AMS Weather Studies Introduction to Atmospheric Science 4

AMS Weather Studies Introduction to Atmospheric Science, 4 th Edition Chapter 12 Tropical Weather Systems © AMS 1

Case-In-Point § Hurricane Katrina – 28 August 2005 – Devastated the Gulf Coast of Louisiana and Mississippi; New Orleans experienced catastrophic flooding – 3 rd most intense landfalling U. S. hurricane – Claimed 1, 300 lives and is the most destructive hurricane in terms of economic loss – New Orleans topography made the city particularly vulnerable § The city occupies a bowl between the Mississippi River and Lake Pontchartrain, much of which is up to 1. 8 m (6 ft) below sea level § New Orleans relies on levees and pumps to keep water out § Levee system was breached, pumps failed, and city was flooded to depths of up to 6 m (20 ft) – New Orleans residents were warned in advance, but thousands did not evacuate – Katrina was followed by Hurricane Rita less than a month later § Landfall was well to the west of New Orleans (near Sabine Pass, TX), but flood waters up to 1. 5 m (5 ft) deep spread over parts of the city due to new breaks in the levees – Hurricane Gustav hit the Gulf Coast in 2008, causing $4. 3 billion in damage © AMS 2

Driving Question § What conditions are required for the development of tropical cyclones? – This chapter will describe: § Weather in the tropics § Characteristics of tropical cyclones § Geographical and seasonal distribution § Associated hazards § Tropical cyclone life cycle § Forecasting efforts § Unsuccessful experiments to modify these storms © AMS 3

Weather in the Tropics § The tropics is the belt between the Tropic of Cancer (23. 5 degrees N) and the Tropic of Capricorn (23. 5 degrees S) § Weather exhibits very little seasonal variation with uniformly high temperatures § Diurnal temperature variation is typically greater than the range of monthly mean temperatures over the course of a year § No fronts or frontal weather – Air masses are uniformly warm and humid § Thunderstorm activity – Thunderstorms may align in tropical non-squall clusters – More intense cells (tropical squall clusters) can form that look like middle latitude squall lines – ITCZ stimulates thunderstorm activity and follows the sun, so that summer is the rainy season and winter is the dry season § Very little horizontal pressure gradient – Isobaric analysis is of little value – Streamline analysis is used instead © AMS § A streamline is a line drawn on a map that is parallel to the wind direction § Can be used to identify regions of divergence and convergence, such as that associated with an easterly wave 4

Hurricane Characteristics § Definition – an intense cyclone that originates over tropical ocean waters, usually in late summer to early fall and has a maximum sustained wind speed ≥ 119 km per hr (74 mph) § Contrast with an Extra-tropical Cyclone – No associated fronts or frontal weather due to its origin over uniformly warm and humid conditions – Sea level pressure and steep horizontal air pressure gradient are typically greater than that of an extra-tropical cyclone – A hurricane is a much smaller system – A mature hurricane is a warm-core low whose circulation weakens with altitude; an extra-tropical cyclone is a coldcore low whose circulation strengthens with altitude © AMS 5

Hurricane Characteristics § The eye is at the center of a hurricane – Almost cloudless skies, subsiding air, light winds – Diameter ranging from 10 to 65 km (6 to 40 mi) – Eye shrinks as hurricane intensifies § The eye wall (borders the eye of a mature storm) – Ring of cumulonimbus clouds that produce heavy rains and very strong winds – The most dangerous and potentially most destructive part of a hurricane is the eye wall on the side of the advancing system where the wind blows in the same direction as the storm’s forward motion § In the Northern Hemisphere, this occurs on the right side of the hurricane when facing in the direction of the system’s forward motion § Cloud bands spiral inward towards the eye wall and produce heavy convective showers and hurricane-force winds § At high altitudes, cirrus or cirrostratus spiral outward © AMS 6

Hurricane Characteristics © AMS 7

Hurricane Characteristics © AMS The eye wall of Hurricane Katrina on 28 August 2005 while the storm was over the Gulf of Mexico 8

Where and When § Necessary conditions formation: – Relatively high sea surface temperatures (SST) § SST of at least 26. 5 °C (80 °F) through an ocean depth of 45 m (150 ft) or more § Sustains circulation by the latent heat released when water vapor, evaporated from the ocean surface, is conveyed upward and condenses § Strong tropical cyclone winds can induce Ekman transport and lead to lower SST § Cyclones may intensify over warm-core rings and weaken over cold-core rings § SST requirement makes formation seasonal – Most Atlantic hurricanes develop in late summer and early autumn when ocean surface waters are warmest – Official season runs from 1 June to 30 November – Peak threat to U. S. coastline is from mid-August to late October – Adequate Coriolis Effect § With rare exception, tropical cyclones do not form within 5 degrees of the equator – Weak vertical wind shear § Allows cluster of cumulonimbus to form § Wind shear would tear these apart © AMS – Relatively humid air in the mid-troposphere 9

Tropical Cyclone Breeding Grounds © AMS 10

Where and When Frequency of Atlantic basin tropical storm and hurricanes © AMS 11

Where and When § During El Niño, Atlantic hurricanes are infrequent due to strong high altitude winds § Strong wind shear is the main reason why hurricanes rarely form off the coasts of South America § Worldwide average of 80 tropical cyclones per year – ½ strengthen to hurricanes – The western Pacific Ocean is the most active area § About 27 systems each season, about 17 of which intensity into typhoons (~ 4 supertyphoons) § Only hurricanes spawned in the tropical Atlantic, Caribbean and Gulf of Mexico pose a serious threat to coastal North America – Average – 10. 6 named tropical storms, 6 become hurricanes (2. 4 major hurricanes), 2. 5 hurricanes strike U. S. coast each year – 2005 season set a record with 27 named tropical storms – Every Atlantic/Gulf coast state from TX to ME has been hit § FL is most hurricane prone, TX 2 nd, LA 3 rd – The Pacific coast is rarely a hurricane target due to NE trades © AMS – The Hawaiian Islands are sometimes threatened 12

Life Cycle of a Hurricane § 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 © AMS § 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 13

Life Cycle of a Hurricane § Tropical Disturbance – Only a small percentage of convective cloud clusters in the tropical Atlantic evolve into full-blown hurricanes § Subsidence of air on the eastern flank of the Bermuda-Azores anticyclone and trade-wind inversion inhibit deep convection § Vertical wind shear is usually too great § Atmospheric conditions that inhibit cyclone formation appear to be associated with the Sahara air layer (SAL), an elevated mass of dry, dusty, stable air originating over the Sahara Desert that travels many thousands of kilometers westward over the Atlantic – If conditions favor hurricane development, the surface air pressure falls and a cyclonic circulation develops © AMS § Water vapor condenses within the storm, releases latent heat, and the heated air rises. Expansional cooling of rising air triggers more condensation and release of latent heat § Rising temperatures in the storm’s core and divergence of air aloft trigger a sharp drop in surface air pressure and increased surface convergence § If favorable conditions persist, cycle continues and the tropical disturbance intensifies and its winds strengthen 14

Life Cycle of a Hurricane § Tropical Depression – Maximum sustained winds reach 37 km per hr (23 mph) or higher § Tropical Storm – Winds reach at least 63 km per hr (39 mph) – Assigned a name § Hurricane – Winds reach 119 km per hr (74 mph) or higher § As storm weakens, it is downgraded by reversing this classification system © AMS 15

Life Cycle of a Hurricane § Tropical cyclone trajectories are often erratic, however, cyclones typically initially drift westward and curve toward the north and northeast when they reach the western Atlantic § Upon reaching about 30 degrees N, a hurricane may begin to acquire extra-tropical characteristics as colder air is drawn into the system and fronts develop § Some hurricanes fueled by the warm Gulf Stream may maintain tropical characteristics far up the Atlantic coast § New England has been the target of many strong hurricanes © AMS 16

Hurricane Hazards § Heavy rains and inland flooding – Freshwater flooding was responsible for 60% of deaths from 1970 – 1999 attributed to tropical cyclones or their remnants § Strong winds – Responsible for 12% of deaths during the same period § Tornadoes § Storm surge – Caused most of the 1, 300 fatalities associated with Hurricane Katrina – Remains the most serious potential impact of a landfalling hurricane © AMS 17

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 mid. Atlantic 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 © AMS 18

Hurricane Hazards § Inland Flooding – The image shows radar-determined cumulative rainfall over southeast Texas produced by the remnants of Tropical Storm Allison – Allison ranks as the most deadly and costly tropical storm to strike the U. S. mainland © AMS 19

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 20

Hurricane Hazards § 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. 3 ft) surge at Pass Christian, MS – East Pakistan (now Bangladesh) in 1970 – storm surge killed ~300, 000 © AMS 21

Hurricane Hazards § What causes a storm surge? – Strong onshore winds combined with low air pressure § Low air pressure causes the water to rise about 0. 5 m for every 50 mb drop in pressure (or about 1 ft for every 1 in. of mercury drop in pressure) – The storm surge is superimposed on top of normal tides – A surge of 1 – 2 m (3 – 6. 5 ft) can be expected with a weak hurricane; that of a violent hurricane may top 5 m (16. 4 ft) – The greatest potential for a surge occurs with strong onshore winds, a shallow sloping shoreline, during high tide, and in densely-populated areas lacking coastal buffers – Storm surges are accurately predicted with a numerical model called SLOSH (Sea, Lake, and Overland Surges from Hurricanes) © AMS 22

Storm Surge © AMS 23

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) © AMS 24

Saffir-Simpson Hurricane Intensity Scale © AMS 25

Saffir-Simpson Hurricane Intensity Scale © AMS 26

Saffir-Simpson Hurricane Intensity Scale © AMS 27

Trends in Hurricane Frequency § The figure shows the recent upward trend in the number of intense (category 4 and 5) hurricanes worldwide, even though the overall number of hurricanes worldwide has declined since the 1990 s § Most of the increased frequency occurred in the North Pacific, SW Pacific, and Indian Ocean, with only a small increase in the North Atlantic © AMS 28

Trends in Hurricane Frequency § The North Atlantic is the only region of the globe where overall tropical cyclone activity has increased recently § Several factors contribute: – Higher SST in the tropical Atlantic related to the Atlantic Multidecadal Oscillation (AMO) – An amplified ridge over the central and eastern North Atlantic – Weaker vertical wind shear in the deep tropics over the central North Atlantic – Favorable African easterly jet stream © AMS 29

Hurricane Threat to the Southeast § The infrequency in major hurricanes during the 1970 s and 1980 s lulled many coastal residents of the southeast U. S. into a false sense of security and encouraged coastal development and growth § In 2005, NOAA reported that the coast is home to 53% of all Americans § Population growth is most rapid from Texas through the Carolinas, especially in Florida § Public safety officials are concerned about the trend toward more Atlantic tropical cyclones © AMS 30

Hurricane Threat to the Southeast © AMS Tracks of three of the four hurricanes that struck Florida in 2004 31

Hurricane Threat to the Southeast § Barrier islands are particularly at risk – A barrier island is an elongated, narrow accumulation of sand oriented parallel to the coast and separated from the mainland by a lagoon, estuary, or bay § Padre Island in Texas is the longest in the U. S. measuring more than 180 km (112 mi) – A constantly changing system § Sea waves dissipate their energy by shifting the sands and modifying the shape of the island § Gradually migrate toward the mainland § Face an open ocean and absorb the brunt of ocean storms – Cities such as Atlantic City, NJ, Miami Beach, FL, and Virginia Beach, VA are built entirely on barrier islands – The photographs show a barrier island at Pine Beach, FL before and after Hurricane Ivan caused a breach – Evacuation becomes critical here, as well as other coastal areas © AMS 32

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 – Cost of evacuation amounts to about $1 million per mile of coastline – Vertical evacuation may be an option © AMS 33

Evacuation § Other strategies to minimize loss of life and property: – Stringent building codes – Preservation of mangrove swamps – Elimination of federal floodplain insurance § The photograph shows a home designed so that the first floor will give way to storm-surge floodwaters. The second and third floor living areas are supported by wooden beams driven deeply into the sand © AMS 34

Long-Range Forecasting of Atlantic Hurricanes § Since the early 1980 s, Prof. William M. Gray and his colleagues at Colorado State University have issued seasonal hurricane activity forecasts for the Atlantic basin – First forecast issued 6 months before the hurricane season and then updates are made – Original basis of forecasts was apparent linkage between the frequency of hurricanes in the tropical Atlantic and rainfall in West Africa – Also factored in stratospheric quasi-biennial oscillation (QBO) – After this scheme didn’t work well for several years after giving skillful results, they developed a new one based on empirical relationships between Atlantic Basin hurricane activity and a combination of atmospheric factors in various parts of the world, including the QBO – In order to improve skill, a modified statistical forecast scheme was recently developed using data from 1950 -2007, and first implemented in the December 2007 forecast § NOAA has also issued a Seasonal Outlook for Atlantic basin hurricane activity since 1998 – Gives probabilities of overall seasonal activity compared to normal, and likely ranges of named tropical cyclones, hurricanes, major hurricanes, and © AMS Accumulated Cyclone Energy (ACE) 35

Hurricane Modification § Project STORMFURY (1961 -1983) – Spurred by 6 destructive hurricanes that affected U. S. East Coast during the mid-1950 s – Working hypothesis: seeding hurricanes with silver iodide crystals would reduce wind strength § This was supposed to increase latent heat and enhance convection just beyond the eye wall § A new eyewall would then form farther out and the hurricane’s circulation would theoretically weaken – Apparent modest successes were dismissed because convective clouds in hurricanes were found to have too little supercooled water for seeding to be effective © AMS 36