|
Hurricane Science |
|
What is a hurricane? Written by Lisa Tacoronte Formation of a Tropical Cyclone Tropical Cyclone A tropical cyclone is a warm-core, low pressure system with an organized circulation that forms over tropical or subtropical waters. In the Atlantic/East Pacific Oceans, it is called a hurricane. In the West Pacific Ocean, it is a typhoon. In the Indian Ocean, it is known as a cyclone. [National Weather Service. (Last mod. 02-Sep 2006). National Hurricane Center. Retrieved September 11, 2006, from: http://www.nhc.noaa.gov/] |
|
Conditions to Form There must exists several favorable conditions for a tropical cyclone to form: ▪ Warm ocean water (at least 80˚F/ 27˚C) through a depth of about 150 feet ▪ An atmosphere that cools fast enough with ascension that it is potentially unstable for moist convection. ▪ Moist air near mid-level troposphere ▪ A minimum distance from the equator of about 300 miles (but not within 5˚ latitude) ▪ A disturbance near the surface Low vertical wind shear between surface and troposphere (of less than 23mph). Note: Wind shear is the change in wind speed with height. Additionally, a tropical cyclone must be close to the equator to form, but, except rare cases, do not form within 5˚ latitude of the equator because of the lack of Coriolis Force. This force is what causes the cyclone to spin. The direction it spins depends on the hemisphere it which it is located. In the Northern hemisphere, it spins counter-clockwise; in the Southern hemisphere it spins clockwise. (National Weather Service. [Last mod. 02-Sep 2006). National Hurricane Center. Retrieved September 11, 2006, from: http://www.nhc.noaa.gov/] Seven Regions of Formation Storms form within seven regions of the earth: ▪ Atlantic basin ▪ Northeast Pacific Basin ▪ Northwest Pacific Basin ▪ North Indian Basin ▪ Southwest Indian Basin ▪ Southeast Indian/Australian basin Australian/Southwest Pacific Basin Our interest lies in the Atlantic Basin, which includes the North Atlantic Ocean, the Gulf of Mexico, and the Caribbean Sea. Hurricane season in this region “officially” begins June 1 and last until November 20. The season peak is in mid-September. [National Weather Service. (Last mod. 02-Sep 2006). National Hurricane Center. Retrieved September 11, 2006, from: http://www.nhc.noaa.gov/]
Seven Regions of Tropical Cyclone Formation [National Weather Service. (Last mod. 02-Sep 2006). National Hurricane Center. Retrieved September 11, 2006, from: http://www.nhc.noaa.gov/] Disturbances (Causes) Several disturbances work to trigger the formation of a tropical cyclone. Tropical Cyclone Disturbances
Strength of a Cyclone Once a tropical cyclone system with organized circulation forms, it can become more organized if it stays over warm waters and upper level winds (wind shear) remain weak. Key to its power is warm water and war moist air. A storm is propagated when water vapor cools and rises. It condenses into clouds, releasing heat which warms the surrounding atmosphere. The air becomes lighter and continues to rise and more air rushes in its place, which are the strong winds we feel from storms. When the eye of the storm moves over land, it lacks the moisture and heat that power the storm and convection. It weakens and eventually diminishes. [National Weather Service. (Last mod. 02-Sep 2006). National Hurricane Center. Retrieved September 11, 2006, from: http://www.nhc.noaa.gov/]
*Convection that Powers Tropical Cyclones [National Weather Service. (Last mod. 02-Sep 2006). National Hurricane Center. Retrieved September 11, 2006, from: http://www.nhc.noaa.gov/] Why do Hurricanes form? Why is it important that they form? The tropical cyclones role is to take heat stored in the ocean and transfer it to the upper atmosphere where upper level winds carry the heat to the poles. This is important because its is what keeps the Polar Regions from being cold as can be and keeping tropics from overheating. Tropical cyclones help regulate the earths temperature. Therefore, it would not be wise to disrupt a cyclone’s intensity because oceans will retain that heat. Over time, a heat build-up could enhance successive storm and increase their frequency and intensity. [National Weather Service. (Last mod. 02-Sep 2006). National Hurricane Center. Retrieved September 11, 2006, from: http://www.nhc.noaa.gov/] Tropical Cyclone Structure “Imagine standing at the center of a Roman coliseum that is 20 miles across, with walls that soar 10 miles into the sky, towering walls with cascades of ice crystals falling along its brilliantly white surface. That's what it's like to stand in the eye of a hurricane.” Kerry Emanuel in Divine Wind Emanuel K. (2005). Divine Wind: The History and Science of Hurricanes. (New York, NY) Oxford University Press, Inc. Main Parts of a Hurricane The main parts of a tropical cyclone include the rainbands, the eye and the eyewall. A cyclone is basically air that spirals to the center, up through the eyewall, and out the top in the opposite direction. In the center, air also sinks in attempt to fill the empty space that consists of the eye. The eye is a cloud-free, calm, clear region with light winds (no more than 15mph). It usually forms when the storm reaches 74mph winds or more. The eye is formed by a combination of angular momentum and centrifugal force. Because of conservation of momentum, as air approaches the center, its speed increases. The increased speed of spiraling air creates a centrifugalforce that wants to pull it outward, thus forming an clear eye with strong winds in the eyewall. Rising eye in the eye create the eyewalls. [National Weather Service. (Last mod. 02-Sep 2006). National Hurricane Center. Retrieved September 11, 2006, from: http://www.nhc.noaa.gov/] The eyewall consists of bands of clouds, strong winds and heavy rains. It is the strongest of the storm with rings of tall thunderstorms. The rainbands, or feederbands, are bands of showers, gusty winds, and often heavy rain with gaps into bands. They become more pronounced as a storm intensifies. These bands are curved and trail away for the center in a spiral-like fashion. Rainbands often include bursts of rain and wind and tornadoes. [Accuweather.com Hurricane Center. (Copyright 2006). Hurricane Facts. Retrieved September 22, 2006, from: http://hurricane.accuweather.com/hurricane/facts.asp] [National Weather Service. (Last mod. 02-Sep 2006). National Hurricane Center. Retrieved September 11, 2006, from: http://www.nhc.noaa.gov/]
Structure of a Tropical Cyclone [National Weather Service. (Last mod. 02-Sep 2006). National Hurricane Center. Retrieved September 11, 2006, from: http://www.nhc.noaa.gov/] The size of a hurricane averages near 300 miles wide with great variations. The size of the storm is not an indication of the hurricane’s intensity. Hurricane force winds are felt out to about 150 miles from the center. Tropical-force winds extend out 300 miles from the center. Tropical Cyclone Classification A tropical cyclone is classified according to wind speed values. A system with organized circulation with wind speeds less than or equal to 38 mph is called a tropical depression. Once it reached 39 mph, it is called tropical storm and given a name. Wind speeds of 74mph or higher is classified as a hurricane.
Tropical Depression Tropical Storm Hurricane For all three: [Accuweather.com Hurricane Center. (Copyright 2006). Hurricane Facts. Retrieved September 22, 2006, from: http://hurricane.accuweather.com/hurricane/facts.asp] The name hurricane comes from the Carib Indians of the West Indies who called the storm hurcan it is said that Taino trabis of Central America caleled their god of evil “Huracan.” The Spanish colonists modified the word to hurricane. Hurricanes are classified according to the Saffir-Simpson scale (Category 1-5) based on the wind speeds of the hurricanes. The scale estimated the potential property damage and flooding along the coast from hurricane landfall. Storm surge values are highly dependent on the slope of the continental shelf in the landfall region.
[National Weather Service. (Last mod. 02-Sep 2006). National Hurricane Center. Retrieved September 11, 2006, from: http://www.nhc.noaa.gov/] Hurricane Names Once a tropical cyclone is considered a hurricane, it is given a name. There are six lists of name that rotate every hurricane season. There are no names that begin with the letter Q, U, X, Y, and Z because of the scarcity of the names. Once the list of names is used up for the season, the Greek alphabet is used. The names of major land falling storms that may have a major economic impact are retired. Examples include Katrina, Rita, Wilma, Andrew and Camille. Hurricane Names
[National Weather Service. (Last mod. 02-Sep 2006). National Hurricane Center. Retrieved September 11, 2006, from: http://www.nhc.noaa.gov/] Hurricane Hazards Hurricanes cause several hazards, directly and indirectly. They cause storm surge, wind and squalls, inlands floods, flash floods, urban/area floods, river floods, and tornadoes. Storm surge is basically water pushed towards the shore by the force of winds swirling around the storm. Gusts are short but rapid bursts of wind speed in winds. Squalls, often associated with bands of thunder storms which make up the spiral bands of a storm, are longer period of increased wind speeds. A weak slow moving hurricane can cause more damage due to flooding than a more powerful, fast-moving hurricane. Tornadoes usually form in the right-front quadrant, relative to the storms motion, or can be found embedded in rainbands. [National Weather Service. (Last mod. 02-Sep 2006). National Hurricane Center. Retrieved September 11, 2006, from: http://www.nhc.noaa.gov/] Other Environmental Disturbances that Affect Hurricanes El Niño/Southern Oscillation (ENSO) ENSO refers to the warming of ocean waters along the equator in the Eastern Pacific. It results in more rainfall in the Eastern Pacific while the West Pacific is relatively dry. Normally, however, sea surface temperatures are 14˚F higher in the Western Pacific, with cool temperature off of South America due to upwellings of cold, nutrient-rich water from the deeper level of water off the northwest coast of South America.
[National Weather Service. (Last mod. 02-Sep 2006). National Hurricane Center. Retrieved September 11, 2006, from: http://www.nhc.noaa.gov/] During El Niño years, trade winds decrease in the central and western Pacific which leads to a reduced upwelling cooler water. This cuts of the e supply of nutrient-rich water. Consequently, there is a rise in sea surface temperature and a large decline in the fishing industry. El Niño has a significant impact on tropical cyclones in the North Atlantic. During El Niño years there is a: ▪ Reduction in the number of cyclones ▪ 60% reduction in the number of hurricane days Overall reduction in the system intensity This occurs because of stronger than normal westerly winds that develop in the western North Atlantic and Caribbean during El Niño. [National Weather Service. (Last mod. 02-Sep 2006). National Hurricane Center. Retrieved September 11, 2006, from: http://www.nhc.noaa.gov/] Global Warming Global warming is defined as the progressive gradual rise of the earth's surface temperature thought to be caused by the greenhouse effect and responsible for changes in global climate patterns. It results in an increase in the near surface temperature of the Earth and has occurred in the distant past as the result of natural influences, but the term is most often used to refer to the warming predicted to occur as a result of increased emissions of greenhouse gases. [Weather and Climate Glossary. (2006) Netcent Communications Weather Guide. Retrieved November 20, 2006, from: weather.ncbuy.com/glossary.html] Many scientists recently believe that hurricanes are becoming more intense because of rising sea surface temperature. Leading hurricane authority Kerry Emanuel explains that an upswing in the tropical North Atlantic sea surface temperature, possibly due to increasing anthropogenic greenhouse gases, increase the energy available to tropical cyclones. It is possible and likely that global warming could increase the intensity of hurricanes in the time to come. How global warming affects hurricane frequency is still uncertain. [Emanuel K. (January 2006). Anthropogenic Effects on Tropical Cyclone Activity. Kerry Emanuel’s Home Page. Retrieved November 20, 2006, from: http://wind.mit.edu/~emanuel/anthro2.htm] |
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
Questions or Comments? Email them to neworleans1@mit.edu |