Inside Tropical Cyclones--Part 1: Nomenclature, Classification, and the ITCZ

The period of time from June 1st to November 30th is officially known as "Hurricane Season" for the Atlantic Basin (encompassing the Gulf of Mexico, Caribbean Sea, and Atlantic Ocean). While tropical cyclones can and have occurred outside of these calendar dates, the atmospheric conditions that are conducive for the development of these storms are usually present during the summer and fall months.

NOMENCLATURE AND CLASSIFICATION:

A cyclone is an area of low pressure in the atmosphere in which winds converge at the center and then diverge with vertical height. In the Northern Hemisphere, the flow around a cyclone is COUNTERCLOCKWISE. There are several types of cyclones we discuss meteorology ranging from mid-latitude cyclones that flow from west to east along the jet stream to mesocyclones which are in important part of rotating supercell thunderstorms and tornado development. A TROPICAL CYCLONE, however, is a cyclone that originates in the tropical regions over warm bodies of water. A tropical cyclone has a closed, low-level circulation and a warm core, meaning the tremendous of amounts of heat are released from the system. Powerful thunderstorms (deep convection) wraps around the center of circulation, and as these storms grow with height, they release incredible amounts of heat energy into the atmosphere. According to NOAA's Hurricane Research Division, the total energy generated by the convection in a hurricane 6.0 x 10^14 Watts, which is "equivalent to 200 times the world-wide electrical generating capacity." Hurricanes also produce an incredible amount of energy from the winds within the system, but this is far less than the convection generated.

Tropical cyclones are classified by different names depending on the intensity of the winds. A tropical depression is a tropical cyclone with winds <38 mph. The next level up from a tropical depression is a tropical storm with maximum sustained winds ranging from 39-73 mph. Once the maximum sustained winds reach 74 mph or higher, the tropical cyclone becomes a hurricane. When the conditions are favorable, a few hurricanes strengthen to become major hurricanes (111 wind or greater). In other parts of the world, you may hear hurricanes referred to by different names. For example, in the West Pacific Ocean, hurricanes are referred to as "typhoons". In the Indian Ocean, they are simply called "cyclones". It is VERY IMPORTANT TO REMEMBER that tropical cyclones have many other facets to them besides wind and that other impacts, such as catastrophic inland flooding and storm surge may cause just as much havoc and devastation as wind.

THE INTERTROPICAL CONVERGENCE ZONE (ITCZ)

During the Northern Hemisphere summer and fall seasons, certain conditions line up best for the tropics to become active. The most important feature to watch for genesis of tropical cyclones is the intertropical convergence zone, or ITCZ for short. The location of the ITCZ is affected very much by global circulation patterns in earth's climate system. The goal of earth's large scale circulation pattern is to redistribute energy to parts of the earth that would normally receive less incoming solar radiation from the sun.

Most of the incoming solar radiation from the sun is directed toward locations near the equator. A basic overview of Earth's circulation pattern would have a large cell in each hemisphere with air rising near the equator (where most of the incoming solar radiation is directed) and this flow being transported to the polar regions and back to equator.

This basic overview of the planetary circulation cell can be broken down into smaller cells with the Hadley Cell being one of the most important for the location of the ITCZ and tropical cyclone development. The Hadley Cells are located from ~5N/S to ~30N/S (latitude) and essentially help to focus the location of the ITCZ seasonally. Air converges and rises between the Northern and Southern Hemisphere cells. On the other end of the cells, which are closer to 30N/S latitude, the air subsides forming a region of high pressure. In the Atlantic Basin, we have an area of high pressure located ~30N called the "Bermuda High" which can steer tropical systems westward with the trade winds across the Atlantic Ocean. Seasonally, the ITCZ basically follows the location of where the most solar radiation is located. By the Summer Solstice, the sun is over the Tropic of Cancer, which is north of the equator and does not go back to being directly over the equator until the Autumnal Equinox. Therefore, during what we in the northern hemisphere call "summer" and the first half "fall", the ITCZ is located farther north than where it would be during the winter or early spring. Due to the continuous rising air in the ITCZ, this is a common region for the development of showers and storms. As these storm complexes shift north away from the equator, the Coriolis Effect becomes stronger, and these showers and storms can organize into what we know as tropical cyclones. Tropical cyclones generally form between 5N and 20N due to the need of some Coriolis Effect to develop that circulation.

The next portion of this series will cover the typical life cycle of a tropical cyclone and how to determine if the system is weakening or strengthening.