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Chapter 15 hurricanes Tropical Weather • Noon sun is always high, seasonal temperature changes small • Daily heating and humidity = cumulus clouds and afternoon thunderstorms • Non-squall clusters, tropical squall line, tropical wave • Seasons defined by precipitation as opposed to temperature Figure 15.1 A tropical wave (also called an easterly wave) moving off the coast of Africa over the Atlantic. The wave is shown by the bending of streamlines—lines that show wind flow patterns. (The heavy dashed green line is the axis of the trough.) The wave moves slowly westward, bringing fair weather on its western side and rain showers on its eastern side. Tropical Disturbance – An organized cluster of cumulonimbus clouds over tropical seas that has a surface center of low pressure; usually triggered by the ITCZ – Easterly Wave • A ripple in the tropical easterlies featuring a weak trough of low pressure • Forms over East Africa and propagates westward • Precursors of ~ 65% of named Atlantic tropical cyclones Anatomy of a Hurricane • Intense storm of tropical origin with winds greater than 64kts (74 mph) Stepped Art Fig. 15-3, p. 414 Hurricane Formation and Dissipation The Right Environment Tropical waters with light wind 26.5°C sea surface temperatures (June-November) Surface winds must converge , need trigger Coriolis effect zero at equator 5-20º latitude areas of development Inhibited by Trade wind - inversion near 20deg latitude Strong upper level wind shear Strong El Nino event Figure 15.7 Hurricanes form over warm, tropical waters. This image shows where sea surface temperatures in the tropical Atlantic exceed 28°C (82°F)—warm enough for tropical storm development—during May, 2002. The total number of hurricanes and tropical storms (red shade) and hurricanes only (yellow shade) that have formed during the past 100 years in the Atlantic Basin—the Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico. (NOAA) Figure 15.9 The top diagram shows an intensifying tropical cyclone. As latent heat is released inside the clouds, the warming of the air aloft creates an area of high pressure, which induces air to move outward, away from the high. The warming of the air lowers the air density, which in turn lowers the surface air pressure. As surface winds rush in toward the surface low, they extract sensible heat, latent heat, and moisture from the warm ocean. As the warm, moist air flows in toward the center of the storm, it is swept upward into the clouds of the eyewall. As warming continues, surface pressure lowers even more, the storm intensifies, and the winds blow even faster. This situation increases the transfer of heat and moisture from the ocean surface. The middle diagram illustrates how the air pressure drops rapidly as you approach the eye of the storm. The lower diagram shows how surface winds normally reach maximum strength in the region of the eyewall. • The Storm Dies Out – Cold water, land • Hurricane Stages of Development – Tropical Disturbance • Mass of Tstorms – Tropical Depression (22-34kts) • Several closed isobars around center – Tropical Storm (35-64kts) • Isobars packed closer together – Hurricane (> 65kts) Figure 15.10 Visible satellite image showing four tropical systems, each in a different stage of its life cycle. Figure 15.11 Regions where tropical storms form (orange shading), the names given to storms, and the typical paths they take (red arrows). • Hurricanes and Mid-latitude Storms different – Hurricane warm core low – Mid-latitude cold-core low – Hurricane + upper level trough May become a mid-latitude cyclone Figure 1 Surface weather map for the morning of September 23, 2005, showing Hurricane Rita over the Gulf of Mexico and a middle-latitude storm system north of New England. Hurricane movement General track: west, northwest, northeast Much variation Figure 15.13 An extremely rare tropical cyclone (with no name) near 28°S latitude spins clockwise over the south Atlantic off the coast of Brazil during March, 2004. Due to cool water and vertical wind shear, storms rarely form in this region of the Atlantic Ocean. In fact, this is the only tropical storm ever officially reported there. Naming Hurricane and Tropical Storms • Process has changed over the years: – Latitude and longitude – Letters of the alphabet around WWII – Alphabetical female names NWS 1953 – Alphabetical, alternating female and male names -1978- 79 – Retirement (Katrina, Camille) if a Cat3 for 10yrs Devastating Wind, Storm Surge, and Flooding • Highest winds on the eastern side of storm (wind + speed of storm) • Swell • Storm surge on north side of storm – Caused most of the 1,300 fatalities associated with Hurricane Katrina – Remains the most serious potential impact of a landfalling hurricane • Heavy rains and inland flooding • • • • – Freshwater flooding was responsible for 60% of deaths from 1970 – 1999 attributed to tropical cyclones or their remnants Coastal flooding River flooding Hurricane spawned tornadoes Saffir-Simpson scale – 1 weakest, 5 strongest A hurricane moving northward will have higher sustained winds on its eastern side than on its western side. If the hurricane moves from east to west, highest sustained winds will be on its northern side. Hurricane Hazards • Inland Flooding – Rains are typically 13 - 25 cm (5 - 10 in.) – Heavy rains persist as the storm tracks inland – Case study – Hurricane Agnes (1972) • Rains accounted for most of the property damage • Devastating floods in midAtlantic region, especially central Pennsylvania, when heavy rain fell on already saturated grounds and hilly terrain – Hurricane Mitch (1998), Tropical Storm Alberto (1994), and Tropical Storm Allison (2001) also caused devastating flooding Hurricane Hazards • Wind – Wind pressure, the force per unit area caused by air in motion, increases with the square of the wind speed – Debris transported by wind increases damage potential – Small but powerful whirlwinds (spin-up vortices) embedded in a hurricane’s circulation may be responsible for the most severe property damage – Winds diminish rapidly upon storm landfall © AMS • Hurricane over land is no longer in contact with warm ocean water, its energy source • Frictional resistance slows wind and shifts wind direction toward the center; causes the storm to fill and weaken • The system may still produce tornadoes after making landfall, partially due to strong wind shear between the surface and aloft 21 • Storm surge – A dome of ocean water 80 – 160 km (50 – 100 mi) wide that sweeps over the coastline near the hurricane’s landfall – Caused by strong winds and low barometric pressure and is most likely on the side of the hurricane with onshore winds – Wind-driven waves on top of the dome of water, armed with floating debris, are responsible for much of the structural damage – Prior to 1970, was responsible for the majority of hurricane-related fatalities. Awareness, warnings, and evacuation have generally been much better since then. – From 1970 to 1999, there were only 6 storm surge deaths. Then there was Hurricane Katrina and the death and destruction caused by its surge. – 1895 unnamed hurricane killed an estimated 2,000, and left 20,000 to 30,000 homeless due to the storm surge – The most deadly U.S. natural disaster was the hurricane that hit Galveston, TX (1900) when 8,000 people perished – Hurricane Camille (1969) produced a 7.3 m (24.3ft) surge at Pass Christian, MS – East Pakistan (now Bangladesh) in 1970 – storm surge killed ~300,000 Figure 15.15 Storm Surge When a storm surge moves in at high tide it can inundate and destroy a wide swath of coastal lowlands. Figure 15.16 The changing of the ocean level as different category hurricanes make landfall along the coast. The water typically rises about 4 feet with a Category 1 hurricane, but may rise to 22 feet (or more) with a Category 5 storm. Saffir-Simpson Hurricane Intensity Scale • Provides an estimate of potential coastal flooding and property damage from a hurricane landfall • Wind speed is the primary determining factor for a hurricane’s rating • Storm surge is just an estimate – It depends on underwater topography and other factors in the region of landfall • Property damage rises rapidly with rating – 100-300 times greater damage from a category 4 or 5 hurricane than a category 1 • From 1901 to 2004, about 37% of landfalling hurricanes were classified as major (category 3 or above) The number of hurricanes (by each category) that made landfall along the coastline of the United States from 1900 through 2007. All of the hurricanes struck the Gulf or Atlantic coasts. Categories 3, 4, and 5 are considered major hurricanes. Figure 15.18 HUGO 1989 A color-enhanced infrared satellite image of Hurricane Hugo with its eye over the coast near Charleston, South Carolina. Figure 15.19 Andrew 1992 Color radar image of Hurricane Andrew as it moves onshore over south Florida on the morning of August 24, 1992. The National Hurricane Center (NHC) is located about 30 km (19 mi) from the center of the eye. Figure 15.21 IVAN 2004 Visible satellite image of Hurricane Ivan as it makes landfall near Gulf Shores, Alabama, on September 15, 2004. Ivan is a major hurricane with winds of 105 knots (121 mi/hr) and a surface air pressure of 945 mb (27.91 in.). Figure 15.23 Hurricane Katrina just after making landfall along the Mississippi/Louisiana coast on the morning of August 29, 2005. Shown here, the storm is moving north with its eye due east of New Orleans. At landfall, Katrina had sustained winds of 110 knots, a central pressure of 920 mb (27.17 in.), and a storm surge over 20 feet. • Observation: Atlantic Hurricanes 2004-2005 – Abnormally warm ocean water and weak vertical sheer allowed for high frequency of hurricanes Tracks of three of the four hurricanes that struck Florida in 2004 Hurricane Watches, Warnings, and Forecasts • Watch issued 24-48 hours before hurricane expected to make landfall • Warning issued when storm expected to strike coast within 24 hours and probability of strike in a given location provided. Hurricane Katrina over the Gulf of Mexico with sustained winds of 145 mi/hr on August 28, 2005, at 1 a.m. CDT. The current movement of the storm is west-northwest at 8 mi/hr. The dashed orange line shows the hurricane’s projected path; the solid purple line, the hurricane’s actual path. Areas under a hurricane warning are in red. Those areas under a hurricane watch are in pink, while those areas under a tropical storm warning are in blue. Hurricanes in a Warmer World No clear answer, need more data Intensity and frequency most likely to be impacted. Figure 5 Sea-surface temperature departures from the twelve-year average (1985–1997) on May 30, 2005. Notice that the darker the red, the warmer the surface water. (NOAA) Figure 6 The total number of tropical storms and hurricanes (red bars), hurricanes only (yellow bars), and Category 3 hurricanes or greater (green bars) in the Atlantic basin for the period 1851 through 2007. (NOAA) Modifying Hurricanes • Operation STORMFURY: 1961- 1983 – seed clouds to create rain, weaken hurricane, and reduce winds; no conclusive evidence it was effective • Oil or film on water to reduce evaporation and latent heat available to storms Evacuation • Effectiveness of coastal evacuation plans was tested in 1985 when category 3 Hurricane Elena (bottom figure) followed an erratic path over the Gulf of Mexico • The potential downside of evacuation was illustrated by Hurricane Floyd (1999), a very large hurricane approaching the Southeast – 2 million were evacuated and massive gridlock occurred • Greater uncertainty with forecast track translates into a broader evacuation zone and greater economic losses – Vertical evacuation may be an option