METEOROLOGY

GEL-1370

Chapter ElevenChapter Eleven

HurricanesHurricanes

What we are going to learn?

We are going to learn the answers to the following questions– What are hurricanes?

– How do they form?

– Why they strike the east coast of the US than the west coast of the US?

– Conditions necessary for the hurricane development

– Why are skies are clear in a hurricane’s eye?

– What is the fuel that derives the hurricane?

– What factors tend to weaken hurricane?

– Why hurricanes are more likely to strike New Jersey than Oregon?

– Why do hurricanes move westward over tropical waters?

Anatomy of a hurricane

• Hurricane: An intense storm of tropical origin, with sustained winds >64 knots (74 mi/hr)

• Other Names: Typhoon: Western North Pacific• Beguio: In Phillipines• Cyclone: In India and Australia• International agreement: Tropical cyclone• Tropics: region between 23.5° north – 23.5° south; noon

sun is always high in the sky, and so diurnal and seasonal changes in temperature are small; daily heating of the surface & high humidity favor the development of cumulus clouds & afternoon thunderstorms

A tropical wave as shown by the bending of streamlines; the waves move westward, bringing fair weather on its western

side & showers on the eastern side

Tropical weather

• Most of tropics are marked by seasonal differences in amount of precipitation, as temperature remains warm all year long

• Greatest cloudiness & precipitation occur during high-sun period (when the ITCZ moves into the region)

• Winds blow from the east, northeast or southeast – the trade winds

• Streamlines: depict wind flow direction; useful to see where surface air converges and diverges

• Tropical wave: A wavelike disturbance in the tropical easterlies; have wavelengths ~2,500 km & travel from east to west at speeds 10-20 knots

Hurricane Elena over the Gulf of Mexico, about 130 km southwest of Apalachicola, FL (1985); winds flow

counterclockwise about its center; central press. 955 mb

Hurricane Anatomy• Average diameter of a hurricane ~500 km

• Elena’s eye is ~ 40 km wide; within the eye, the winds are light and clouds are mainly broken

• Surface winds increase in speed as they blow counter-clockwise and inward toward the center

• Eye wall: A wall of dense thunderstorms that surrounds the eye of a hurricane; within the eye wall, heaviest precipitation & strongest winds are found; extend up to ~15 km above sea level

• Observations when we cross a hurricane (W to E): – When we approach, sky is overcast with cirrostratus clouds;

– Pressure drops slowly at first & then more rapidly as we move closer to the center

– When we approach the eye, winds blow from the north and northwest with increasing speed; heavy rain showers

Hurricane anatomy – contd.

• Inside the eye: air temperature rises, winds slacken, rainfall ceases & sky brightens; lowest pressure reading (~50 mb lower than just outside the storm)

• East side of the eye wall: heavy rain, strong southerly winds

• As we move away from the eye wall, pressure rises, winds diminish, rain decreases, & eventually sky clears

• Why no thunderstorm inside the eye: In the eyewall, severe thunderstorms lead to release of latent heat which in turns warms the eye --- leading to higher pressure which initiates a downward air motion (sinking air) --- sinking air warms by compression and absence of thunderstorms (so much heat is added to the air that surface air temp remains uniform in the hurricane)

A model vertical view of air motions, clouds & precipitation in a typical hurricane

Hurricane formation & dissipation• Hurricanes form over tropical waters (5-20 °) where

winds are light, surface water temp is warm (>26.5°C) over a vast area, high humidity; warm surface water must extend >200 m in the water; 2/3 hurricanes in 10-20 °

• These conditions prevail over tropical and subtropical North Atlantic and North Pacific oceans during summer and early fall (June-November)

• The surface winds must converge for a mass of unorganized thunderstorms to develop into a hurricane; surface winds along ITCZ; when a wave forms along the ITCZ, convection becomes organized leading to the formation of hurricanes

Hurricane formation and dissipation

• Hurricanes do not where the surface winds are strong• During El Nino event, strong winds aloft typically occurs

over the tropical Atlantic --- fewer Atlantic hurricanes than normal

• During El Nino event, warmer water of El Nino in the northern tropical Pacific favors the development of hurricanes in that region

• Energy for a hurricane come from the direct transfer of sensible het from the warm water into the atmosphere and from the transfer of latent heat from the ocean surface; if thunderstorm start to organize along the ITCZ or along a tropical wave, and if the trade wind inversion is weak, conditions will lead to the formation of a hurricane

Hurricane formation & dissipation - contd

• Tropical disturbance: A thunderstorm with a slight wind circulation

• Tropical depression (TD): When Tropical disturbance becomes TD when winds speeds reach 20-34 knots & several closed isobars appear about its center in a surface weather map

• Tropical Storm: When TD wind speed reaches 35-64 knots & when the isobars are packed together

• Hurricane: When tropical storm speed exceeds 64 knots• Stages: Tropical disturbance crossing over Panama;

tropical disturbance speed 25 knots; tropical storm speed < 40 knots; hurricane >110 knots

Visible satellite image of 4 tropical systems, each in a different stage of its life style

Hurricane movement

• Hurricanes form in tropical oceans (except in the S.Atlantic & eastern South Pacific: water temp is too cold); hurricanes formed over the N. Pacific & N. Atlantic are steered by easterly winds and move west or northwestward

• Actual path of a hurricane is determined by the structure of the storm & the storm’s interaction with the environment

• Off coast of Mexico over the N. Pacific spawns ~8/yr

• Tropical North Atlantic ~ 6/yr• Hawaiian Islands (20-23°N) is in direct path of

eastern Pacific hurricanes & tropical storms

Regions where tropical storms form, their names and typical paths

Some erratic paths taken by hurricanes

Hurricane and middle latitude storms

• Hurricane Middle latitude storms• Energy from warm water & horizontal temp gradients

latent heat of condensation

Hurricane weakens with height Intensifies with height

Contains an eye where air is Characterized by centers of

sinking rising air

Strongest near the surface Strongest winds found aloft in the jet stream

Around the hurricanes, isobars are more circular, pressure gradient is much steeper & winds are stronger

Destruction & warning• A hurricane moving northward over the Atlantic will normally

survive for a longer time than will the one from the same latitude over the eastern Pacific – surface water of Atlantic is warmer than the Pacific

• When a hurricane is approaching from the east, its highest winds are usually on its north side (winds that push the storm along add to the winds on the north & subtract from the winds on the south)

• When wind blows over water, the water beneath is set in motion; bending of water with depth (Ekman spiral) causes a net transport of water (Ekman transport)

• High winds of hurricanes generate large waves (10-15 m in height); waves move outward in the form of swells – storm’s energy is carried; effects of storm is felt days before the hurricane arrives

Destruction and warning – contd.

• Region of low pressure allows the ocean level to rise (~50 cm); 1mb drop ~ 1 cm rise

• Storm surge is produced by a combination of high winds, high waters and the net transport of water toward the coast – inundates low-lying areas and turns beachfront homes into piles of splinters

• Hurricane watch: issued ~24-48 hrs before the storm arrives

• Hurricane warning: When storm appears to strike an area within 24 hrs, warning is issued; issued for a large coastal area ~550 km (5 degrees) in length

• After landfall, hurricanes become tropical storms with peak winds < 50 knots

12 Most intense hurricanes from 1900-1999

• .Rank Hurricane Year Cen. P Catego. Death

1 Florida (Keys) 1935 892 mb 5 408

2 Camille 1969 909 5 256

3 Andrew 1992 922 4 53

4 Florida/South TX 1919 927 4 >600

5 FL-Lake Okeechobee

1928 929 4 1836

6 Donna 1960 930 4 50

7 Texas (Galveston) 1900 931 4 >6000

8 Louisiana (Grand Isle)

1909 931 4 350

9 Louisiana (New Orleans)

1915 931 4 275

10 Carla 1961 931 4 46

11 Hugo 1989 934 4 49

12 Florida (Miami) 1926 935 4 243

A composite IR images of Hurricane Georges (September 18-28, 1998); its trek across Caribbean

and northward into US

Hurricane Gloria on Sept. 27, 1985; moving northward @25 knots, winds of 100 knots on its right & 50 knots on left side;

945 mb at the central

When a storm surge moves in at high tide it can inundate and destroy a wide swath of coastal lowlands

Color-enhanced IR satellite image of Hurricane Hugo near Charleston, SC

Saffir-Simpson Hurricane damage-potential scale

• .Scale # Central

Pressure

(mb)

Winds

Knots

Storm

Surge (m)

Damage

1 >980 64-82 ~1.5 Damage mainly to trees, shrubbers and unanchored mobile homes

2 965-979 83-95 ~2.0-2.5

Some trees blown down; some damage to to exposed mobile homes; some damage to roofs

3 945-964 96-113 ~2.5-4.0

Large trees blown down; mobile homes destroyed; some structural damage

4 920-944 114-135 ~4.0-5.5

All signs blown down; extensive damage to roofs, windows and doors; destruction of mobile homes

5 < 920 >135 >5.5 Most serious damage to buildings; < 4.5 m above sea level within 500 m shore

Beach homes at Folly Beach, South Carolina before hurricane Hugo

Beach homes at Folly Beach, South Carolina after Hurricane Hugo

IR satellite image of Hurricane Andrew moving across South Florida on the morning of August 24, 1992; central

pressure 932 mb and sustained winds of 126 knots

Number of hurricanes (3,4,5 are major) in each category that made landfall along the coastal US from 1900-1999; all

hurricanes struck the Gulf or Atlantic coasts

Naming Hurricanes• A name is assigned when a storm reaches tropical storm

strength• Beginning 1953 (-1977), the National Weather Service

began using female names• Beginning 1978-1979, alternately male and female names

were assigned

Chapter Summary• What are streamlines

• Hurricane-eye, eyewall, where the pressure is higher & lower; vertical structure, rotation of hurricanes, source of energy, conditions for the formation of hurricanes

• Commonness between middle latitude cyclones & hurricanes

• Various stages of hurricane, areas where more hurricanes

• Why hurricanes don’t form in certain regions?

• Saffir-Simpson scale

• How hurricanes can be modified?

• Damage by hurricanes

• Other names of hurricanes