NATS 101

Lecture 20Hurricanes

Supplemental References for Today’s Lecture

Aguado, E. and J. E. Burt, 2001: Understanding Weather & Climate, 2nd

Ed. 505 pp. Prentice Hall. (ISBN 0-13-027394-5)

Danielson, E. W., J. Levin and E. Abrams, 1998: Meteorology. 462 pp.

McGraw-Hill. (ISBN 0-697-21711-6)

Types of Tropical Cyclones

Cyclone Type Winds

Tropical Depression 25-39 mph

Tropical Storm 40-74 mph

Hurricane/Typhoon 75 mph

Most Depressions do not develop into Storms

Majority of Storms reach Hurricane status

Some Hurricane Extremes

Lowest Central Pressure Pressure

Pacific: Typhoon Tip 1979 870 mb

Atlantic: Hurricane Wilma 2005 882 mb

Costliest Hurricanes Cost-Loss

Hurricane Andrew 1992 $25 billion

Hurricane Katrina 2005 $200 billion?

Bangladesh Cyclone 1970 300,000 dead

Andrew 1992 Time Sequence

Link to Older NASA Satellite Animations

2005 Atlantic Hurricanes NASA

Note cooler water in wake of Dennis, Emily and Katrina

U.S. Hurricane Deaths and Costs

Williams, The Weather Book

Hurricane Lecture Overview

• What are the primary differences between hurricanes and extratropical cyclones?

• When and where do hurricanes form?

• How do hurricanes intensify?

• What is the structure of a hurricane?

• What kind damage do hurricanes inflict?

• When and where do hurricanes dissipate?

Strong FrontsCold at Storm Center AloftStrongest Winds AloftForms outside TropicsDiameter of 500-1000 milesEnergy Source: Horizontal

Temperature Contrast

Differences Between Tropical and Extratropical Storms

Williams, The Weather Book

No FrontsWarm at Storm Center AloftStrongest Winds near SurfaceForms over Tropical OceansDiameter of 200-500 milesEnergy Source: Energy Fluxes

from Warm Ocean

Where Hurricanes Form?

Williams, The Weather Book

HurricanesHurricanes

TyphoonsTropical Cyclones

Hot Bed!Hot Bed!

• Hurricanes go by different names in different regions of the world.• Form over warm tropical waters, equatorward of 20 latitude… Not on equator (poleward of 5 ) b/c non-zero Coriolis is needed.• Occur most frequently over Western North Pacific Ocean.

Atlantic Hurricane Frequency

Occur in Warm SeasonMaximum Likelihood

when Sea Surface Temperatures are Warmest-September

Average of ~6 Per Year Large Yearly Variability

Fewer in El Nino YearsMore in La Nina Years

Danielson et al. Fig. 13.2

Atlantic Hurricane Tracks

• Atlantic hurricanes tend to form in the Middle Tropical Atlantic Ocean and Caribbean Sea

• They usually propagate westward before turning northward

• They dissipate rapidly over land

Danielson et al. Fig. 13.12

Hurricane Steering

Williams, The Weather Book

Large-scale flow controls where hurricanes go.

Hurricane Necessary Ingredients

Williams, The Weather Book

Warm Water with T 82oF Deep Warmth > 200 ft

Converging Surface Winds Seedling Low Required

Conditionally Unstable Air Supports Deep Convection

Widespread, Deep Humid Air Supplies More Latent Heat

Weak Vertical Wind Shear Shear Shreds Storm Apart

Diverging Winds Aloft

Where do Seedling Vortices Come?Lots of Places and Ways

Remnant mid-lat circulation

Vortices along ITCZ Easterly Waves

Remnant MCC circulationDanielson et al. Fig. 13.14

3D Flow within Hurricanes

Aguado and Burt, Fig. 12-3

Surface winds spiral inward c’clockwise

Winds aloft spiral outward clockwise

Eyewall winds spiral upward c’clockwise

Winds inside eye spiral downward

clockwise

Thermal Structure of Hurricane

Aguado and Burt, Fig. 12-4

Radar of Andrew’s Landfall

Most intense rainfall is along the eyewall.

Fastest surface winds are along the eyewall.

Region inside of eye is dry with light winds

Danielson et al. Fig. 13.25

Floyd hourly rain loop from RSMAS

Rita TRMM "Hot Towers"Rita TRMM Rain Rate

Eye of Hurricane Luis 1995Luis Visible Eye Animation

Asymmetry of Hurricane Winds

20 kts

80 kts80 kts

60 kts

100 kts

Aguado and Burt, Fig. 12-10

Region of Maximum

Storm Surge

Hurricane Intensity Scale

Williams, The Weather Book

(> 980 mb) (965-980 mb) (945-964 mb) (920-944 mb) (< 920 mb)

Primary Hurricane Hazards

• Wind Damage

Large-Scale Hurricane Circulation Itself

Embedded Tornadoes

• Flooding

Heavy Rains Far Inland, 5”-10” Common

Storm Surge along Shoreline

Hurricanes Spawn Tornadoes

• Tornadoes embedded within a hurricane after landfall tend to be weak (category F1-F2)

• But they are embedded within an environment with 65+ kt winds.

• Causes hurricane wind damage to be localized.

Aguado and Burt, Fig. 12-11

Inland Flooding-Agnes 1972

• Even weak hurricanes can be catastrophic, hundreds of miles inland.

• Agnes 1972, category 1 storm for a few hours.• Agnes merged with a slow-moving ET cyclone. • Up to 15” of rain in 24 h fell over Pennsylvania.• Previous flood records exceeded by 6 ft.• Damage > $10B in inflation adjusted dollars.• Costliest U.S. storm prior to Andrew and Katrina.

Storm Surge I

Williams, The Weather Book

• Low atmosphere pressure raises a mound of water inside eye. • Water rises about 1 cm for every 1 mb decrease in pressure. • Inward spiraling winds push more water toward hurricane eye. • Deep hurricanes only raise about 1 meter of water over deep ocean. Water can sink downward and flow away from the surface.

Storm Surge II

Williams, The Weather Book

• In shallow water near land, water can not flow away under surface.• But winds continue to push water inward towards storm’s center. • Winds along hurricane’s right flank also push water against shore.• Water piles up along shoreline and rushes inland. The big effect!• Effect is worse where ocean floor slopes gently - Gulf of Mexico!

Link to COMET Surge Animation

Storm Surge III

Williams, The Weather Book

• If hurricane hits at high tide, the two effects superimpose.• A 2 ft tide plus a 10 ft surge rises water 12 ft above mean sea level.• Penetration of storm-whipped waves inland worsens damage.• Waves cause far more destruction than the high water alone.

Winds and Storm Surge

Danielson et al. Fig. 13.20Floyd wave height forecast from RSMAS

Surge Damage

• Richelieu Apartments before and after landfall of Camille 1969.• Camille was a Category 5 hurricane. • Sustained winds > 180 mph!• Storm surge was 24 feet along the coast! • Many tired citizens took refuge in apartments. Sadly, many died.

http://en.wikipedia.org/

Hurricane Decay

Hurricanes weaken when they make landfall (or go over cool water).Intense surface energy fluxes are cut off and friction increases.

Andrew Central Pressure

Danielson et al. Fig. 13.26

Summary: Hurricanes

• What are differences between hurricanes and extratropical cyclones?

Many significant ones! See earlier slide.• Where and when do hurricanes form?

5-20 latitude over oceans during warm season• How do hurricanes intensify?

Energy source is surface energy fluxes from the underlying warm ocean

Summary: Hurricanes

• What is the structure of a hurricane?

Eyewall - strongest winds, heaviest rain

Eye - dry with light winds

• What kind damage do hurricanes inflict?

Can be catastrophic due to high winds, torrential rains, and coastal storm surges

• When and where do hurricanes dissipate?

At landfall or when they go over cold water

Assignment for Next Lecture

• Topic – Air Pollution • Reading - Ahrens, pg 317-324, 327-340

• Problems - 12.1, 12.5, 12.14, 12.15, 12.19, 12.23