an initial low-level disturbance is needed, such as a tropical wave or frontal low. Once a surface area of low pressure is established, air rises and in turn makes the atmosphere surrounding the disturbance more unstable. As the instability increases, air rises faster and results in the continued development of clouds and eventually thunderstorms. The end result is a positive feedback process whereby clouds and thunderstorms continue to develop as long as the ocean temperature remains warm enough for sufficient heat release to occur. This process is outlined below.
The second theory for development is known as WISHE (Wind Induced Surface Heat Exchange) and differs from CISK in the fact that initial instability is not needed. Instead, evaporation from the ocean, enhanced by wind, provides the instability. As long as there is a preexisting disturbance, such as an area of low pressure, clouds form and heat is released into the atmosphere. As the area of low pressure strengthens, convergence at the surface is enhanced, which results in faster wind and more evaporation. Similar to CISK, a positive feedback process is established whereby the cyclone will continue to strengthen depending on the ocean temperature. An idealized schematic of the WISHE theory is depicted below.
No matter which method you choose, the end result is still the same. In both theories, the circulation of the tropical cyclone is aided by heat released from the ocean. The main difference is that in WISHE surface wind is the driving mechanism, whereas convection itself is the driving mechanism in CISK.
**Graphics are courtesy of UCAR's COMET Program, which works to further the education of meteorologists online. Find out more about this free program here: http://comet.ucar.edu/