Tropical Cyclones and Hurricanes Tropical Cyclones and Hurricanes Read Chapter 11Read Chapter 11

Katrina's landfall

IvanIvan

CharleyCharley

The HurricaneThe Hurricane A low pressure system of A low pressure system of

tropical origin that produces tropical origin that produces sustained surface winds sustained surface winds >=74mph>=74mph

Organized convection Organized convection around centeraround center

Tropical cyclone is generic Tropical cyclone is generic name given low-pressure name given low-pressure systems forming over warm, systems forming over warm, tropical seas (typhoon in tropical seas (typhoon in western pacific, super western pacific, super typhoon if winds >150 mph)typhoon if winds >150 mph)

The Hurricane: Birth RegionsThe Hurricane: Birth Regions Not over landNot over land Not high latitudes, but not at the equator Not high latitudes, but not at the equator

eithereither(Graphic by Robert Simmon, NASA (Graphic by Robert Simmon, NASA

GSFC)GSFC)

Active Season last year, Active Season last year, Inactive this yearInactive this year

20062006

88

55

22

A Recipe for Hurricane A Recipe for Hurricane FormationFormation 1) Sea surface temperature (SST) >80°F 1) Sea surface temperature (SST) >80°F

(26.5°C) & deep layer of warm water(26.5°C) & deep layer of warm water 2) Conditional/absolute instability through a 2) Conditional/absolute instability through a

deep layer of troposphere- Important for deep layer of troposphere- Important for thunderstorms!thunderstorms!

3) Moist air in mid-troposphere3) Moist air in mid-troposphere 4) Weak vertical wind shear4) Weak vertical wind shear 5) Genesis region at least 5° away from 5) Genesis region at least 5° away from

equatorequator 6) Source of low-level cyclonic spin6) Source of low-level cyclonic spin

Ingredient #1: Warm waterIngredient #1: Warm water

Warm water ensures Warm water ensures high evaporation rates, high evaporation rates, thus high low level dew thus high low level dew points…promotes points…promotes instability & instability & convection!convection!

Tropical cyclones can Tropical cyclones can rapidly strengthen if rapidly strengthen if they move over patch they move over patch of warm waterof warm water

Atlantic Hurricane Season Peaks ~ Atlantic Hurricane Season Peaks ~ Sept.10Sept.10 – around when ocean is – around when ocean is

warmestwarmest

Why is a relatively deep layer of warm Why is a relatively deep layer of warm water needed?water needed?

Strong hurricane wind can lead to Strong hurricane wind can lead to upwelling of colder water from below asupwelling of colder water from below as

high winds at surface mix the ocean waterhigh winds at surface mix the ocean water– Cooler surface waters lead to less Cooler surface waters lead to less

evaporation, more stabilized atmosphereevaporation, more stabilized atmosphere

Ingredient # 2: Conditional Instability- How Ingredient # 2: Conditional Instability- How convection leads to pressure fallsconvection leads to pressure falls

Animation on CD – let’s take a lookAnimation on CD – let’s take a look

Ingredient #3: Mid-Tropospheric Ingredient #3: Mid-Tropospheric MoistureMoisture

Dry mid-level air promotes hurricane Dry mid-level air promotes hurricane decay by leading to precipitation decay by leading to precipitation evaporating => downdrafts => cooler, evaporating => downdrafts => cooler, more stable air near ground => less more stable air near ground => less convectionconvection

#4) Vertical wind shear separates #4) Vertical wind shear separates low level circulation from tall low level circulation from tall

thunderstormsthunderstormsLess wind shear => Less wind shear => better environment for better environment for hurricane formationhurricane formation

Ingredient #5: Cyclonic vorticity (spin) Ingredient #5: Cyclonic vorticity (spin)

Earth vorticity is increased as one head toward poles, Earth vorticity is increased as one head toward poles, it’s zero at equator (related to Coriolis force)it’s zero at equator (related to Coriolis force)

Earth vorticity needed for cluster of t-storms to obtain Earth vorticity needed for cluster of t-storms to obtain cyclonic spincyclonic spin

Tropical cyclone formation at least 5° from equatorTropical cyclone formation at least 5° from equator

Ingredient #6: Tropical WavesIngredient #6: Tropical Waves

Emerge every 3-4 days off Emerge every 3-4 days off coast of Africa from June-coast of Africa from June-October, about 60/yearOctober, about 60/year

When waves spawn When waves spawn hurricanes in east-central hurricanes in east-central Atlantic, storm is termed Atlantic, storm is termed “Cape Verde” storm…“Cape Verde” storm…Cape Verde season is Cape Verde season is from August to Octoberfrom August to October

Intertropical Convergence Zone Intertropical Convergence Zone (ITCZ): The source for tropical (ITCZ): The source for tropical

waveswaves

Stages of Development IStages of Development I

Tropical disturbance: disorganized blob of Tropical disturbance: disorganized blob of thunderstorms, often forms within the ITCZthunderstorms, often forms within the ITCZ– If atmosphere and ocean conditions favor If atmosphere and ocean conditions favor

development, we enter a development, we enter a positive feedback looppositive feedback loop Latent heat release in thunderstorms through Latent heat release in thunderstorms through

condensationcondensation Increased pressure aloftIncreased pressure aloft Upper-level divergence, latent heating & Upper-level divergence, latent heating &

compressional warming lead to lower surface compressional warming lead to lower surface pressurepressure

Increased inflow deflected to right and cyclonic Increased inflow deflected to right and cyclonic circulation enhanced, more evaporation into air circulation enhanced, more evaporation into air =>more fuel for t-storms…=>more fuel for t-storms…

Animation of this process on CDAnimation of this process on CD

……->Tropical Depression->Tropical Depression

organized thunderstorms, circular wind flow organized thunderstorms, circular wind flow around center, wind speed less than 39 mpharound center, wind speed less than 39 mph– Given number-letter tag (i.e. Tropical Given number-letter tag (i.e. Tropical

Depression 5-A)Depression 5-A)

Stages of Development IIStages of Development II

Tropical Storm: sustained wind speeds Tropical Storm: sustained wind speeds from 39-73mph with higher gustsfrom 39-73mph with higher gusts– Storm gets a nameStorm gets a name

Six-year cycle Six-year cycle (http://www.nhc.noaa.gov/aboutnames.shtml)(http://www.nhc.noaa.gov/aboutnames.shtml)

Names reused or retired if storm is historicNames reused or retired if storm is historic If all names used in a season, storms named If all names used in a season, storms named

with Greek alphabetwith Greek alphabet

– If conditions remain favorable, positive If conditions remain favorable, positive feedback loop continues, leading to…feedback loop continues, leading to…

Stages of Development: Stages of Development: HurricaneHurricane

Wind speed (sustained) at Wind speed (sustained) at least 74 mphleast 74 mph

An eye formsAn eye forms Spiral bandsSpiral bands Conservation Conservation

of of angular angular momentummomentum

Strength dictated by SSTsStrength dictated by SSTs

Strength of tropical cyclone related to Strength of tropical cyclone related to number & intensity of thunderstorms number & intensity of thunderstorms

around eyearound eye

Winds fastest near surrounding calm eye Winds fastest near surrounding calm eye of hurricane…known as eye wallof hurricane…known as eye wall

Relative Relative VelocityVelocity

Strongest winds in Strongest winds in a tropical cyclone a tropical cyclone are in the right are in the right front quadrantfront quadrant

How does the eye form?How does the eye form?

As air spirals into center of As air spirals into center of storm, outward acting storm, outward acting centrifugal force centrifugal force increases…eventually, increases…eventually, centrifugal force equals centrifugal force equals PGF and air stops PGF and air stops spiraling inward….leads to spiraling inward….leads to increased convergence increased convergence and rising air motionand rising air motion

Diameter ~ 25 miDiameter ~ 25 mi Smaller eye-stronger Smaller eye-stronger

hurricanehurricane

Loop of Hurricane Katrina showing Loop of Hurricane Katrina showing cyclonic flowcyclonic flow

http://http://www.atmo.arizona.edu/products/radar/kat_liwww.atmo.arizona.edu/products/radar/kat_lix_rad.gifx_rad.gif

Importance of anticyclonic flow aloft Importance of anticyclonic flow aloft above a hurricane…increased upper-above a hurricane…increased upper-

level divergencelevel divergence

Hurricane FelixHurricane Felix

Vertical Wind Profile in a Vertical Wind Profile in a HurricaneHurricane

Speedy ring of air at Speedy ring of air at surface in eye wall surface in eye wall expands outward as it expands outward as it rises due to upper-rises due to upper-level divergencelevel divergence

Wind speed decreases Wind speed decreases with height conserving with height conserving angular momentumangular momentum

Hurricane: Saffir-Simpson Hurricane: Saffir-Simpson Scale (in need of revision?)Scale (in need of revision?)

This graphic was created by Lynn A. Dombrowski, Ed. D.This graphic was created by Lynn A. Dombrowski, Ed. D.

Storm SurgeStorm Surge

Water, not wind, is by far the biggest killer Water, not wind, is by far the biggest killer (includes (includes both storm surge and fresh water flooding)both storm surge and fresh water flooding)

Right front quadrant is most dangerous (N.Hemisphere)Right front quadrant is most dangerous (N.Hemisphere)

– Strongest winds (rotation + translation velocities)Strongest winds (rotation + translation velocities)

– Winds blow on-shore which piles up waterWinds blow on-shore which piles up water

Apartment Complex before CamilleApartment Complex before Camille

Apartment Complex after CamilleApartment Complex after Camille

DemiseDemise

Loss of fuelLoss of fuel– Moves into colder waterMoves into colder water– Moves over landMoves over land

Wind shearWind shear– Tilts the thunderstormsTilts the thunderstorms– Lowest pressure not concentrated, pressure Lowest pressure not concentrated, pressure

gradient relaxes and winds weakengradient relaxes and winds weaken Increased friction (landfall)Increased friction (landfall)

Symbols on Weather MapsSymbols on Weather Maps

Tropical Storm SymbolTropical Storm Symbol

Hurricane SymbolHurricane Symbol

Hurricane Watches/WarningsHurricane Watches/Warnings

Watch Watch - Issued when there is a possibility - Issued when there is a possibility that a part of the coastline could experience that a part of the coastline could experience hurricane conditions within 36 hourshurricane conditions within 36 hours

WarningWarning – Issued when hurricane – Issued when hurricane conditions are expected for a part of a conditions are expected for a part of a coastline within 24 hourscoastline within 24 hours

Understanding National Hurricane Understanding National Hurricane Center (NHC) Forecasts Center (NHC) Forecasts

http://www.nhc.noaa.gov/archive/2005/KAThttp://www.nhc.noaa.gov/archive/2005/KATRINA_graphics.shtmlRINA_graphics.shtml