© 2012 Pearson Education, Inc. Lecture Presentation Chapter 10 Hurricanes and Extratropical Cyclones

© 2012 Pearson Education, Inc.

Lecture Presentation

Chapter 10

Hurricanes and Extratropical

Cyclones


Learning Objectives

Understand the weather conditions that create, maintain, and dissipate cyclones

Understand the difficulties in forecasting cyclone behavior

Know what geographic regions are at risk for hurricanes and extratropical cyclones

Understand the effects of cyclones in coastal and inland areas


Learning Objectives, cont.

Recognize linkages between cyclones and other natural hazards

Know the benefits derived from cyclones

Understand adjustments that can minimize damage and personal injury from coastal cyclones

Know the prudent actions to take for hurricane or extratropical cyclone watches and warnings


Introduction to Cyclones

An area or center of low pressure with rotating winds Counter-clockwise in Northern Hemisphere Clockwise in Southern Hemisphere

Tropical or extratropical Based on origin and core temperature

Characterized by intensity Sustained wind speeds and lowest atmospheric

temperature


Tropical and Extratropical Cyclones

Tropical Cyclones Form over warm tropical or subtropical ocean water (5°–20°) Have warm central cores Tropical depressions, tropical storms, hurricanes High winds, heavy rain, surges, and tornadoes Derive energy from warm ocean water and latent heat

Extratropical Cyclones Form over land or water in temperate regions (30°–70°) Associated with fronts and cool central cores Strong windstorms, heavy rains, surges, snowstorms,

blizzards Most do not produce severe weather Derive energy from temperature contrasts along fronts


Classification

Nor’easter Extratropical cyclone that moves along northward along East

Coast U.S.

Hurricanes Tropical cyclones in Atlantic and eastern Pacific Oceans

Typhoons Tropical cyclones in Pacific Ocean west of International

Dateline and north of the equator

Cyclones Tropical cyclones in Indian Ocean

Saffir-Simpson Scale classifies hurricanes based on wind speed

© 2012 Pearson Education, Inc.Table 10.1


Naming

Extratropical storms are sometimes named after their origins Example: Alberta Clipper

Hurricanes named by international agreement through World Meteorological Organization Named once winds exceed 63 km (39 mi.) per hour Names assigned sequentially each year from list for each

origin Male/Female names alternated Names are reused every 6 years Names of big storms are retired (example: Katrina)


Cyclone Development: Tropical Disturbance

A organized mass of thunderstorms persisting for > 24 hours

Typically 200 to 600 km (120 to 370 mi.)

Has a weak rotation due to Coriolis effect

Formed by Lines of convection Upper-level low pressure troughs Cold front remnants Easterly waves of converging and diverging winds

Atlantic Ocean hurricanes


Tropical Depressions and Tropical Storms

Tropical Depression Tropical disturbance wind speeds increase and

begins to spin A low pressure center is formed

Tropical Storm Winds increase to 63 km (39 mi.) ph Storm is given a name Wind speeds are not at hurricane strength, but

rainfall can be intense


Hurricanes

Not all tropical storms develop into hurricanes Classified when winds reach 119 km (74 mi.) per hour

Environmental conditions Thick layer of warm ocean water

Water must be warm and there must be deep

Steep vertical temperature gradient Atmosphere must cool quickly with increasing altitude

Weak vertical wind shear Strong winds aloft prevent hurricane development.


Hurricane Structure

Rain bands Clouds that spiral inward around center

Counterclockwise in Northern Hemisphere Increase in intensity towards the center of the hurricane

Eyewall Innermost band of clouds Contain the greatest winds and rainfall

Eye Area of calm at center of the hurricane Narrow at surface and wider at top


Hurricane Structure, cont.

Warm, moist air spirals upward around eyewall Air rises, it loses moisture

Upward rotation draws air from eye, causing dry air to sink back into center

Upward rotation also causes air to flow out the top of the storm concentrated in exhaust jets Allows additional warm air to feed bottom of the

storm

© 2012 Pearson Education, Inc.Figure 10.14


Hurricane Paths and Demise

Movement is controlled by the Coriolis effect and steering winds In Northern Hemisphere storms deflect to the right Track west in trade winds and curve northwest and

then northeast Hurricanes can make a loop In North Atlantic, steered by Bermuda High

As hurricane moves over land, it loses energy (warm water) Can become extratropical cyclone


Extratropical Cyclones

Necessary conditions Strong temperature

gradient at surface usually along cold, warm or stationary fronts

Strong upper level winds provided by jet stream

Polar jet stream Subtropical jet stream

Figure 10.18


Polar and Tropical Jet Stream

Polar jet stream shifts from crossing the United States in the winter to crossing southern Canada in the summer Subtropical jet stream crosses Mexico and Florida and is

strongest in the winter

Large high-pressure ridges and low-pressure troughs cause jet streams to bend and producing waves or meanders May also split in two around isolated high-pressures and

reunite

Extratropical cyclones often develop in curves or divergences in jet streams


Polar and Tropical Jet Stream, cont.

Bending or splitting cause the polar jet stream to dip south and the subtropical jet stream to flow northeast

The southern branch of a split polar jet stream in the Pacific Ocean brings warm moist air out of the tropics West Coast forecasters refer to the flow of warm moist

air as the Pineapple Express, because of its origin near Hawai’i

Nor’easters form when bends of the polar and subtropical jet streams begin to merge off the southeastern coast of the United States


Extratropical Cyclone Development

Low-pressure center develops along frontal boundary Cold front on southwest, warm front on east Conveyor belt of cold air circulates counter-clockwise

Warm air is wedged to the east Conveyor belt of warm air rises on the southeast side creating

a comma Conveyor belt of dry air aloft feeds the cyclone from behind

the cold front

Cold front wraps around the warm front, causing an occluded front develop trapping warm air aloft

Cold air completely displaces the warm air, pressure gradient weakens and storm dissipates




Geographic Regions at Risk for Cyclones: North America Hurricanes threaten

contiguous United States, Puerto Rico, the Virgin Islands, and U.S. territories in the Pacific Ocean

They are a lesser threat to Hawai’i and Atlantic Canada

On the Pacific coast, hurricanes strike Baja California and the west coast of the Mexican mainland

Figure 10.22


Atlantic Hurricane Paths

West toward East coast of Florida, sometimes passing over Caribbean Move out into the Atlantic

Ocean to the northeast

Westward over Cuba and into the Gulf of Mexico to strike the Gulf Coast

Westward to the Caribbean and then northeastward skirting the East Coast May strike the continent

from central Florida to New York

Figure 10.17



Geographic Regions at Risk for Cyclones: Worldwide

Northwest Pacific is much more active than North Atlantic

Indian Ocean is also a very active hurricane zone

South Atlantic and southeast Pacific, rarely have hurricanes because of cold ocean water

Hurricanes do not form close to the equator because of the absence of the Coriolis effect



Geographic Regions at Risk from Cyclones, Summary

Tropical cyclones East and Gulf Coasts Hawaii and Atlantic Canada Baja California and West Coast Mexico

Extratropical cyclones Winter windstorms in Pacific Coast Winter snow Sierra Nevada, Rocky Mountains and

east Spring and summer thunderstorms and tornadoes in

United States and Canada


Cyclone Effects: Storm Surge

Local rise in sea level resulting from storm winds

Can be > 3 m (10 ft.)

Because of spinning, surge is greatest in right quadrant of storm as it makes landfall

Height is greatest near time of maximum winds

Height is also greater if landfall coincides with high tide



Effects on Storm Surge Magnitude

Largest effect from stress exerted by wind on water Fetch refers to the area over which the wind blows Larger fetch results in larger storm surge

Smaller effect from low atmospheric pressure in storm pulling up on water surface

Also depends on shape of coastline

Water level tends to increase continually as storm approaches


Cyclone Effects: High Winds

Described by Saffir-Simpson Scale

Decrease exponentially with landfall

Strongest recorded winds in United States from extratropical cyclone Responsible for strong winds in blizzards and

tornadoes


Cyclone Effects: Heavy Rains

Average hurricane produces trillion gallons of water

Rainfall from cyclones can cause inland flooding

Flooding affected by: Storm’s speed Land elevation over which the storm moves Interaction with other weather systems Amount of water in soil, streams and lakes prior to

storm


Links to Other Natural Disasters

Coastal erosion

Flooding

Mass wasting

Other types of severe weather Tornadoes, severe thunderstorms, snowstorms,

and blizzards


Natural Service Functions of Cyclones

Source of precipitation

Redistribute warm air from tropics

Maintain ecosystems Winds carry plants, animals, and

microorganisms Waves stir up deeper, nutrient-rich waters Winds topple weak and diseased trees in forests Waves break apart some corals


Human Interaction with Cyclones

Urbanization of vulnerable coastlines increases magnitude of the effect of cyclones

Destruction of sand dunes makes areas more susceptible to hurricane winds

Construction of seawalls and bulkheads reflect waves and contribute to beach erosion

Poor building materials and practices can make hurricanes more dangerous to people

Global warming may contribute to higher intensity and frequency of hurricanes in the future


Minimizing the Effects of Cyclones: Forecasting and Warnings Forecast includes:

If it will make landfall Where and when Wind strength Rainfall amount Storm surge

Monitored by U.S. Hurricane Center, Canadian Hurricane Center

Hurricane watch means likely hurricane in 36 hours

Hurricane warning given when hurricane is likely within 24 hours or less


Forecasting Tools

Weather satellites Detect early warning signs Can not show wind speed

Aircraft U.S. Air Force, NOAA airplanes fly into the storm to

collect data

Doppler radar Give information on rainfall, wind speed, and

direction of the storm


Forecasting Tools, cont.

Weather buoys Automated weather stations that give

information at their locations

Computer models Predict and track hurricane progress Have vastly improved hurricane information Still lacking in predicting storm intensity



Storm Surge Predictions

Predict the time and elevation of surge Forecasters use wind speed, fetch and average

water depth

Need detailed information on topography Different elevations on land affect the storm

surge

Computer models use central pressure, size, forward speed, track, wind speed, and seafloor topography


Hurricane Prediction and the Future

Deaths have decreased dramatically because of better forecasting, improved education, and greater public awareness

However, coastal populations are increasing, increasing risk


Perception of and Adjustment to Cyclones

Perception of hazard depends on personal experience More experienced people may take hazard more

seriously More seasoned people may also take less precautions

Community adjustments to cyclone hazard Warning systems Evacuation plans and shelters Insurance Building design


Perception of and Adjustment to Cyclones, cont.

Personal adjustments to cyclone hazard Be aware of hurricane season Prepare homes and property for hazard Obtain flood insurance Install heavy shutters that can be latched Learn evacuation route Make a family emergency plan Collect emergency supplies


End

Hurricanes and Extratropical Cyclones

Chapter 10

Documents

© 2012 Pearson Education, Inc. Lecture Presentation Chapter 10 Hurricanes and Extratropical Cyclones