Lecture Presentation Chapter 10 Hurricanes and Extratropical Cyclones

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

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 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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) © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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. © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

Figure 10. 20 © 2012 Pearson Education, Inc.

Figure 10. 21 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

Figure 10. 24 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

Figure 10. 25 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

Figure 10. 26 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

Links to Other Natural Disasters § Coastal erosion § Flooding § Mass wasting § Other types of severe weather § Tornadoes, severe thunderstorms, snowstorms, and blizzards © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

Figure 10. 32 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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