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Tropical Cyclones and Hurricanes Read Chapter 11 Charley Ivan Katrina's landfall The Hurricane A low pressure system of tropical origin that produces sustained surface winds >=74mph Organized convection around center Tropical cyclone is generic name given low-pressure systems forming over warm, tropical seas (typhoon in western pacific, super typhoon if winds >150 mph) The Hurricane: Birth Regions Not over land Not high latitudes, but not at the equator either (Graphic by Robert Simmon, NASA GSFC) Active Season last year, Inactive this year 2006 8 5 2 A Recipe for Hurricane Formation 1) Sea surface temperature (SST) >80°F (26.5°C) & deep layer of warm water 2) Conditional/absolute instability through a deep layer of troposphere- Important for thunderstorms! 3) Moist air in mid-troposphere 4) Weak vertical wind shear 5) Genesis region at least 5° away from equator 6) Source of low-level cyclonic spin Ingredient #1: Warm water Warm water ensures high evaporation rates, thus high low level dew points…promotes instability & convection! Tropical cyclones can rapidly strengthen if they move over patch of warm water Atlantic Hurricane Season Peaks ~ Sept.10 – around when ocean is warmest Why is a relatively deep layer of warm water needed? Strong hurricane wind can lead to upwelling of colder water from below as high winds at surface mix the ocean water – Cooler surface waters lead to less evaporation, more stabilized atmosphere Ingredient # 2: Conditional Instability- How convection leads to pressure falls Animation on CD – let’s take a look Ingredient #3: Mid-Tropospheric Moisture Dry mid-level air promotes hurricane decay by leading to precipitation evaporating => downdrafts => cooler, more stable air near ground => less convection #4) Vertical wind shear separates low level circulation from tall thunderstorms Less wind shear => better environment for hurricane formation Ingredient #5: Cyclonic vorticity (spin) Earth vorticity is increased as one head toward poles, it’s zero at equator (related to Coriolis force) Earth vorticity needed for cluster of t-storms to obtain cyclonic spin Tropical cyclone formation at least 5° from equator Ingredient #6: Tropical Waves Emerge every 3-4 days off coast of Africa from JuneOctober, about 60/year When waves spawn hurricanes in east-central Atlantic, storm is termed “Cape Verde” storm…Cape Verde season is from August to October Intertropical Convergence Zone (ITCZ): The source for tropical waves Stages of Development I Tropical disturbance: disorganized blob of thunderstorms, often forms within the ITCZ – If atmosphere and ocean conditions favor development, we enter a positive feedback loop Latent heat release in thunderstorms through condensation Increased pressure aloft Upper-level divergence, latent heating & compressional warming lead to lower surface pressure Increased inflow deflected to right and cyclonic circulation enhanced, more evaporation into air =>more fuel for t-storms… Animation of this process on CD …->Tropical Depression organized thunderstorms, circular wind flow around center, wind speed less than 39 mph – Given number-letter tag (i.e. Tropical Depression 5-A) Stages of Development II Tropical Storm: sustained wind speeds from 39-73mph with higher gusts – Storm gets a name Six-year cycle (http://www.nhc.noaa.gov/aboutnames.shtml) Names reused or retired if storm is historic If all names used in a season, storms named with Greek alphabet – If conditions remain favorable, positive feedback loop continues, leading to… Stages of Development: Hurricane Wind speed (sustained) at least 74 mph An eye forms Spiral bands Conservation of angular momentum Strength dictated by SSTs Strength of tropical cyclone related to number & intensity of thunderstorms around eye Winds fastest near surrounding calm eye of hurricane…known as eye wall Relative Velocity Strongest winds in a tropical cyclone are in the right front quadrant How does the eye form? As air spirals into center of storm, outward acting centrifugal force increases…eventually, centrifugal force equals PGF and air stops spiraling inward….leads to increased convergence and rising air motion Diameter ~ 25 mi Smaller eye-stronger hurricane Loop of Hurricane Katrina showing cyclonic flow http://www.atmo.arizona.edu/products/radar/ kat_lix_rad.gif Importance of anticyclonic flow aloft above a hurricane…increased upperlevel divergence Hurricane Felix Vertical Wind Profile in a Hurricane Speedy ring of air at surface in eye wall expands outward as it rises due to upperlevel divergence Wind speed decreases with height conserving angular momentum Hurricane: Saffir-Simpson Scale (in need of revision?) This graphic was created by Lynn A. Dombrowski, Ed. D. Storm Surge Water, not wind, is by far the biggest killer (includes both storm surge and fresh water flooding) Right front quadrant is most dangerous (N.Hemisphere) – Strongest winds (rotation + translation velocities) – Winds blow on-shore which piles up water Apartment Complex before Camille Apartment Complex after Camille Demise Loss of fuel – Moves into colder water – Moves over land Wind shear – Tilts the thunderstorms – Lowest pressure not concentrated, pressure gradient relaxes and winds weaken Increased friction (landfall) Symbols on Weather Maps Tropical Storm Symbol Hurricane Symbol Hurricane Watches/Warnings Watch - Issued when there is a possibility that a part of the coastline could experience hurricane conditions within 36 hours Warning – Issued when hurricane conditions are expected for a part of a coastline within 24 hours Understanding National Hurricane Center (NHC) Forecasts http://www.nhc.noaa.gov/archive/2005/KAT RINA_graphics.shtml