G109: Weather and ClimateHurricanesHurricanesTopics1. Characteristics2. Location3. Structure4. Developmenta. Tropical Disturbanceb. Tropical Depressionc. Tropical StormReadings: A&B Ch. 12d. Hurricanee. Influencesf. Pathg. Dissipation5. Destruction & Damage6. Detection & Tracking7. Saffir-Simpson ScaleG109: Weather and Climate 13: HurricanesCharacteristics of a HurricaneCharacteristics of a Hurricane•Hurricanes: Also called typhoons (in western Pacific) or cyclones (in Australia and Indian Ocean) Form over tropical oceans Have rotary circulation• Low pressure system• Counter-clockwise in northern hemisphere Have sustained wind speeds > 120 km/h• Sustained winds up to 350 km/h Diameter: 100-1500 km across (average 600 km) Pressure at center: ~870-990 mb (average 950 mb)G109: Weather and Climate 13: HurricanesHurricanes Around the WorldHurricanes Around the WorldG109: Weather and Climate 13: HurricanesStructure of HurricaneStructure of Hurricane• Eye: center of storm; ~30km in diameter Lowest pressure Warmest part of storm Air descends gradually, heating up by compression Precipitation ceases ~30 km in diameter When eye is shrinking, hurricane is intensifying• Eye wall: ~10-20 km from center of eye Greatest winds Thickest cloud Heaviest rainG109: Weather and Climate 13: HurricanesStructure of HurricaneStructure of Hurricane• Formed from large number of thunderstorms arranged in a pinwheel formation Bands of heavy convection and precipitation separated by areas of weaker uplift and less precipitation Below ~7.5 km: inward spiraling motion around low pressure Above ~7.5 km: cirrus clouds spiral outward around high pressure centerG109: Weather and Climate 13: HurricanesStructure of HurricaneStructure of Hurricane• Hurricanes are “warm-core cyclones” Unlike mid-latitude cyclones, temperature increases toward center of storm Core is kept warm by release of latent heatG109: Weather and Climate 13: HurricanesStructure of a HurricaneStructure of a Hurricane• In center: centrifugal force is very strong Divergence and sinking air Mostly clear skies in the eye of the hurricane• Near the eye wall Air rises, cools and releases latent heat of vaporization• Latent heat is the primary source of energy for the development of a hurricane Requires ocean water > 27°C Ocean temperature influences• Length and activity in a hurricane season• Location of hurricanesG109: Weather and Climate 13: HurricanesDevelopment of HurricaneDevelopment of Hurricane• Tropical Disturbance: a disorganized group of thunderstorms over the eastern part of a tropical ocean basin In the Atlantic basin, most originate as easterly waves – undulations in the trade wind patternG109: Weather and Climate 13: HurricanesDevelopment of HurricaneDevelopment of Hurricane• Tropical Disturbances ~90% die out before becoming organized, as they move over cold Canary current along western Africa ~10% continue westward and organize: pressure decreases and cyclonic rotation develops →Tropical Depression• Tropical Depression: classified if the low pressure has at least one closed isobar Wind speeds < 60 km/hG109: Weather and Climate 13: HurricanesDevelopment of HurricaneDevelopment of Hurricane• Tropical Storm: develops if a tropical depression intensifies Wind speeds > 60 km/h At this point, the system is named ~80-100 tropical storms per year in world oceans ~10 tropical storms per year in the Atlantic Ocean• Hurricane: develops from tropical storm Wind speeds > 120 km/h ~40 hurricanes per year in the world oceans ~6 hurricanes per year in the AtlanticG109: Weather and Climate 13: HurricanesInfluences on Hurricane DevelopmentInfluences on Hurricane Development• Conditions necessary for hurricane development Ocean surface must be > 27°C, to provide enough latent heat• No hurricanes form pole-ward of 20° Coriolis force must be sufficiently strong to prevent low pressure center from filling • No hurricanes form equator- ward of 5° Unstable conditions through entire depth of troposphere• No hurricanes form in E. Atlantic over cold current• Hurricanes develop between 5-20° (N/S) over westernmost portion of warm oceansG109: Weather and Climate 13: HurricanesHurricane PathHurricane Path• Hurricanes and tropical storms are highly erratic in movement Changing speed or direction, occasionally back-tracking• Hurricane path is influenced by Hadley cell: easterly trade winds in tropics Westerly winds in mid-latitudes Warm ocean currentsG109: Weather and Climate 13: HurricanesHurricane DissipationHurricane Dissipation• A hurricane will dissipate: When it moves over land• Loses source of moisture and latent heat• Rougher surface causes a reduction in wind speed → pressure gradient force dominates Low pressure center fills When it moves further pole-ward• Cooler sea surface temperatures Most hurricanes weaken as they track northwards along the east coast of the U.S. When it reaches a location where flow aloft is unfavorable (counteracting hurricane movement)G109: Weather and Climate 13: HurricanesHurricane DestructionHurricane Destruction• High wind speeds: by definition > 120 km/h• Heavy rain and flooding Up to 10 m/day of precipitation from eye wall Up to 10 in./day at any particular location• Tornadoes Most often in right forward quadrant• Storm surges – high water level due to: Piling up of water dragged by heavy winds Low pressure• Debris carried by water contributes to damageG109: Weather and Climate 13: HurricanesHurricane DestructionHurricane Destruction• Hurricane winds and storm surges are most intense on the right-hand side of the storm, relative to direction of travelG109: Weather and Climate 13: HurricanesDetection and Tracking of HurricanesDetection and Tracking of Hurricanes• National Hurricane Center (NHC, Miami FL)  Track hurricanes and tropical storms in the Atlantic and eastern Pacific  Use:• Flights into hurricanes• Satellite imagery• On-ground instrument packages• Computer models Hurricanes are very erratic; prediction is difficult  Issue:• Hurricane Watch: when landfall is predicted in more than 24 hours• Hurricane Warning: when landfall is predicted within 24 hoursG109: Weather and Climate 13: HurricanesSaffir-Simpson Hurricane Intensity ScaleSaffir-Simpson