What is a Tropical Cyclone?A tropical cyclone is a low pressure system that develops over the tropical oceans of the worldTropical cyclones often move into the middle latitudes or over land areasThey are the most destructive storms on the planetTropical Cyclones (Atlantic)Tropical Disturbance/Tropical Wave – Cluster of showers and thunderstormsTropical waves also known as African Easterly WavesTropical Depression – Winds less than 39 mph (34 knots)Tropical Storm – Tropical cyclone with winds between 40 and 73 mph (34-64 knots)Hurricane – Strong tropical cyclone with winds greater than 74 mph (65 knots) and eyewall developmentHurricane season peaks in SeptemberHurricane NamesLists are used every six yearsAlternating male and female namesSeparate list for each ocean basinA storm is only named when it reaches tropical storm strengthIf a tropical cyclone caused a lot of damage or loss of life, its name is retiredExamples: Andrew, Hugo, Ike, Ivan, Katrina, Rita, WilmaHurricane IngredientsSea-surface temperatures (SSTs) greater than 26.5ºCDeep layer of warm water in upper oceanCluster of thunderstormsUsually in the form of a tropical disturbance or tropical waveMoist air in the mid-troposphereWeak vertical wind shear (<15 kts)Latitude poleward of 5ºCoriolis force is zero at equatorAllows system to rotateWarm WaterWarm water adds heat and moisture to the lower troposphereThis aids thunderstorm developmentThe layer of warm water must also be deep because hurricanes stir up the ocean as they move across itLarge waves mix warm surface water with cold deeper waterOcean temperature decreases with depthIntertropical Convergence Zone (ITCZ)NE Trade winds in Northern Hemisphere converge with SE Trade winds in Southern HemisphereBelt of thunderstorms found near equatorVertical Wind ShearVertical wind shear – The change in wind direction or wind speed with heightStrong vertical shear is usually associated with strong winds aloftStrong vertical wind shear is BAD for hurricanesRips them apartFavorable for thunderstorms, tornadoes, mid-latitude cyclonesCoriolis ForceWithout the Coriolis Force, tropical cyclones will not rotateWinds would flow directly toward the center of low pressureThe low will dissipate quicklyCoriolis effect forces winds to spiral in to center of stormAllows system to rotateTropical cyclones must be at least 50 latitude away from EquatorHurricane DevelopmentStronger winds = More heat and moisture (evaporation) transferred from ocean to atmosphereThunderstorms form near center of developing stormThunderstorms = RISING AIRAir diverges aloft from thunderstormsDivergence aloft creates low pressure at the surfaceAt surface, air flows counter-clockwise and inward toward center of lowAs air moves over warm ocean, heat and moisture transferred from ocean to airMore heat/moisture rising means stronger thunderstorms, more divergence aloft, and lower pressure at centerLower pressure means stronger PGF and stronger winds, and more heat/moisture transfer from seaAnother feedback mechanismHurricane DissipationStrong vertical wind shearRips storm apartMoves over landOcean is source of heat and moistureMountains rip storm apartMoves over cold waterLoses source of heat and moistureRemains stationary for too longUpwelling of cold water weakens stormHurricane StructureThe eye (center; lowest pressure)Clear and calm (and WARM)The eyewall (strongest pressure gradient)Strongest winds, heaviest rain, worst weatherSpiral bandsOuter portion of the stormAirflowAir spirals inward toward the eye at the surfaceRequires Coriolis forceAs air reaches the eyewall, it ascends vertically through the eyewall to the tropopauseAir cannot penetrate tropopause because it is very stableAt upper levels, most of this air spirals outward anticyclonically (clockwise in NH)Divergence aloft lower pressure at center of stormSome of this air sinks in eye (sinking air prohibits cloud formation)Conservation of Angular MomentumFrom physics:Wind velocity x radius = ConstantRadius = Distance from center of stormAs air spirals inward toward center of storm, radius decreases, so wind speed must increase!Stadium effect: Eye smallest diameter near surface yields strongest wind speeds near surfaceOpposite of mid-latitude cyclones and jet streamLeft and RightIf you are moving with the hurricane, your right is the hurricane’s rightYour left is the hurricane’s leftMap notes:The words “right” and “east” are never, ever interchangeableThe words “up” and “north” are never, ever interchangeableDestructive Forces in a HurricaneStrong windsStorm surgeInland floodingTornadoesIntense WindsWinds are strongest on RIGHT side of circulation because storm motion and rotational winds are in same directionWind speeds are additiveThey are in opposite directions on LEFT side of stormSubtractWeaker windsHurricanes: Formation and Structure 11/06/2012What is a Tropical Cyclone?-A tropical cyclone is a low pressure system that develops over the tropical oceans of the worldoTropical cyclones often move into the middle latitudes or over land areas-They are the most destructive storms on the planetTropical Cyclones (Atlantic)-Tropical Disturbance/Tropical Wave – Cluster of showers and thunderstormsoTropical waves also known as African Easterly Waves-Tropical Depression – Winds less than 39 mph (34 knots)-Tropical Storm – Tropical cyclone with winds between 40 and 73 mph (34-64 knots)-Hurricane – Strong tropical cyclone with winds greater than 74 mph (65 knots) and eyewall development-Hurricane season peaks in SeptemberHurricane Names-Lists are used every six yearsoAlternating male and female names-Separate list for each ocean basin-A storm is only named when it reaches tropical storm strength -If a tropical cyclone caused a lot of damage or loss of life, its name is retiredoExamples: Andrew, Hugo, Ike, Ivan, Katrina, Rita, WilmaHurricane Ingredients-Sea-surface temperatures (SSTs) greater than 26.5ºC-Deep layer of warm water in upper ocean-Cluster of thunderstormsoUsually in the form of a tropical disturbance or tropical wave-Moist air in the mid-troposphere-Weak vertical wind shear (<15 kts)-Latitude poleward of 5ºoCoriolis force is zero at equatoroAllows system to rotateWarm Water-Warm water adds heat and moisture to the lower troposphereoThis aids thunderstorm development-The layer of warm water must also be deep because hurricanes stir up theocean as