• tropical • landfall • • • hurricane • • supertyphoon • severetropical cyclone • severecyclonic storm • tropical depression • tropical storm • eyewall • tropicaldisturbance • majorhurricane • easterlywave • tropicalwave • Africaneasterly wave • MiddleLevel African EasterlyJet • horizontalwind shear • harmattanwinds • lntertropicalFront • CapeVerde storm • extratropicallow- pressuresystem • subtropicalcyclone • right-front quadrant • mesovortex • dropsonde • spiral bands • eye-wall LEARNING OBJECTIVES replacementcycle After reading this chapter, students will: • Be able to identify the seven ocean basins where occurs. • B_ea ble to identify the generic naming conventions used to describe the hierarchy of tropical in each of the seven basins (tropical depression, tropical storm, hurricane, etc.). Gainan appreciation for the history and protocol for generating the lists of specific names used in each of the seven ocean basins. Bea ble to assess the six criteria for tropical cyclogenesis in the setting of real eather data. 463 464 CHAPTER 11 Tropical Weather, Part II: Hurricanes CHAPTER 11 Tropical Weather, Part II: Hurricanes 465 • Understandthe genesis, movementand weather associated with easterly waves. · • Gain an appreciation for why tropical cyclogenesis is rare in the South At 1antic Ocean. • Understandthe positive feedback cycle, Conditionalln~tabilitrot t~e Second Kind,and its role in the development,maintenance and mtens1f1cat1onof tropical cyclones. _ . . I I • Understandthe underpinningscience of storm surge and its relat1onsh1pto landfalling tropical cyclones. • Understandthe physics that explains the existence of a hu_rricane'seye and gain an apprec'iation that winds in the eye are not always 1_1ght._ _ _ • Understandthe role of subtropical high-pressure systems 1nproviding steering currents for tropical cyclones. _ • Gain an appreciation for the role that the National Hurrican~ Ce_nterplays in warning the public, and be able to interpret some of the advisories and 4i@liJjffj (a) Radarreflectivity shows the eyeof Hur'.i~aneIke ~circula~blue r~gion) starting to moveover the TexasCoast at 0654Z products issued by NHC. on September13, 2008; (b) Dopplerveloc1t1es associated with HurricaneIke at 0404Zon September13, 2008. Fastw inds in the lowertroposphere blowing toward the radarat Houston,TX (shadesof blue anda little purple)were representative of the strong,onshore surfacewinds that produceda destructivestorm surge along the upperTexas coast (courtesy of NOAA).

On animations of satellite images , tropical cyclones On March 28, 2004, a rare hurricane over the South At­ TABLE11.1 Saffir-SimpsonHurricane Damage Potential Scale display a noticeable cyclonic circulation. They also have lantic Ocean ~truck the east coast of near the to:'111 an organized area of convection (thunderstorms) around of Torres abo;ut 800 km ( 500 mi) south of Rio de Janeiro Category Wind(knots; mph) StormSurge (m; f t) or near their center. Figure 11.2a is a radar reflectivity ( see Figure 11.1). Wind speeds estimated by the U. ~. 1: Minimal 64-82 74-95 1.0-1.7 4-5 image from Houston , TX around 07002 on September National Hurricane Center at 78 kt (90 mph) unoffi­ 13, 2008. A few minutes later, made land­ 2: Moderate 83-95 96-110 1.8-2.6 6-8 cially ranked the hurricane as a Category 1 on the Saf­ fall (the storm's center crossed over land) on the north­ 3: Extensive 96-113 111-130 2.7-3.8 9-12 fir-Simpson Damage Potential Scale ( see Tabl~ 11.1 ). ern tip of Galveston Island as a Category-2 hurricane . Locally dubbed Hurricane "Catarina" beca~se it came 4: Extreme 114-135 131-155 .3.9-5.6 13-18 Maximum sustained winds were approximately 95 kt ashore in the· province of Brazil, the storm 5: Catastrophic > 135 > 155 > 5.6 >18 (about 110 mph). Note the yellow and orange echoes killed at least two people , destroyed 500 homes, dam­ around the western flank of Ike 's well-defined eye , aged 20,000 others, and left 1,500 people homeless. which is the roughly circular island of generally light wind s at a hurricane's core. In this chapter , you'll learn Before meteorologists had reliable satellite imagery that thunderstorms organizing around (or near) the cen­ (prior to the mid-l 960s ), hurricanes could have formed ter of a are necessary for the low­ pressu re system to intensify. over the South and nobody would_have ever known (unless they hit land, of course) . But if the Tropical cyclones attract a great deal of attention from last four decades are any indication , there weren't many. meteorologists, and for good reasons. They destroy prop­ Indeed, Catarina was the only hurricane ever observ~d erty and take human life-tolls that can reach stagger­ in the South Atlantic Ocean during the modern satelh_;e ing proportion when these powerful storms pass over era Somewhat understandably, Catarina caught Brazi - highly populated, low-lying coastal areas that are vulner­ ian . forecasters by surprise.. There h ave b een only tw.o able to storm surge- the wind-driven rush of the sea other tropical cyclones observed over t h e s ou thAtlant1c into coastal areas as a strong tropical cyclone arrives. Ocean since the 1960s, in April 1991 and January 2_004 . Figure 11.2b is an image of Doppler velocities around For the record tropical cyclone is. t h e generi·c, umver- 04002 on September 13, 2008 , a few hours before the sal name given' to low-pressure systems th· a t form ov,er ~eflectivity image in Figure 11.2a. This velocity image 11 llldicates a swath of fast winds in the lower troposphere warm tropical or subtropical· seas . I. n t h_ is· cha pter ' youeft of blowing toward Houston and the Seaboard (shades A visiblesatellite image of a rarehurricane off the discover why the South Atlantic 1s virtuall y her . es ii@iiJjli• . h Atl t. c sometnn of blue and a bit of purple). These onshore winds pro­ j~@hJIJiiPhotographs of a portionof the upperTexas eastcoast of Brazilon March26, 2004 (courtesy tropical cyclones , while the Nort an i 11 1 duced a formidable storm surge that destroyed parts of ••• •••• •·-•·•-•-• Coastbefore (top) andafter (bottom) Hurricane of NASA). teems with them. th Ike.The yellow arrows point out featuresthat appear in eachimage e coast (see Figure 11.3). From space , the swirling (courtesyof USGS) 466 CHAPTER 11 Tropical Weather, Part II: Hurricanes CHAPTER 11 Tropical Weather, Part II: Hurricanes 467

60N

45N

30N

15N

EQ

15S

30S

45S

60S (a) (b) 0 30E 60E 90E 120E 150E 180 150W 120W 90W 60W 30W @j@jjJIII•(a) Spiral Galaxy Messier 101 bearsa strikingresemblance to somehurricanes. This awesomeimage is a compositeof about50 individualexposures from the HubbleSpace Telescope (courtesy of NASAand ESA); (b) HurricaneIvan on Ni@!ililFITheprimary global breeding grounds for tropical cyclones(shaded areas) and the typicaltracks they take (redarrows). September5, 2004 (courtesyof NASA).

force winds are known simply as severe tropical cyclones Globally each year, approximately 80 to 90 tropical cy­ galactic-arm appearance on satellite images, together 2. Northeast Pacific Basin (from to the Inter­ (Austra lian forecasters refer to "weaker" hurricane­ clones reach tropical storm intensity, with about two-thirds with an eye staring menacingly into space, absolutely national Dateline) strength storms simply as cyclones). Continuing on our of these attaining the threshold of 64 kt (74 mph) to qual­ invites ,investigation by the scientifically curious (see 3. Northwest Pacific Basin (from the International Date­ world tour, we note that forecasters call a strong tropical ify as a hurricane ( or typhoon, etc.). In order to efficiently Figure 11.4). line to Asia, plus the South Sea) cyclone in the North a severe cyclonic communicate information about these systems and avoid 4. North Indian Basin (includes the Bay of Bengal and storm. Finally, in the Southwest Indian Ocean, the generic confusion when more than one is active at the same time, Giii.liii~~~~.::;..t!!:.,...~"""'llllll.. Check out the satellite loop of Hurricane tropical cyclone is the chosen designator. meteorologists give them a number-letter tag when they be­ Isabel (2003) on the companion CD, and the Arabian Sea) come a tropical cyclone. For example, Tropical Depression also a movie of the angry ocean surface under Isabel 5. Southwest Indian Basin (from Africa to about 100°E From "hurricane" to "typhoon" to "severe cyclonic storm," the labels that meteorologists attach to strong trop­ 2-E would be the second tropical cyclone of the season in taken from a hurricane hunter aircraft. longitude) 6. Southeast Indian/Australian Basin (100°E longitude ical cyclones vary widely across the globe. As it turns the Northeast Pacific Basin. This generic naming conven­ As you probably already know, the most intense trop­ to 142°E longitude) out, the lists of names that forecasters use to distinguish tion never changes. ical cyclones that form in the tropical Atlantic and north­ 7. Australian/Southwest Pacific Basin (142°E long i­ storms that form in a particular basin also vary across the Most tropical cyclones receive a name once they reach east Pacific Oceans, with sustained surface winds of at tude to about 120°W longitude) globe. Before we delve into these naming conventions, tropical storm intensity. Table 11.2 shows the six alpha­ least 65 kt (74 mph), are called hurricanes . How about we first need to trace the life cycle of a typical tropical cy­ betized lists ofnames currently used by the National Hur­ the rest of the world? Figure 11.5 contains a lot of information that we will clone because, in some ocean basins, assigning a name ricane Center for the North Atlantic Basin. The lists of explore in this chapter. One observation that we'd like to depends on the cyclone's stage of development. names are recycled. So, for example, the 2009 list will be used again in 2015. The only exception occurs when TROPICALCYCLONES: A GLOBAL make right off the bat is that tropical cyclones do not NamingTropical Cyclones: Ham with Chicken form on the equator. Also note the absence of tropical hurricanes or tropical storms are extremely destructive PERSPECTIVE Livers andMushrooms cyclones in the South Atlantic Ocean. Catarina was a or deadly. In these cases, their names are "retired " from There are seven ocean basins in which tropical cyclones rare bird indeed . On the way to becoming a hurricane, an intensifying the list because using the name again for a future storm routinely form: We recommend that you glance at Fig­ Strong tropical cyclones go by different names in the tropical low-pressure system evolves from a tropical de­ would be inappropriate and/or insensitive. The World ure 11.5 while you read through the following list. To get other five basins. In the Northwest Pacific Basin , fore­ pression to a tropical storm. A tropical depression is a Meteorological Organization, ·a specialized agency of your bearings, we point out that the shaded areas in Fig­ casters use typhoon to label a tropical cyclone whose tropic al cyclone that has an observable cyclonic circu­ the United Nations headquartered in Geneva, Switzer­ ure 11.5 mark the common breeding grounds for tropi­ maximum sustained winds have reached the hurricane lation on satellite imagery and whose maximum sus­ land, then chooses a new name as a replacement. A re­ cal cyclones . The red arrows indicate the directions that threshold of 65 kt (74 mph), and supertyph oon if tained winds are 33 kt (38 mph) or less. A tropical cent example of a retired name is Dean in 2007, which tropical cyclones typically move within and beyond storm is a tropical cyclone with maximum sustained will be replaced by Dorian in 2013. Prior to the 2008 hur­ these breeding grounds. wind speeds reach 130 kt (150 mph). In the Sout~­ west Pacific Ocean west of l 60°E longitude and also in Winds of 34- 63 kt (39- 73 mph). Of these different ricane season , approximately 70 names had been retired 1. Atlantic Basin (the North Atlantic Ocean, the Gulf the Southeast Indian Ocean east of 90° longitude (gets a stages of development, only hurricanes are distinguished from the North Atlantic lists of names, including Agnes of Mexico, and the Caribbean Sea) little dicey, eh?), strong tropical cyclones with hurricane- by the characteristic eye in the center of the circulation. (1972), Andrew (1992), Tropical Storm Allison (2001), 468 CHAPTER 11 Tropical Weather, Part II: Hurricanes CHAPTER 11 Tropical Weather, Part II: Hurricanes 469

TABLE11.2 AtlanticBasin Tropical Storm and Hurricane Names male names. Instead, they included Asian words that re­ ferred to flowers, animals, birds, trees, foods, etc., while 2009 2010 2011 2012 2013 other names are simply descriptive adjectives . Ana Alex Arlene Alberto Andrea Arthur For example, consider Ketsana and Parma, Bill Bonnie Bret Beryl Barry Bertha which patrolled the Northwest Pacific Basin in late Oc­ Claudette ' Colin Cindy Chris Chantal Cristobal tober 2003 (see Figure 11.6). Ketsana (contributed by Danny Danielle Don Debby Dorian Dolly Lao People's Democratic Republic) means "a kind of tree," and Parma ( contributed by , China) means Erika Earl Emily Ernesto Erin Edouard "ham with chicken livers and mushrooms"! Hungry for Fred Fiona Franklin Florence Fernand Fay a tropical cyclone, anyone? Table 11.3 shows the names Grace Gaston Gert Gordon Gabrielle Gustav for this basin- you'll note that they do not appear in al­ Henri Hermine Harvey Helene Humberto Hanna phabetical order. Rather, the contributing nations are Ida Igor Irene Isaac Ingrid Ike listed in alphabetical order. This list of countries deter­

'~ 1 Joaquin Julia Jose mines the order in which names are assigned . / Joyce Jerry Josephine ? Kate Karl Katia Kirk Karen Kyle Before October 2004 , tropical cyclones that formed in the North Indian Ocean were not named from a tradi­ Larry Lisa Lee Leslie Lorenzo Laura tional list. For this basin, forecasters simply used a label Mindy Matthew Maria Michael Melissa A visiblesatellite image of SuperCyclonic Storm Marco consisting of a two-digit number and letter that the cy­ @i@Mil•• Gonuat 09Zon June4, 2007.On this date, Nicholas Nicole Nate Nadine Nestor Nana clone received once it attains tropical depression status. Gonu'smaximum sustained winds reached 140 kt (161mph), Odette Otto Ophelia Oscar Olga Omar For example, "Tropical Cyclone 02A" would be the sec­ makingit the strongesttropical cyclone ever to developin the North Peter Paula Phillippe Patty Pablo Paloma ond tropical cyclone of the season to form over the Ara­ IndianOcean (courtesy of MODISRapid Response Project at NASA/GSFC). Rose Richard Rina Rafael Rebekah Rene bian Sea. "Tropical Cyclone O1 B" would be the first Sam Shary Sean Sandy tropical cyclone of the season to form over the Bay of Sebastien Sally Bengal. Teresa Tomas Tammy Tony Tanya Teddy One of the most infamous named storms in the North cyclone ever to develop in the North Indian Ocean. As Victor Virginie Vince Valerie Van Vicky Indian Ocean was Severe Cyclonic Storm Gonu, which Gonu moved toward the Arabian Peninsula, the storm Wanda Walter Whitney William Wendy Wilfred developed over the Arabian Sea in early June 2007 (see weakened considerably as it drew dry air over the desert into its circulation. Figure 11. 7). On June 4, Gonu's maximum sustained J winds reached 140 kt (161 mph), compelling forecast­ The notion that dry air circulating into Gonu would cause the storm to weaken is part of a much broader dis­ Isabel (2003) , Charley, Frances, Ivan and Jeanne (2004), ers to upgrade Gonu to a super cyclonic storm. As of States. From 1950 to 1952, meteorologists named trop ­ cussion about the conditions that are favorable ( or un­ Dennis, Katrina, Rita, Stan and Wilma (2005), and Dean, this writing, Gonu stands as the most powerful tropical ical cyclones in the Atlantic Basin according to the pho­ favorable) for the genesis and development of tropical Felix and Noel (2007). netic alphabet (Able, Baker, Charlie, etc.). Then, in 1953, cyclones. Now that we've played the "name game," let's The custom of naming tropical storms and hurricanes the U.S. Weather Bureau switched to an alphabetized list investigate these conditions. has an interesting history. Contrary to popular belief, offemale names. In 1979, the the practice of naming Atlantic hurricanes dates back a amended their lists to also include male names. RECIPEFOR HURRICANES: few hundred years to the West Indies (the three main In 1959, forecasters monitoring the central Pacific Ocean groups of islands that comprise the West Indies are the started to use female names to designate tropical storms SIX INGREDIENTSIN JUST Bahamas, the Greater Antilles, and the Lesser Antilles) . and hurricanes that formed near Hawaii. The rest of the THERIGHT MEASURE In the nineteenth century, islanders began to name hur­ Northeast Pacific Basin followed suit in 1960. In 1978, So far you've learned that tropical cyclones have an ricanes after saints. Indeed, when hurricanes arrived on the lists of names for tropical storms and hurrican es in observable cyclonic circulation on loops of satellite a day commemorating a saint, locals christened the these basins were amended to include male names as well. imagery. storm with the saint's name. For example, fierce Hurri­ In the Northwest Pacific Ocean, forecaste rs began cane Santa Ana struck Puerto Rico on July 26, 1825. using female names for tropical cyclones in 1945. In ~ C~e~k out th~ companion C~ to watch a stnkmg satellite loop of a hurncane. Hurricane San Felipe (the first) and Hurricane San Fe­ tandem with the 1979 change in the , they ~ lipe (the second) hit Puerto Rico on September 13, 1876 also revised their lists to include male names. On Janu­ There are also organized thunderstorms concentrated and September 13, 1928, respectively . ary 1 2000, however, in a dramatic departure from tra- around or near their centers . ·w e will elaborate on the During World War II, Navy and Army Corp forecast­ dition' , forecasters from the nations. and ternton · ·es m · the.f role that organized convection plays in the intensifica­ ers informally named Pacific storms after their girl­ Northwest Pacific Basin agreed to use a markedly di - tion of tropical cyclones later in this chapter. First, how­ friends or wives . The spirit of this informal naming ferent naming convention. From that day forward, ~e , A visiblesatellite image of TyphoonsParma and ever, we will discuss the six ingredients required for the - ~ ~ Iii Ketsanaon October24, 2003.The southern tip convention apparently started a trend in the United new lists did not (for the most part) contam. ma1 e 0 r 1e- of appearsin the upperleft of the image(courtesy of NASA) genesis of tropical cyclones. For students interested in a tropical cookbook, here 's the recipe: 470 CHAPTER 11 Tropical Weather, Part II: Hurricanes CHAPTER 11 Tropical Weather, Part 11:Hurricanes 471

TABLE11.3 NorthwestPacific Tropical Storm and Typhoon Names li@i;Jii•:jfhe long-term averageof sea­ Thenames for tropicalstorms and typhoons that developin the NorthwestPacific Basin are Asian words submitted surfacetemperatures (in °C) in by countriesin the region.The names are compiled into "runninglists": Afterall the namesin onelist are used,the the AtlanticBasin from June1 to nameof the nextstorm that developsis simplythe first word in the nextlist. November30 (courtesyof NOAA). NORTHWESTPACIFIC BASIN

Contributor II Ill IV V Damrey Kong-rey Nakri Krovanh Sarika China Haikui Yutu Fengshen Dujuan Haima DPRKorea Kirogi Toraji Kalmaegi Mujigae Meari HK,China Kai-Tak Man-yi Fung-wong Choi-wan Ma-on Japan Tembin Usagi Kanmuri Koppu Tokage ~LaoPDR Average Sea-SurfaceTemperature (°C) - June to November Bolaven Pabuk Phanfone Ketsana Nock-ten Macau Sanba Wutip Vongfong Parma Muifa Malaysia 22 23 24 25 26 27 28 29 Jelawat Sepat Nuri Melor Merbok Ewiniar Fitow Sinlaku Nepartak Nanmadol Malaksi Danas Hagupit Lupit Talas Don't get nervous. Although these ingredients might Any way you slice it, high SSTs pave the way for deep ROKorea Gaemi Nari Jangmi Mirinae Noru seem daunting at first, they all relate to the two charac­ convection, which newly forming and intensifying trop­ Prapiroon Wipha Mekkhala Nida Kulap teristics that all tropical cyclones display-organized ical cyclones require around their centers. U.S.A. Maria Francisco Higos Omais Rake convection around or near their centers and an observable To close the deal on the pivotal role that high sea-sur­ Son-Tinh Lekima Bavi Canson Sonca cyclonic circulation on satellite imagery. Keep these two face temperatures play in the genesis and development of Cambodia traits in mind as we explore each of the six ingredients. tropical cyclones, we point out that Hurricane Charley Bopha Krosa Maysak Chanthu Nesat rapidly intensified to a Category-4 storm (110 kt to 125 China Wukong Haiyan Haishen Dianmu Haitang kt in just three hours) as it bore down on the west coast DPRKorea Sonamu HighSea-Surface Temperatures: The Podul Noul Mindulle Nalgae of Florida on August 13, 2004 (see Figure 11.11). Fig­ HK,China Foundationfor OrganizedThunderstorms Shanshan Lingling Dolphin Lionrock Banyan ure 11.12 shows a satellite-based analysis of sea­ Japan Figure 11.8 shows average sea-surface temperatures Yagi Kaziki Kujira Kompasu Washi surface temperatures on August 12, 2004. Note the very LaoPDR (SSTs) over the North Atlantic Ocean from June 1 to Leepi Faxai Chan-horn Namtheun Pakhar warm water just off Florida's southwest coast. Indeed, November 30, the period that corresponds to official Macau Bebin ca Peipah Linfa Malou Sanvu hurricane season in this basin. Note the corridor of water Malaysia Rumbia Tapah Nangka Meranti Mawar temperatures of 26.5°C (80°F) or higher that extends 0 ;: ... N ;: 0 ~ 0 0 Micronesia Soulik Mitag ... N ..... N ~ ... N Soudelor Fanapi Guchol from the west coast of Africa across the tropical Atlantic Qi WI 1111 .... u u C ::, 0. .... > Qi Qi ::, Qi u u 0 Philippines Cimaron Hagibis Malave Malakas Talim to the Caribbean Sea and the Gulf of Mexico (roughly, ~ < < v> 0 0 z Q Q ROKorea the yellows and oranges). This corridor marks "hurri­ 110 I'.!• Jebi Neoguri Goni Megi 100 : : Doksuri > Thailand Mangkhut cane alley," where tropical cyclones routinely develop 90 Rammasun Morakot Chaba Khanun 80 ~ ' during hurricane season. 70 ~ U.S.A. Utor Matmo Etau Aere 0. Vicente The heart of season is August to 60 Vietnam 50 e Trami Halong Vamco Songda Saola October, when there's a marked upturn in the frequency 40 ...~ of tropical cyclones. In light of Figure 11.9, which shows 30 o : 20 1:: 10 E ' the daily frequency of Atlantic hurricanes and tropical ::, : 0 :z storms, it's obvious that hurricane season peaks during Hurricanes and Tropical 5 tor ms the second week of September. Not coincidentally , sea­ 1. Sea-surface temperatures of26.5°C (80°F) or higher 4. Weak vertical above the newly forming • Hurricanes NOAA surface temperatures in the North Atlantic are also at and a relatively deep layer of warm water beneath tropical cyclone their highest during this month ( see Figure 11.10). the ocean surface 5. A genesis location that lies at least 5 ° latitude away l"ll!lll""'!l"W!laThedaily frequency of hurricanes(yellow) and High sea-surface temperatures ensure high evapora­ 2. Conditional instability through a deep layer of the from the equator If \M'lilllfP bothhurricanes and tropical storms (red) in the tion rates and thus high dew points in the lower tropo­ AtlanticBasin, per 100years. Climatologically , the peakof the troposphere 6. A group of initially disorganized showers and 3. Moist air in the middle troposphere sphere. High water temperatures also help to destabilize tropicalseason in the AtlanticBasin is aroundSeptember 10 thunderstorms the lower troposphere by providing a "warm bottom." (courtesyof NOAA). CHAPTER 11 Tropical Weather, Part II: Hurricanes 472 CHAPTER 11 Tropical Weather, Part II: Hurricanes 473

Thelong-term FIGURE11.10 averageof sea­ 30N · ~-IOM- ClRES/ Cllrnot,,- OlognosU..._,.,,oa Cent.er surfacetemperatures (in °C) in the AtlanticBasin during September Mexico (courtesyof NOAA).

20N Pacific Ocean

Average Sea-SurfaceTemperature (°C)- September

22 23 24 25 26 27 28 29 Sea-Surlace Temperature Anomaly (°C) , October 1, 1997

SSTs in the neighborhood of 32°C (almost 90°F) lay in layer of water below the surface must also be warm. -2 -1 0 2 the path of Hurricane Charley. The stage was set for rapid Let's investigate. FIGURE11.13 A visiblesatellite image of HurricaneNora off Thedeparture of sea-surfacetemperature FIGURE11.14 intensification once Charley moved over these very warm the westcoast of Mexicoin September1997 (in °C) from averageon October1 , 1997,still waters ( other factors also likely contributed to Charley's WarmWater Below: When the WindStirs (courtesyof NOAA). reflectedthe coolingof the oceansurface along the track of Hurri­ ). the Sea caneNora . Themore southern pocket of cooler-than-averagewater Not only must sea-surface temperatures be high for marksthe regionwhere Nora stalled. The second pocket of relatively tropical cyclones to form and develop, but a fairly deep A newly forming but slowly moving tropical cyclone can ing this time, Nora's maximum sustained winds decreased low sea-surfacetemperatures just southof BajaCalifornia reflects sputter and fade before it ever really develops. To see how, from 90 kt (104 mph) to 65 kt (75 mph). Why would this the upwellingof cool waterassociated with Nora'sre-intensification let's start on the high end of tropical cyclones by focus­ weakening occur? With Nora's winds continuing to agi­ on September21 when115-kt (130-mph) winds reallystirred up ing our attention on Hurricane Nora, which patrolled the tate and mix the same area of sea, cooler water upwelled the sea(courtesy of NOAA). Northeast Pacific Basin from September 16-26, 1997, from depths of several tens of meters (while warmer sur­ off the southwest coast of Mexico (see Figure 11.13). face water also mixed downward). In response to the up­ From midday on September 18 to early on September welling, sea-surface temperatures in this region decreased Incidentally,hurricanes and tropical storms that pass over 20, the storm essentially stalled just off the coast. Dur- by as much as 2°C (3.6°F), creating a pocket of anom­ the cool wake of a previous storm can also lose strength. alously cool water off the southwestern coast of Mexico Again, the organized convection around the center of such that persisted for days (see Figure 11.14). As a result of a tropical cyclone would likely weaken, with the degree of cooler surface water, evaporation rates lowered, the at­ weakening depending on how cool the water was and how mosphere stabilized a bit, and organized, tall thunder­ long the storm stayed over the cool wake. storms around Nora's eye began to collapse. In other These arguments about the role of water temperatures words, the hurricane weakened. in the intensification (or weakening) of tropical cyclones With this example in mind, you can imagine what might assume, of course, that the other ingredients on the trop­ happen if a newly forming tropical cyclone encountered ical cyclone recipe don't affect the storm's intensity. tropical seas where the layer of warm water beneath the Speaking of other ingredients, let's move on to the second surface was relatively shallow. Indeed, as winds began to item-the atmosphere must _be conditionally unstable strengthen, cooler water would quickly upwell to the sea through a deep layer of the troposphere . surface, in effect limiting further development. After all, if fully developed hurricanes such as Nora take a big hit, ConditionalInstability: Paving the Sea-SurfaceTemperature (°C)-August 14, 2004 imagine the effects on a newly forming and rather frag­ Wayfor TallThunderstorms 32 33 ile tropical cyclone. So you can see why tropical meteo­ 26 27 28 29 30 31 Truth be told, high sea-surface temperatures go hand­ FIGURE11.11 A compositeradar image of HurricaneCharley rologists stipulate that high sea-surface temperatures must alongFlorida's southwest coast at 21Z on August A satellite-basedanalysis of sea-surfacetem­ in-hand with a conditionally unstable troposphere. In FIGURE11.12 be accompanied by a relatively deep warm layer below the 13, 2004,just afterthe stormcame ashore near Cayo Costa with peratureson August12, 2004.Note the very ocean surface in order to promote genesis of tropical other words, the two ingredients are not independent. 5t maximumsustained winds near 125 kt (145mph) (courtesyof WSI warmwater (shown here in yellow)off Florida'ssouthwest coa cyclones. That 's because high SSTs ensure high evaporation Corporation). (courtesyof NOAA). CHAPTER 11 Tropical Weather, Part II: Hurricanes 475 474 CHAPTER 11 Tropical Weather, Part II: Hurricanes Someair parcels eye of Gonu represents cloud-top temperatures of -80 °C Even though they have relatively weak updrafts, tall FIGURE11.15 reachingthe top (-l 12°F) or lower, which, in turn, indicates a ring of eye-wall thunderstorms are integral cogs in the power­ of thunderstormsaround the center very tall thunderstorms around the eye, formally called ful hurricane machinery (more details later in the chap­ of a newlyforming tropical cyclone the eye wall . ter) . For now, just keep in mind that high sea-surface sink into the centralair column. As ferocious as the tall, eye-wall thunderstorms might temperatures, a relatively deep layer of warm water Becausethey are practicallydevoid seem, the updrafts that sustain these thunderstorms are below the surface, and conditional instability through a of watervapor, these sinking air relatively tame. To explain this apparent paradox, recall deep layer of the troposphere, are primary ingredients parcelswarm dramaticallyat a rate from Chapter 9 that the strength of a thunderstorm's up­ for cooking up a tropical cyclone. There's one other ther­ closeto dry adiabatic. draft depends largely on the degree of positive buoy­ modynamic ingredient (related to temperature and mois­ ancy--essentially , the difference between the temperature ture) crucial to the development of a tropical cyclone. of a rising air parcel and the temperature of the envi­ ronment. Over warm tropical seas, the environmental lapse rate in the middle troposphere is pretty close to Ingredient13: A MoistMiddle Troposphere the moist adiabatic rates that rising air parcels typically The color-enhanced water vapor image in Figure 11.18a density and weight. In other words, there is relatively follow there. As a result , positive buoyancy is often sur­ shows Super .Cyclonic Storm Gonu over the Arabian rates, which , in tum -, ensure that the air above the warm low pressure at the surface. prisingly small inside eye-wall thunderstorms. Thus, the Sea at 18Z on June 4, 2007. At the time, Gonu's maxi­ ocean is teeming with water vapor. In Chapter 8, yo~ The taller and more organized the thunderstorms be- updrafts that sustain them tend to be rather weak. mum sustained winds were 140 kt (161 mph) , making learned that high dew points in parcels ~ear the earth s come around the center of a tropical cyclone, ~he greater Relatively weak updrafts in the eye wall help to ex­ it the strongest tropical cyclone ever to form over the surface mean that net condensation readily occurs aft~r the compressional warming during descent m the cen­ plain the general lack of lightning around the eye of a North Indian Ocean. In this image, the blue shading in­ a relatively short ascent. Such rising parcels can remam tral column. Thus, as thunderstorms around the center of hurricane. These weak updrafts are rather inefficient at dicates dry air in the middle and upper troposphere. positively buoyant to great altitudes because lar~e re­ a tropical cyclone grow taller, the surface pressu~e tends separating electrical charges- that is, creating regions Within the next several hours, as Gonu moved westward leases oflatent heat keep parcels warmer than their en­ to decrease. Figure 11.16 is a color-enhanced mfrared with large positive and large negative charges that are toward the Arabian Peninsula and started to draw dry, vironment through a deep layer of the troposphere. Such satellite image of Super Cyclonic Storm ?onu at l SZ on necessary to produce lightning. As an example , the eye mid-tropospheric air into its circulation , the historic ascent to great altitudes is pivotal for tall thunderstorms June 4, 2007 . The circular yellow shadmg around the wall of , the last Category-5 hurri­ storm took a big hit. By 1230Z on June 5 (Figure to form and organize around the center of a newly form- cane to hit the United States, produced cloud-to-ground 11.1Sb), Gonu's maximum winds had weakened to ing tropical cyclone. _ . lightning at arate less than ten strikes per hour between 105 kt (121 mph). The tropical cyclone continued to Although high sea-surface temper~tures and_condit1onal the time it was over the Bahamas and the time it made weaken as it ingested more and more of this low-humid­ instability are not independent , we hst Ingred1_ent #2 sep­ its second landfall in Louisiana ( see Figure 11.17). There ity air from high over the deserts of the Middle East. arately here because it gives us the opportumty to sho~ were even hours without any strikes at all! By compar­ The message here should be loud and clear- dry air in you how tall thunderstorms around the center of a tropi­ ison, thunderstorms 100 km (60 mi) or more from a hur­ the middle troposphere is poisonous to hurricanes. cal cyclone help the cyclone to develop. ricane's core can produce cloud-to-ground lightning at Logically , if hurricanes take a hit when they ingest By the time air parcels reach the tops of thundersto~s rates as high as several hundred strikes per hour. dry air in the middle troposphere, imagine the effect of (we're talking very high altitudes here), most of their water vapor has been depleted. Hold _this thought fo_ra moment. Figure 11.15 indicates that air parcels reach~ng the tops of thunderstorms near the center of a developmg Thetrack of Hur­ tropical cyclone take one of two routes. Some parc_els FIGURE11.11 move outward away from the developing center, creatmg ricaneAndrew, August16-28, 1992.Positions are upper-level divergence, which helps to maintain the low shownevery six hours.The color of weight of the central air columns. the dots correspondsto the tropical cyclone'sintensity, from tropical Check out "Hurricane Structure" on the depressionto tropicalstorm to ei ~ ~ =--11)companion CD; it shows the movement of the five Categorieson the Saffir­ air parcels within a hurricane. Simpsonscale (courtesy of Wikipedia). Alternatively , some parcels sink into the central col­ umn of the tropical cyclone. Because these parc~ls have A color-enhancedinfrared satellite image o f FIGURE11.16 very little water vapor, they warm by compr~ss10n at a SuperCyclonic Storm Gonu at 18Zon June rate close to dry adiabatic. In response, the air h_eatsup 4 2007(from the Meteosat-7geostationary satellite) . Theyell o~oF) shadingwhich represents cloud-top temperatures of -soac (-11f dramatically. As a result, the central air column m a de­ ' · th ye wall o veloping tropical cyclone has the highest average tem­ or lower,marks the tops of tall thunderstormsin e e Gonu(courtesy of NOAAand the NavalResearch Laboratory ). perature and, accordingly , the lowest average column 476 CHAPTER 11 Tropical Weather, Part II: Hurricanes CHAPTER 11 Tropical Weather , Part II: Hurricanes 477

These high-altitude winds essentially helped to push the deep convection southeastward away from the low-lev 1 circulation around the center of Nicholas. e Meanwhile, at the 850-mb level (about 1500 m) the winds blew pretty much from the southeast at m;dest speeds compared to 200 mb (see Figure 11.20b). With modest southeasterlies at 850 mb and stronger northwest­ erly winds at 200 mb, the low-level circulation associ­ ated with Tropical Storm Nicholas got separated from the tall thunderstorms that previously formed around the cen­ ter of the tropical cyclone. Such is the disruptive power of vertical wind shear. In the context of hurricane genesis, the most relevant as­ pect of wind shear is the change in wind speed with alti­ tude. To calculate the vertical wind shear in the layer between 850 mb and 200 mb, we simply subtract the wind vector at 850 mb from the wind vector at 200 mb. Thus, -so -40 -30 -211 wind shear has the same units as wind speed-perhaps (a) (b) FIGURE11.19 A multi-channelsatellite image of TropicalStorm mis or mph. Calculating the direction of the wind shear Nicholasin the AtlanticOcean on October20, (a) A color-enhancedwater vapor image of SuperCyclonic Storm Gonu over the ArabianSea at 18Zon June4, 2007 is a bit more complex because it involves breaking down FIGURE11.18 2003.Yellowish tones indicate clouds with low tops, whilehigh (fromthe Meteosat-7geostationary satellite). Maximum sustained winds were 140 kt (161 mph)at the time.The blue the winds at 850 mb and 200 mb into their east-west and cloudtops appearas whiteor whitishgray. At the time, Nicholas indicatesdry air in the middleand uppertroposphere over the ArabianPeninsula and other parts of the MiddleEast; (b) A color-enhanced north-south components. was northeastof Guyanain SouthAmerica (courtesy of NOAA). watervapor image of Gonuabout 18 hourslater. By this time, the storm hadweakened after ingesting dry, mid-troposphericair overthe We '11dispense with the details and show you Figure MiddleEast. Maximum sustained winds had dropped to 105 kt (121mph) (courtesyof the NavalResearch Laboratory). 1l .20c, which is the average 850-200 mb wind shear for October 20, 2003. As a general rule, values of vertical agery). A single multi-channel satellite image can occa­ wind shear less than 10 m/s (in white here) are suffi­ sionally accomplish the same result when a tropical cy­ ciently low to allow genesis and development of tropical clone's low-level circulation becomes "exposed." This dry, mid-level air on a newly forming and somewhat the disturbance. To the extent that new convection is piv­ cyclones; in other words, these values correspond to happens when thunderstorms near and around the center fragile tropical cyclone. It can't be good for develop­ otal for the development of the disturbance into a tropi­ weak vertical wind shear. Clearly, the northwesterly ver­ of circulation move away from the storm's inner core. ment. To understand why, consider that the seed for most cal cyclone, the die is cast and the disturbance fizzles (or tical wind shear on this date was strong enough to sepa­ Given that we can observe Nicholas 's "exposed" low­ tropical cyclones is typically a group of disorganized at least doesn't develop any further). rate Nicholas' deep convection from the storm's low-level level circulation on Figure 11.19, we can infer that the showers and thunderstorms that meteorologists call a Now that we've discussed all three of the thermody­ center of circulation. storm was in a "highly sheared environment" at this time tropical disturbance . Until thunderstorms become bet­ namic ingredients, let's move on to the dynamic ingre­ Figure 11.21 shows the long-term average 850-200 (we'll explain what this means in just a moment). ter organized around a single, well-defined center of cir - dients (related to air motions) needed for the genesi s mb vertical wind shear over the Atlantic Basin during the First, a final point about Nicholas. As you have already culation, the system's development will be limited. and development of tropical cyclones. heart of hurricane season, from August 15 to October learned, the lack of deep convection around the inner core That's because the disorganized warming associated with 15. In the areas in white, vertical wind shear is less than of a tropical cyclone suggests that the system is doomed. air sinking around disorganized thunderstorms near the 10 mis, indicating that favorable wind-shear conditions Not surprisingly, the National Hurricane Center down­ core of the disturbance tends to cause pockets oflow pres­ VerticalWind Shear: A LittleGoes prevail during this time ( on average) over much of the graded Nicholas to a tropical depression on October 23. sure to jump around like a hot baked potato being tossed a LongWay basin. Given the region's favorable thermodynamic en­ If you had an inking that a "highly sheared environ­ around the dinner table. Figure 11.19 is a multi-channel ( combination visible/ vironment, tropical cyclones commonly form and in­ ment" referred to vertical wind shear, then you are right When a newly forming tropical cyclone mixes infrared) satellite image of Tropical Storm Nichola s at tensify here during this time. Keep in mind that Figure on the money. As you recall from Chapter 9, vertical wind in, or entrains, dry air from the middle troposphere, l 5 l 5Z on October 20, 2003. On this type of image, the 11.21 shows average conditions during a two-month pe­ shear is a change in wind speed and/or wind direction chances are that it will fizzle. Why is that? Mid-level dry yellowish shades represent low clouds, while bright riod-wind shear over any specific region at any spe­ with increasing altitude. In Nicholas' case, it was north­ air entrained into thunderstorms enhances evaporational white typically marks the tops of cumulonimbus clouds cific time can be more or less favorable for tropical westerly shear that exposed the storm's low-level circula­ cooling, so parcels become more negatively buoyant. This (a duller, bluish white usually indicates cirrus clouds). cyclogenesis (tropical cyclqne formation and/or inten­ tion and set the stage for the tropical cyclone to dissipate. leads to stronger downdrafts , which are able to penetrate Note the swirl of yellowish low clouds that marks the sification), depending on the prevailing weather pattern. Consider the analysis of winds at the 200-mb level (about farther down into the lower troposphere, bringing cooler, low-level circulation of Tropical Storm Nicholas (keep The bottom line here. is that vertical wind shear over 12,500 m) on October 20, 2003, in Figure 11.20a, three drier air along with them. This downward intrusion acts in mind that all tropical cyclones have a cyclonic ci~cu­ a tropical disturbance should be weak if the disturbance hours before the satellite image. Note the northwesterly to snuff out new convection in the vicinity of the core of lation that's typically observable on loops of satellite un- is ever going to develop into a tropical cyclone. This de­ Winds (represented by arrows) in the vicinity ofNicholas. bate about vertical wind shear becomes moot, however, 478 CHAPTER 11 Tropical Weather, Part II: Hurricanes CHAPTER 11 Tropical Weather, Part II: Hurricanes 479

air to the right ( or to the left in the Southern Hemi­ sphere) . Without a sufficiently strong Coriolis force, air would move almost directly toward the center of the low rapidly adding mass to the air column over the low and thus causing air pressure to increase. In effect, a tropi­ cal disturbance too close to the equator would not have the opportunity to develop a low-level circulation and become a tropical cyclone.

Average 850-200 mb Wind Shear (m/s), August 15- October 15 Figure 11.22 dramatically illustrates our point. Here, the Indian and Pacific Oceans were partitioned into 10 14 18 22 26 30 boxes measuring one degree latitude by one degree lon­ gitude. Then the number of times that a tropical cyclone FIGURE11.21 Thelong-term average vertical wind shearbe­ tween850 mb and200 mb from August15 passed through each box during the 30-year period from to October15. Arrowsrepresent the directionof the wind shear, 1972 to 200 I was counted. This chart displays that fre­ whilethe magnitudeof the wind shearis color-codedin m/s quency on an annual basis, with the dark red splotches (courtesyof NOAA). marking the m·ost active areas for tropical cyclones, dark blue indicating relatively low activity, and white repre­ /) I \_ senting areas that, for all practical purposes, had zero 200-mb Winds (m/s} if the tropical disturbance is too close to the equator, activity. Note the absence of tropical cyclones on and which leads us to our next ingredient. near the equator . As a generally steadfast rule, tropical cyclones do not RotationNeeded: Stay Away from the Equator! form within 5° latitude of the equator - the Coriolis 5 10 15 20 25 30 35 40 Force is simply too weak there. If you look very closely Throughout the last few sections, we focused our atten­ at Figure 11.22, you can see an exception to this rule tion on the two basic characteristics that all developing along the southern tip of Malaysia near 1.5°N latitude, tropical cyclones display: an observable low-level circu­ l 00°E longitude . That narrow blue swath represents the lation, and organized thunderstorms around the center of track of Typhoon Vamei in late December 2001, which that circulation. We haven't yet given much ink to the cy­ spun up only about 160 km ( 100 mi) from the equator clonic circulation itself, primarily because we defined a (see Figure 11.23a). Despite an almost-nil Coriolis tropical cyclone using the words "low-pressure system," force at 1.5°N latitude, Typhoon Vamei was so close to so the two already go hand-in-hand. the equator that its winds howled in both hemispheres But there's an underlying assumption here that is not simultaneously (they were stronger in the Northern obvious: For the circulation to form in the first place, Hemisphere, of course). How could this formidable cir­ the Coriolis force must be strong enough to deflect the culation develop at such a low latitude?

FIGURE11.22 Theaverage annualfrequency of tropicalcyclones over the Indian and PacificOceans from 1972to FIGURE11.20 Thewind direction(arrows) and wind speed 850-200 mb Wind Shear (m/s) (color-codedin m/s) at 12Zon October20 , 2001.The dark red splotchesmark 2003 in the vicinity of TropicalStorm Nicholas at the (a) 200-mb the mostactive areas for tropical level,,and(b) 850-mblevel; (c) Theaverage 850-200 mb wind cycl,bnes;dark blue indicates very 10 14 18 22 26 30 shearfor October20, 2003(courtesy of NOAA). low activity,while white represents (c) areasvirtually unaffected by tropical cyclones.Note the absenceof activityon and nearthe equator (courtesyof ChrisCantrell , Joint 120E 140E 160E 180 160W 140W 120W 100W · BOW TyphoonWarning Center). 480 CHAPTER 11 Tropical Weather, Part II: Hurricanes CHAPTER 11 Tropical Weather, Part II: Hurricanes 481 Speaking of longshots, can a tropical cyclone ever cross the equator? To our knowledge, none has, but a few have come close, particularly in the North Indian Ocean ( check back to Figure 11.22). Is it possible? We would say "yes" because a tropical cyclone's large cyclonic cir­ culation would not initially be affected by the weak change in the Coriolis force as it crossed into the oppo­ site hemisphere. But there's another factor related to the variation of the Coriolis force with latitude that works against any crossover. Although this "other factor" (called the Beta effect) is beyond the scope of this textbook, we at least wanted to address the issue because we often get questions about hurricanes crossing the equator from in­ quisitive students. With these extreme but interesting issues put to rest, FIGURE11.24 (a) A visiblesatellite image of the easterntropical Atlantic and Africa at 12Zon September1, 2004. An easterlywave that let's fa k generic. What is the primary source for the . ha_d_iust em_erged off Af'.icawould be the catalystfor the seedlinglow-lei/el circulation of HurricaneIvan (courtesyof groups o disorganized showers and thunderstorms that DundeeSatellite Receiving Station); (b) An infraredsatellite image from September7, 2004shows Hurricane Ivan approaching the Caribbean. can move to th~odynamically favorable environ­ Theremnants of once-HurricaneFrances, which also developed from an easterlywave, were moving into the southeasternUnited States at the ments and increase the chance for tropical cyclones to time (courtesyof NOAA). FIGURE11.23 An infraredsatellite image at 0335Zon December spin up over the North Atlantic and Northeast Pacific '~ } 27,2001, shows Typhoon Vamei centered at 1.5°N originates north of the equator over Africa and then latitudenear Singapore. At this time,Vamei's circulation spanned both Basins? And, lest we forget, why aren't there any hurri­ winds stretching across Africa and indicated by the black moves westward across the tropical North Atlantic hemispheres.Surface streamlines (in black)indicate how the lay of the canes in the South Atlantic Ocean, save for that rare arrow is the Middle Level African Easterly Jet (MLAEJ Ocean (and sometimes into the Northeast Pacific landconspired to producea low-levelspin that, when combined with storm that hit Brazil in March 2004? Let's investigate. Ocean). Meteorologists sometimes call an easterly wave for short); the 600-mb pressure level lies in the rnid­ anarriving group of showersand thunderstorms, set the stage for the troposphere, thus the "Middle Level" in the name of this veryunusual development of a typhoonnot far fromthe equator a , though, more formally, it is an African easterly wave. The bottom line here is that easterly seasonal jet. During August, the core of the MLAEJ spans (courtesyof CRISP/NationalUniversity of Singapore). EasterlyWaves: Out of Africa from roughly 30°E to 30°W longitude. waves often serve as the catalyst for the seedling low­ Recall that a group of disorganized showers and thun­ As it turns out, the MLAEJ is a major source for derstorms over the tropics is formally called a tropical level cyclonic circulation that's needed for the genesis of tropical cyclones. African easterly waves. Most form on the south side of The best answer is simply: "the lay of the land and disturbance. Think of a tropical disturbance as a spark the axis of the MLAEJ, where large horizontal wind water." Prior to the formation ofVamei, persistent north­ During the period from June to October, an easterly that can ignite a favorable thermodynamic environmen t shear imparts a cyclonic circulation. In the context of northeasterly winds blew over the narrowing South China wave comes off the west coast of Africa into the eastern and pave the way for a tropical cyclone to form. the MLAEJ, horizontal wind shear is a change in Sea (streamlines are shown in Figure 11.23b). This fun­ Over the North Atlantic Ocean, approximately 60 tropical Atlantic every three or four days. On Septem­ ber 1, 2004, for example, a strong easterly wave had speed of the easterly winds over some specified hori­ neling of the air led to a strengthening of the wind, like percent of the tropical storms and "minor" hurricane s zontal distance in and near the jet. Think of a newly toothpaste squirting from the tube after you squeeze it. (Categories 1 and 2 on the Saffir-Simpson Scale) are emerged from Africa (see Figure 11.24a). This tropical Moreover, the Malaysian Peninsula channeled the air into initiated by tropical disturbances that move westwa rd wave eventually served as the catalyst for the low-level a cyclonic pattern, creating an "artificial" source oflow­ from Africa. These African disturbances also initia te circulation of Hurricane Ivan, which is shown approach­ level cyclonic circulation. Note the winds cyclonically nearly 85 percent of all Atlantic hurricanes that reach ing the eastern Caribbean a week later in Figure 11.24b. swirling near the southern tip of Malaysia in Figure Category-3 strength or higher, which the National Hur­ On average, approximately 60 easterly waves emerge 11.23b. This "artificial" low-level spin, in tandem with ricane Center classifies as major hurricanes . If that's from Africa each year. Given that the long-term aver­ a timely cluster of showers and thunderstorms that mi­ not enough to convince you that these tropical dis~u~­ age annual number of named Atlantic storms is ten ( ap­ proximately six of which become hurricanes), it stands grated over this region, set the stage for an unusual storm bances from Africa are worthy of our scien tlf 1c that defied all the textbooks. Vamei was compact by ty­ scrutiny, consider that some meteorologists believe that to reason that most easterly waves do not initiate a trop­ phoon standards-the area of open water where it devel­ nearly all the tropical cyclones that form in the N or~h­ ical cyclone . At the very least, some easterly waves pro­ oped was only about 500 km (310 mi) in breadth. But east Pacific Basin owe their existence to tropica l dis­ duce clusters of thunderstorms that affect the Caribbean Islands or the Bahamas . Vamei will live in infamy in Singapore, where govern­ turbances from Africa. Average 600-mb Wind (mis), August . What causes easterly waves? This is a good ques­ ment officials told residents that the bad weather was the These tropical disturbances that move westward from 10 15 20 25 30 35 40 result of an "an unusual cluster of severe thunderstorms." Africa are commonly called easterly waves. Forrnall~, an tion whose answer requires a deeper understanding of Averagewind direction(arrows) and speed At least they got the "unusual" right. Scientists calcu­ easterly wave is a tropical disturbance ( disorg anized the dynamics of the atmosphere over Africa. Figure FIGURE11.25 lated that the odds of such a storm developing are about 11.25 shows the average wind direction and speed at the (color-codedin m/s) at the 600-mblevel (about showers and thunderstorms) that has a cyclonic circula­ 6 4000 m) duringthe monthof August,showing the averageposition one in every 100 to 400 years . th 00-mb level (an altitude of about 4000 meters) during tion in the lower part of the middle troposphere, and at of the MiddleLevel African Easterly Jet as the blackarrow the month of August. The narrow ribbon of fast 600-mb (courtesyof NOAA). 482 CHAPTER 11 Tropical Weather, Part II: Hurricanes CHAPTER 11 Tropical Weather, Part II: Hurricanes 483 relatively large temperature gradient that stretches roughly east-west across Africa. Local African meteorol­ FIGURE11.28 Averagesurface ogists call the boundary between these two contrasting temperatures air masses the Intertropical Front . acrossAfrica in August,in °c.Note The bottom line here is that a front stretches east­ the narrowribbon of relativelylarge west across Africa, with hotter air to the north and temperaturegradient that marksthe cooler air to the south- in essence, a reversal of the lntertropicalFront. Arrows show "normal" north-south temperature gradient. This front how relativelycool, moist winds crossingthe equatorduring Africa's is relatively shallow, with north-south temperature gra­ 10N summermonsoon meet hot anddry dients vanishing near 600 mb. This lower tropospheric northeasterlywinds from the Sahara thermal gradient produces a height gradient that max- Desert(courtesy of NOAA). 1m1zes ound 600 111bwith higher 600-mb heights to EQ the north a lower 600-mb heights to the south (as FIGURE11.26 Easterlywaves form on the cyclonicshear side shown in Figure 11.29). It follows that there are fairly

(southernside) of the MiddleLevel African ~ strong winds that blow from the east just below the Average Surface Temperature(°C), August EasterlyJet. 600-mb level. These strong easterlies mark the Mid­ dle Level African Easterly Jet, a unique source for east­ 22 24 26 28 30 erly waves. 32 34 36 38 40 forming easterly wave as a cyclonically circulating Atlantic tropical storms and hurricanes that originate e.¢:dy tl~at spins up as a result of this shear (see Figure 1 as easterly waves within 1000 km (600 mi) or so 11.26). Given the pivotal role that easterly waves play of the Cape Verde Islands are called Cape Verde storms Average600-mb in the genesis of tropical cyclones over the North At­ 30N FIGURE11.29 (these islands lie just off the west coast of Africa). It heightsduring lantic and Northeast Pacific Oceans, it behooves us to takes some time and distance from land for an easterly learn more about their source. Augustover Africa and the adjacent wave to develop into a hurricane. Indeed, few easterly easternAtlantic Ocean. With high During North Africa 's hot summer, trade winds in the waves intensify into hurricanes over the eastern Atlantic 20N heightsto the northand low heights Southern Hemisphere cross the equator and penetrate Ocean. To our knowledge, only a handful of tropical cy­ to the south(a reverseof the to the southern edge of the Sahara Desert. As it turns clones have been classified as a hurricane east of25°W "normal"north-south gradient) , a out, this cross-equatorial flow is a component of Africa's longitude in the deep tropics. The farthest east that a ribbonof strongeasterly winds (the summer (see Figure 11.27). At the southern tropical cyclone has been classified as a major hurri­ 10N MLAEJ)sets up in the largegradient edge of the Sahara, moist and relatively cool air associ­ cane in the Atlantic Basin was roughly 30°W longitude, of 600-mbheights (courtesy of NOAA). ated with this monsoon meets hot and dry air flowing where Hurricane Frances rapidly intensified in 1980. from the northeast over the Sahara Desert (these north­ The season for Cape Verde storms typically runs from EQ easterlies are the infamously scorching harmattan about August to early October, essentially the heart of winds ). Figure 11.28 shows average surface tempera­ hurricane season. tures in August and a few arrows that represent these With the six ingredients for the genesis and develop­ Average 600-mb Heights {m), August hot winds and the cooler southwesterlies . Note that av­ ment of tropical cyclones in hand, we 're finally ready erage temperatures approach 40°C ( 104 °F) over the to completely address the reasons why the South At­ western Sahara Desert. Also note the narrow ribbon of 4400 4420 4440 lantic Ocean is virtually devoid of tropical cyclones. 4460 4480 4500

Dramaticsea­ THESOUTH ATLANTIC OCEAN: THE BASIN waves over the South Atlantic Ocean to provide the FIGURE11.21 sonalshifts in WITHOUTA HURRICANE SEASON seedling low-level circulation needed to kick-start trop­ wind directionoccur in a zonef rom ical cyclones. Africato SoutheastAsia and Now that we have easterly waves under our belts, we've taken a big step towards understanding why the South Moreover, check out Figure 11.30, which shows the northernAustralia, qualifying th e vertical wind shear between 850 mb and 200 mb during Atlantic Ocean is virtually bereft of tropical cyclones. highlightedarea as the world'sma jor what would theoretically be·the "peak" of hurricane sea­ The Middle Level African Easterly Jet, which is the monsoonalregion . This region son in the South Atlantic Basin, February to April. No­ occupiesa largeportion of the major source of tropical waves, is strictly a Northern tice that wind shear is a bit too strong (greater than 10 tropicaleastern hemisphere . Hemisphere feature. In other words, the tropical region mis), on average, over the tropical Atlantic off the east of Africa south of the equator doesn't have any source coast of Brazil. So, although sea-surface temperatures for easterly waves. As a result, there aren't any tropical there are favorably high ( see Figure 11.31) , this region 484 CHAPTER 11 Tropical Weather, Part II: Hurricanes

CHAPTER 11 Tropical Weather, Part II: Hurricanes 485 ture provided the group of disorganized showers and thunderstorms (Ingredient #6), and what happened to EQ the typically unfavorable vertical wind shear?

In a nutshell, a 500-mb low approached a stationary 20S ridge of high pressure cut off from the mid-latitude west­ 10$ erlies (see Figure 11.32). This 50 -mb low was accom­ 30S panied by a corre~peri ·ui::;;-~>--rucelow-pressure system with classic cold and warm fronts (not shown). 20S This stag11qnt500-mb pattern provided three key in­ 40S gredients . First, winds aloft were weak, indicated by the relatively lax 500-mb height gradient off the coast of 30$ 500-mb Heights {m) 122 March 25, 2004 Brazil. As a result, vertical wind shear was unusually 500-mb Heights (m) 12Z March 26, 2004 5280 5400 weak there. For proof, focus your attention on Figure 5520 5640 5760 5880 5280 5400 5520 5640 (a} 5760 5880 Average 850-200 mb Wind Shear (mis), Feb -Apr 11.33, which shows the average 850-200 mb wind shear (b) during the two-day period March 25-26, 2004. The FIGURE11.32 10 14 white area off the coast of Brazil represents favorable 18 22 26 30 The500-mb height analyses at (a) 12Zon March25, 2004,and (b) March26, 2004,show a troughcutting off from strong wind-shear values less than 10 mis. mid-latitudewesterlies (black arrow) (courtesy of NOAA). FIGURE11.30 Theaverage vertical wind shear between 850 mb Second, because the 500-mb low cut off over warm and200 mb duringthe three-month period from water, the troposphere off the Brazil Coast destabilized, We point out that in making the transition from a mid­ Subtropical cyclones also form over the North Pa­ Febr:uaryto April.The only region over the SouthAtlantic Ocean with as relatively cold air aloft associated with the 500-rnb latitude low-pressure system (with fronts) to a tropical favorcible~ ind shear(less than 10 m/s,here in white)consistently low stacked above warm, moist air near the ocean sur­ cyclone (no fronts), there was likely a time when Catarina cific and North Atlantic Oceans. On April 20, 2003, for example, the National Hurricane Center christened resid;s'at low latitudeseast of 20°Wlongitude (courtesy of NOAA). face. This set-up paved the way for thunderstorms to had both tropical and extratropical characteristics. For Subtropical Storm Ana while the mongrel system was erupt and to begin organizing around the surface low's the record, an extratropical low-pressure system forms south of Bermuda. The rapidly center of circulation. Finally, the 500-rnb low's rather outside the tropics in an environment where there are tem­ has at least two strikes against it: no easterly waves and, made the transition to a tropical cyclone and officially typically, unfavorable wind shear. broad circulation drew relatively moist air from the mid­ perature gradients . When a system simultaneously has became Tropical Storm Ana at OOZon April 21. Figure dle troposphere over the Amazon eastward into the both tropical and extratropical characteristics, it is some­ But then came Catarina. With the odds stacked against 11.34 is a multi-channel satellite image of Tropical brewing storm. With all six ingredients corning together, times called a subtropical cyclone . In this case, no such it, how did this hurricane ever develop off the east coast Storm Ana. Ana was the first documented tropical of Brazil in late March 2004? In particular, what fea- a rare South Atlantic hurricane developed, and the rest, christening was forthcoming because it caught Brazilian as they say, is history. forecasters a bit off guard. Nonetheless, the storm made a complete transition to a purely tropical cyclone by the time the eye came ashore on March 28, 2004. FIGURE11.31 Averagesea-surface temperatures from Februaryto April overthe 10S SouthAtlantic. Recall that sea-surfacetemperatures EQ of 26.5°G(80°F) or higherare generallyrequired for genesisof tropicalcyclones. Contrary to popularbelief, 20$ SSTsare high enoughto supportthe genesisof tropical 10$ South cyclonesover the SouthAtlantic Ocean (courtesy of NOAA). 30$ America

4 0$ 20S sos_JL_..2!!1- -. i.:....L...:::::=

Average 850-200 mb Wind Shear (mis), March 25-26, 2004 30$ 10 14 18 22 26 30

Average Sea-Surface Temperature {°C), Feb-Apr I FIGURE11.33 Theaverage wind shearbetween 850 mb and 200 mb duringthe two-dayperiod March 25-26 , 2004. Notethe areaof favorablyweak wind shearoff the coastof 22 23 24 25 TropicalStorm Ana at 2oz on April21, 2003.Ana 26 27 28 29 Brazil,where the whitearea represents values less than 10 m/s FIGURE11.34 (courtesyof NOAA). wasjust to the southeastof the islandof Bermuda at this time (courtesyof RaySterner, Johns Hopkins University). 486 CHAPTER 11 Tropical Weather, Part II: Hurricanes CHAPTER 11 Tropical Weather, Part II: Hurricanes 487 storm ever to form over the North Atlantic basin dur- ing April. . Check out the nifty flash animation on the Although we've previously given you an inkling of companion CD to see how a hurricane in­ tensifies in concert with this positive feedback loop. how tropical cyclones intensify, we will now formalize the intensification process. To see firsthand how an increase in the number and or­ ganization of thunderstorms around the core of a tropi­ A TROPICALCYCLONE INTENSIFIES: cal cyclone translates to intensification , let's consider CONDITIONALINSTABILITY OF THE , which first made landfall in south­ SECONDKIND ern Florida in late August 2005 as a Category-I storm and then twice made landfall in Louisiana as a Cate­ During our discussion of the thermodynamic ingredi­ gory-3 hurricane (see Figure 11.36). Given the failed ents needed for the genesis of a tropical cyclone, we levees and severe flooding in New Orleans (Figure pointed out that development hinged on thunderstorms 11.37) and the overall devastation along the central Gulf organizing around the center of a tropical disturbance. Coast , Hurricane Katrina ranks as the costliest and one Within the updrafts of these storms, the large releas e of Thetrack of HurricaneKatrina in lateAugust of the deadliest hurricanes ever to strike the United FIGURE11.36 latent heat inside rising parcels allows parcels to stay States. 2095.Positions are shown every six hours.The positively buoyant to great altitudes. When moisture­ colorof the dotscorresponds to the tropicalcyclone's intensity, from As with all hurricanes , Katrina began as a humble depleted air parcels reach the top of eye-wall thunder­ tropicaldepression to tropicalstorm to thefive categories on the tropical depression . Figure 11.38a, a color-enhanced storms, some parcels gently subside over the core of the Saffir-Simpsonscale . Katrinamade three landfalls: one in southern infrared satellite image at 0615Z on August 24, 2005, disturbance. The resulting large compressional warm­ FIGURE11 .35 Air spiralinginto the centerof a hurricanespeeds Floridaon August25 as a Category-1storm , andtwo in Louisianaon up as it nearlyconserves angular momentum. shows the very cold cloud tops (in red) of several tall , August29 as a Category-3hurricane (courtesy of Wikipedia): ing in the central air column increases its average tem­ disorganized thunderstorms over the Bahamas associ­ perature, and, thus , decreases its average density and ated with Tropical Depression 12. At the time, the max­ weight (refer back to Figure 11.15). In a nutshell, sur­ an increase in the number and organization of thunder­ tal pressure gradient strengthens. In response, wind speeds imum sustained winds ofT.D. 12 were 30 kt (35 mph) . face pressure decreases. storms around the eye. increase, especially lower-tropospheric winds around the In just about four days, the tropical depression intensi­ For the newly forming tropical cyclone to intensify , We can go one step further. Research meteorologists core of the newly forming tropical cyclone. That's be­ fied into a Category-5 hurricane (see Figure 11.38b) , the process must kick into a higher gear. Meteorologi sts have found that a hot tower forming around the eye of cause the horizontal pressure gradient is largest there (the with maximum sustained winds of 150 kt (173 mph). call this higher gear "Conditional Instability of the Sec­ a hurricane means that the storm is twice as likely to in­ large horizontal temperature gradient between the core Note the highly organized ring (in red) of tall eye-wall ond Kind." tensify within the subsequent six-hour period . A hot of the developing tropical cyclone and the surrounding thunderstorms that encircles the eye of Katrina at that tower forming around the eye of Hurricane Bonnie in thunderstorms creates a large pressure gradient). Stronger time . The message here should be loud and clear-the August 1998, visualized in Figure 11.39 by NASA's LatentHeat Feedback: Amplifying the Storm winds then translate to increased low-level convergence strength of a tropical cyclone goes hand-in-hand with TRMM satellite (Tropical Rainfall Measuring Mission), Let's carefully examine the amplification process of a and higher evaporation rates. As a result, the number and strengthening tropical cyclone by starting at the begin­ intensity of thunderstorms increase. ning. As low pressure forms at the ocean surface, air Now the stage is set for an amplifying feedback loop. FIGURE11.37 An aerialview converges toward the center. If the low develops far With the number, strength, and organization of thunder­ of oneof the enough away from the equator , the deflection by the storms increasing, and the supply of water vapor growing, breachedlevees in NewOrleans , LA, Coriolis force creates a cyclonic circulation. the release oflatent heat amplifies around the core of the on August30, 2005, the dayafter As air parcels spiral inward toward the center of low storm and surface pressure further decreases. In turn, sur­ HurricaneKatrina made landfall pressure (see Figure 11.35), they expand. Ordinarily, ex­ face convergence and wind speed increase, systemati­ (courtesyof FEMA). pansion produces cooling (similar to what happens when cally boosting the number and strength of thunderstorms, a parcel of air rises-it also moves toward lower pressure). elevating evaporation rates and upping the release of la­ Of course, any low-level cooling would be detrimental to tent heat. If the pressure continues to decrease, the system further development because it would tend to stabilize the graduates to a tropical storm and eventually a Category- I lower troposphere . But the tendency for inward-spiraling hurricane. If other conditions are favorable (for exam­ par9els to cool as they expand is offset by warm tropical ple, weak vertical wind shear and high sea-surface tem­ seas, which supply more-than-ample heat enegy. Thus, as peratures) , the positive feedback loop continues and the / i{p arcels converge around the center of the low, they re­ hurricane intensifies. This positive feedback loop is called / main warm and moist, setting the stage for their long as- Conditional Instability of the Second Kind, or CISK for / cent within thunderstorms organizing around the eye. short. This process cannot continue unchecked, of course. Meanwhile, as surface pressures decrease in response to Ultimately, sea-surface temperatures impose an upper compressional warming over the low's center, the horizon- limit on the potential intensity of hurricanes. 488 CHAPTER 11 Tropical Weather, Part II: Hurricanes CHAPTER 11 Tropical Weather, Part II: Hurricanes 489

FIGURE11.41 Thefastest winds of a hurricaneand the highest storm surgelie in the right-frontquadrant. In this region, windspush water towards shore (point R will be in this quadrant).At pointL, offshorewinds can actually lower water (b) (a) levelsas the hurricanegoes by. · · 12 (TD 12) t 0615Zon August24 2005.Maximum FIGURE11.38 i~~a~e~~a~~~; (a)A _colr~n~anced 3 i:o~~~~::~~i;~~ln~:~i~~~~;all thund .ersto~ms,whose very cold ~loudtops are sustaine win s were . . or 5 HurricaneKatrina with maximumsustained winds of 150 kt Not surprisingly, the strong winds in the right-front quad­ indicatedin red;(b) ByAugust 2~ , T.D.1 ~ had develo1p5eZd l~to ca;~g h(g~ly organized tall th~nderstorms(ring of red)around the eyeof the (173 mph).This color-enhanced infrared image at 18 s ows e FIGURE11.40 Theobserved surface winds around Hurricane rant of a hurricane produce the storm surge. We'll get hurricane(courtesy of NOAAand the NavalResearch Laboratory). Katrinaat 09Zon August29, 2005.Wind into the details of storm surge in just a moment. speedsare color-coded in knots,and arrows indicate wind Meteorologists sometimes liken a hurricane to a "heat directions.Measurements came from a varietyof sources, engine," which is a device that converts heat to some includingaircraft reconnaissance, ocean buoys, and Doppler radar. Thehurricane was moving north at the time (courtesyof NOAA's type of mechanical work. Let's see if such a comparison HurricaneResearch Division). . makes any sense. Great amounts of water evaporate from warm seas. In turn, the great release of latent heat of condensation in rising air allows parcels to stay pos­ FIGURE11.39 A hot towerfor m­ ing aroundthe was a clue to Bonnie's upcoming intensification from a itively buoyant to great altitudes. When parcels sink eyeof HurricaneBonnie on AuguSt Category-1 to a Category-3 hurricane as it passed north into the eye of the hurricane, they warm dramatically by 22, 1998,was a precursorfor inten­ of the Bahamas. compression. This "warm core" in the eye of a hurri­ sification.The top of the hot tower The strongest wind speeds associated with a hurricane cane paves the way for surface pressure to decrease and was almost18 km (11 mi) aboveth e are typically not evenly distributed around the eye of the for the hurricane to intensify. Given all of these thermo­ oceansurface (courtesy of NASA's storm . Figure 11.40 shows the surface wind speeds dynamic references, you can understand why likening TropicalRainfall Measuring Missi on). ( color-coded, in knots) and wind directions (arrows) hurricanes to heat engines is a pretty good analogy. around the eye of Hurricane Katrina at 0900Z on August With a hurricane's engine humming on all cylinders , 29, 2005, a few hours before Katrina made its first land­ let's look under the hood and inspect the structure of a fall in Louisiana. At the time, the maximum observed mature hurricane. wind speed was 99 kt (114 mph) to the northeast of Ka­ trina 's eye on the storm's forward right flank (looking in the direction of movement). Here, in this right-front FEATURESOF A HURRICANE: quadrant, the storm's forward motion adds to the speed WITHAN EYETO THE SKY of the winds generated by the strong horizontal pressure Figure 11.42a is a visible satellite image of Hurricane gradient ( see Figure 11.41 ). You can think about this Katrina on August 28, 2005. We show you this image boost in wind speed in much the same way as a moving to impress upon you that the eye of a hurricane is not train adds to the total speed of a robber running atop the necessarily clear. Nor is it entirely calm. Indeed, momen­ train toward the engine-the robber's total speed equals tum from fast-moving air in the eye wall, where the hur­ his running speed plus the forward speed of the train. ricane's strongest winds are found, can sometimes mix 490 CHAPTER 11 Tropical Weather, Part II: Hurricanes CHAPTER 11 Tropical Weather, Part II: Hurricanes 491 In reality, this near-balance of forces occurs above the boundary layer, which is the layer in which friction with struments released from Hurricane Hunter aircraft, which the surface operates. This layer typically has a depth of often fly through storms at an altitude of about three kilo­ 5 00--1000 m (about 1600-3200 ft). Near the ocean sur­ meters. Tethered to a parachute , a dropsonde measures face, friction between the air and very turbulent seas sub­ temperature, pressure and relative humidity as 1t de­ stantially reduces the speed of inward-spiraling parcels. scends toward the surface . By tracking the movement of As a result, the centrifugal force is not as large and air a dropsonde using the Global Positioning System, sci­ parcels penetrate closer to the center in response to the entists can also retrieve wind speed and direction. very strong pressure gradient force, whose magnitude Now that you know the source for the data plotted across the eye wall is greatest near the ocean surface. in Figure 11.44, we '11explain why the fastest winds Figure 11.44 shows a typical vertical profile of wind in a hurricane tend to occur near an altitude of about speed in the eye wall of a hurricane. Note that the max­ 500 meters. The "slowest" winds occur right at the ocean imum speeds typically occur at an altitude near 500 me­ surface, where friction is strongest, and winds increase with increasing height above the ocean surface as the (a) (b) ters (1640 ft). By way of background, conventional thinking has held that the strongest winds in a hurricane effect of surface friction decreases. Ordinarily, you would expect winds to continue to increase above the FIGURE11.42 occurred 2-3 km above the surface, but new evidence based on wind data collected by dropsondes now indicate boundary layer, 'but downdrafts in eye-wall thunder­ the level of maximum wind speeds lies at lower altitudes. storms transport momentum from fast winds downward into the boundary layer. This momentum doesn't really Before we discuss the underlying science for Figure have any impact on winds closest to the surface, where 11.44, we first point out that a dropsonde (see Figure friction is strongest. But this momentum does get trans­ 11.45) is a dispensable canister of electronic weather in- inward into the eye, creating swirls of turbulent clouds pressure-gradient force, and air parcels stop crossing is~­ ported down far enough to affect high-rise buildings . In and gusty winds on the periphery of the eye. If you look bars (here we have assumed that the _Coriolis fo~ce is closely at the inset in Figure 11.42a, you can observe a small, and therefore a minor player). With t?e centnfugal 3000 mesovortex inside Katrina's eye. For the re~ord, a force essentially offsetting the pressure gradient force (see mesovortex (in the context of a hurricane's eye) is a re_l­ Figure 11.43), inward-spiraling parcels stop short of the atively small, cyclonic swirl of low cloud~ ( cyclomc center, converging and rising from the stormy eye wall. 2500 - - - - ...------·------means counterclockwise in the Northern Hermsphere). In light of this small but awesome fe~ture, we want you to keep your mind open to some new ideas as we delve fur­ 2000 ------ther into the greatest storms on earth. Just so you don't dismiss everything you've hea~d be­ fore about hurricanes, the central region of the ey~ is, for r '5, 1500 ------·------the most part , an island of relatively light wmds­ ·;- ::r though, again, winds can be stronger inside the eye, par­ ticularly near the eye wall. It is truly a wonder of nature 1000 ·------that light winds characterize the c~ntral part _of the eye Velocity of a hurricane, while ten or so miles away m the eye wall winds could be blowing over 130 kt (150 mph). Centrifugal H~w does the eye of a hurricane form? You've already observed that air parcels spiraling inward toward the low­ est pre~sure at the center accelerate while trying to co~~ serve their angular momentum. As a result of the1r curving paths , air parcels are subject to the outward-act­ Abovethe levelwhere the effectsof friction Wind Speed --~ i11g-'C:ntrifugalforce , whose magnitu~e increases ~s the FIGURE11.43 with the seasurface vanish , thereis a balance /s peed of the inward-spiraling parcels mcreases (t~mk of A plot of wind speedwith altitudein the eye betweenthe pressuregradient and the centnfuga. I f orces close .to the A dropsondeis a pispensablecanister of wall, usingdata gathered by dropsondesin FIGURE11.45 your bottom sliding outward as your car negotiates a centerof a hurricane(assuming that the Coriolisforce is relat,velye more than200 hurricanes,indicates that the maximumwind electronicweather instruments released from curve-that's the centrifugal force at work, and the faster small).So, as air parcelsspiral inward toward the centerof the ey , HurricaneHunter aircraft. Tethered to a parachute,a dropsonde speedsin the eyewall occurabout 500 meters(1640 ft) abovethe you negotiate the curve, the greate~ the o~tward fo~ce on they stoptheir inwardspiral in the eyewall , wherethe two :orces e measurestemperature , pressureand relativehumidity as it surface.Above the levelof maximumwind speeds,winds in the your bottom). At some point , the mcrea~mg cent~1fugal essentiallybalance each other. Thus, a hurricanes· ' fas test . winds ar descendstoward the oceansurface . By trackingthe motionof a eye wall graduallyweaken with increasingaltitude (adapted from force nearly offsets the hurricane 's large, mward-drrected foundin the eyewall, rendering the eyean islandof relativecalm. Franklin,et. al., 2003,Volume 18, Weatherand Forecasting). dropsondeusing the GlobalPositioning System , scientistscan also retrievewind speedand direction (courtesy of the U.S. Air Force). 492 CHAPTER 11 Tropical Weather, Part II: Hurricanes CHAPTER 11 Tropical Weather, Part II: Hurricanes 493

fact, research suggests that people taking refuge on the upper floors of a high-rise building along the coast could experience winds a full Saffir-Simpson category stronger than the winds at the ground! So residents forced to seek refuge in high-rise buildings as a hurri­ cane makes landfall would be safest on lower floors that are high enough to escape the storm surge. The diameter of a hurricane's eye is, on average, about 40 km (25 mi). For rapidly intensifying hurricanes, the eye wall and its radius of maximum winds usually contract inward in response to the increasing pressure gradient (the radius of maximum winds is the distance from the center of the eye to the ring of strongest winds in the eye wall). In the process, this contraction reduces the diameter of the eye. Sometimes the eye becomes so (a) (b) small that it looks like a pinhole from space. For exam­ (a) FIGURE11.41 (a) An infraredsatellite image of HurricaneKatrina at 0430Zon August27, 2005,shows cirrus cloudsobscuring the eye ple, in the 24 hours ending at 06Z on October 19, 2005, of the storm; (b) A colorful microwavesatellite image at approximatelythe sametime showsthe sizeof the eyeand the Tropical Storm Wilma (see Wilma's track in Figure structureof deepconvection around Katrina (courtesy of NASAand the NavalResearch Laboratory). 11.46a) rapidly intensified from 60 kt (69 mph) into a Category-5 hurricane with maximum sustained winds of 150 kt (173 mph). Such a rapid intensification is unprece­ titude in a column of warm air, translating to high pres­ for the science behind this so-called "stadium effect," re­ dented for an Atlantic hurricane. Six hours later (12Z on sure at high altitudes over the oppressively warm eye). visit Figure 11.44 and recall that the screaming message October 19), Wilma's maximum sustained winds reached The circulation around such a lofty high is no different from this graph is that wind speeds in the eye wa11de­ 160 kt (184 mph) and the central pressure dropped to 882 tban around other highs. Indeed, on animations of satel­ crease with increasing altitude above about 500 meters. mb, an all-time record low barometric reading in the At­ lite images of Northern Hemisphere hurricanes, cirrus That's because the magnitude of the horizontal pressure lantic Basin. At the time, aircraft reconnaissance meas­ clouds, acting as a tracer to the pattern of high-altitude gradient across the eye wall also decreases with increas­ ured the diameter of the eye at just two nautical miles (2.3 divergence over a hurricane, often circulate clockwise ing altitude. To understand this assertion, check out Fig­ mi), the smallest eye known to the entire staff at the Na­ and outward away from the storm. The combing of cir­ ure 11.49, which shows various constant pressure surfaces tional Hurricane Center ( see Figure 11.46b ). rus over a storm's bald eye signals that the hurricane has in the eye and eye wall. Because pressure decreases more Cirrus clouds sometimes blow over a hurricane's eye, moved into an environment where there's vertical wind slowly with height in the eye's warm air column, the con­ hiding it from full view on standard visible and infrared shear. If the shear is too strong, surface pressures will in­ stant pressure surfaces become "flatter" with increasing satellite imagery. But a special instrument mounted on crease as the sputtering exhaust system allows some air height, indicating that the magnitude of the horizontal some satellites can get around this issue. This instru­ to pile up aloft. pressure gradient decreases with increasing altitude ment detects microwave radiation from hail and graupel Regardless of whether there's a cirrus "comb-over," (which explains why wind speeds decrease with increas­ (b) ing altitude above 500 meters). With the inward spiral of ( snowflakes coated with ice) in the upper portions of the vertical structure of the eye is often reminiscent of air parcels limited by a horizontal pressure gradient that tall thunderstorms. The small ice crystals in cirrus (a) Thetrack of HurricaneWilma, October 15-25, a stadium, with eye-wall thunderstorms tilting outward FIGURE11.46 clouds essentially are not detectable at this wavelength, 2005. Positionsare shown every six hours.The with increasing altitude ( see Figure 11.48). To get a sense relaxes with increasing height ( and reverses at very high so, for all practical purposes, this instrument can "see colorof the dotscorresponds to thetropical cyclone's intensity , from tropicaldepression to tropicalstorm to thefive Categorieson the through" cirrus clouds. Figure 11.47a is a standard in­ Theclouds and Saffir-Simpsonscale (courtesy of Wikipedia);(b) A visiblesatellite FIGURE11.48 frared image of Hurricane Katrina over the Gulf of Mex­ thunderstorms imageat 1245Zon October19 , 2005,shows the pinholeeye of ico at 0430Z on August 27, 2005 . You can see that clouds in the eyewall leanoutward with HurricaneWilma . At the time, the diameterof the eyewas a meretwo mas~ed the eye from full view at this time. Figure increa~ing altitude.Meteorologists nauticalmiles (2.3 mi), the centralpressure was a recordlow (for the sometimesrefer to this as the 11.4l b: which is a microwave satellite image at about the AtlanticBasin) 882 mb,and maximum sustained winds were 160 kt "stadiumeffect." sa~ ~1me, s~ows the presence of hail a~d gra~pel in (184mph) (courtesy of NOAAand the NavalResearch Laboratory ). /,, ,Jh~ upper P?rt1?ns of thunderstorms associated with Ka­ / tnna (red md1cates the deepest convection). At this wavelength, forecasters were able to "see through" the air rising to high altitudes in the eye wall spread out­ cirrus clouds to get a better idea of the size of the hur­ ward from the center of the storm. This divergenc e of ricane's eye and the structure of the eye wall. high-altitude air is orchestrated by a high-pressure sys­ To understand how cirrus clouds can blow over the tem that forms in the upper troposphere over the eye eye, let's start with the observation that many parcels of (remember that pressure decreases very slowly with al- CHAPTER 11 Tropical Weather, Part 11:Hurricanes 495 494 CHAPTER 11 Tropical Weather, Part 11:Hurricanes where Ike made its turn to the southwest, ever made it such as 500 mb, where winds are close to geostrophic, into the Gulf of Mexico. In fact, had Ike behaved like can serve as a proxy for the mean steering current. nearly all previous storms that had followed a similar Given that the Bermuda high is a semi-permanent fea­ track in the open Atlantic, it would have either made land­ ture during the warm season, it stands to reason that hur­ fall somewhere along the Eastern Seaboard or made a ricanes moving westward over the central Atlantic as far clockwise tum and completely missed the United States. north as 20°N latitude are likely to get caught up its The track of Hurricane Floyd in 1999, shown in Fig­ steering flow. As a result , an eventual turn to the north ure 11.52, is representative of the typical path that trop­ is likely, removing all but the remotest chances of storms ical cyclones take when they pass longitude 61 °W getting into the Gulf of Mexico. longitude heading west, north of 20°N latitude. The truth So why did Ike make that fateful southwestward jog? be told, Floyd passed this longitude line south of Ike's Carefully study Figure 11.53, which shows the 500-mb position. Yet it still made a pronounced northward tum height pattern at 1200Z on September 5, 2008, three (just off the coast of Florida) and, as a result , missed the hours after Ike reached its northernmost latitude in the Gulf of Mexico. The track of Hurricane Floyd is super­ open Atlantic Ocean. Note the lobe of high 500-mb imposed on the average 500-mb heights during the pe­ heights extendi~g southwestward from the subtropical A radarimage at 08Zon September16 , 2004, riod September 8-17, 1999. The most striking feature on high. With the 500-mb flow nearly geostrophic and A schematicof variousconstant pressure FIGURE11.50 showsan intensespiral band to the eastof this 500-mb chart is the closed center of high heights clockwise around the Atlantic subtropical high, steer­ FIGURE11.49 surfacesin the eyeand eye wall of a hurricane. landfallingHurricane Ivan. The spiral band spawned m?re than two sprawled across the Atlantic Ocean (you can't miss it). ing winds for Ike blew from the northeast at this time. Thehorizontal pressure gradient between the eyeand the outer dozentornadoes in southwesternGeorgia and the Floridapanhandle, This high is the 500-mb reflection of the surface As a result, Ike swerved to the southwest, greatly in­ edgeof the eyewall is greatestat the surface.Because pressure severalof themcausing fatalities (courtesy of WSICorporation). Bermuda high, so we '11 also refer to this feature as a creasing its chances of getting into the Gulf of Mexico. decreasesrelatively slowly in the eye'swarm air column, the "subtropical high." If you imagine the clockwise circu­ When gradients in 500-mb heights are weak (and horizontalpressure gradient decreases with increasingaltitude . lation of air around the periphery of this 500-mb high, thus steering winds are weak), tropical cyclones tend (indicatedby the flatteningof the constantpress~re surfaces with tion that Ike reached when it was in the open Atlantic you '11see that the flow of air pretty much mirrors the to move slowly and sometimes erratically. Or they may height). Ocean on September 5. We chose this point because I~e track of Hurricane Floyd. If you're now getting the idea simply stall. Either way, weak steering currents set the turned southwestward around this time , a fateful shift that subtropical high-pressure systems provide the pri­ stage for lethargic tropical cyclones to produce pro­ in track that ultimately sealed the fate of Galveston altitudes , where high pressure forms over the eye)'. the mary steering currents for hurricanes, then you're "on tracted heavy rain and flooding. A good example is vertical structure of the eye takes on the look of a stadmm. Island. . the right track." Tropical Storm Fay in 2008. After mak ing landfall on Why was Ike's eventual entry into the Gulf ofMexic~ Having thoroughly examined the eye and eye wall, w_e To get a sense of the subtropical high's role in the the Florida Keys on August 18, Fay took aim at the such a longshot? As it turns out, only a handful of~um­ will end this discussion about the features of a hum­ movement of tropical cyclones, we first note that the Florida Peninsula (see Fay's track in Figure 11.54; note canes and tropical storms on record that developed m t?-e cane with spiral bands . These tentacles of thu~der­ steering current for hurricanes is the average wind in a that the storm made landfall in Florida a record four Atlantic, moved westward, and passed near the pomt storms that pinwheel around the center of a hurncane layer of air that spans from the lower to the upper tropo­ correspond to areas of enhanced surface convergence sphere. As a result, the flow at mid-tropospheric levels and strong updrafts (see Figure 11.50) The weather as­ sociated with spiral bands is best described as "squally," with fitful but temporary rains, very gusty winds , and sometimes tornadoes. Indeed, the Storm Prediction Cen­ ter in Norman , OK, routinely issues watches whenever a hurricane is expected to make landfall along the Southeast Coast or the coast of the Gulf of Mexico. Speaking of making landfall , what controls a hurri­ cane 's movement? Read on.

HurricaneMoJement: Subtropical Highs 500-mb Heights (m) 122 September 5, 2008

5280 5400 5520 5640 5760 5880 at the S~g _Wheel_ . . Average 500-mb Heights (m) September 8-17, 1999 Hurnc a-ne Ike with maximum sustamed wmds of 95 Thetrack of HurricaneIke in September200 8· z: ' . FIGURE11.51 The500-mb height analysis at 12Zon September Positionsare shownevery six hours.The c olor 5280 5400 5520 5640 5760 5880 FIGURE11.53 kt ( 109 mph) , slammed into the upper Texas ~oast Just 5, 2008,three hours after Ike reached 23.7°N , after 07Z on September 13, 2008, devastatmg Gal­ of the dots correspondsto the tropicalcyclone's intensity, from th 61°w. Steering currents associated with theAltantic subtropical high tropicaldepression to tropicalstorm to the five Catego_rieson : N veston Island. A week earlier, landfall anywhere on the Thetrack of HurricaneFloyd in September1999 blewfrom the northeastin the vicinityof Ike,helping to sendthe Saffir-Simpsonscale. The southwestward turn nearlatitude ~ 3-; ' Gulf Coast seemed like a huge longshot. Check out Fig­ superimposedon a chartof 500-mbheights stormon its southwestwardjog. Herewe assume500 mb is a proxy longitude61 °w pavedthe wayfor the stormto enterthe Gui 0 ure 11.51, which shows the track of Hurricane Ike. Foc~s averagedover the periodSeptember 8-17, 1999(courtesy of NOAA). for the steeringlevel of Ike(courtesy of NOAA). your attention on 23.7°N, 61 °W, the northernmost posi- Mexico(courtesy of Wikipedia). 496 CHAPTER 11 Tropical Weather, Part II: Hurricanes CHAPTER 11 Tropical Weather, Part II: Hurricanes 497

\ I \ \

- 3 - 5 - 1 - 10 - 15 - 20 - 25

Thetrack of TropicalStorm Fay in August2008. FIGURE11.54 Positionsare shown every six hours.The color of the dots correspondsto the tropicalcyclone's intensity, from tropicaldepression to tropicalstorm to the five Categorieson the FIGURE11.55 Totalrainfall (in inches)produced by Tropical Saffir-Simpsonscale (courtesy of Wikipedia). StormFay in August2008. The black line marks the trackof Fay(courtesy of NOAA). times) . On August 20, as Fay neared the east coast of Hourof Danger:Landfall and Storm Surge Florida, it dramatically slowed down as it encountered (b) weak steering currents. For all practical purposes, the As Hurricane Katrina bore down on Louisiana on August FIGURE11.56 (a) A satelliteimage from the a~ernoonof August28, 2005,just afterHurricane Katrina had intensified into a category-5 tropical storm essentially stalled for a short time (note 28, 2005, it intensified into a Category-5 storm, with max­ imum sustained winds reaching 150 kt (173 mph) at st?r~ (courtesyof MODISRapid Response Project at NASA/GSFC);(b) A TRMM-satelliteima e showin a · the bunching of blue circles near Florida's east coast in of thunderstormsenc_1rchng the ~yewall ?fKatrina around 02Z on August29 , 2005.The spiral band (arrows pain; to it) rob~ed:;~~~~nd 1800Z (see Figure 11.56a). Later that night, as a spiral Figure 11.54). The following is a verbatim excerpt tNhAuSnAdersdtorhmsNofmoisture,causing Katrina to weakento a strongCategory-3 hurricane before its first landfallin Louisiana(courtesy of from the late morning discussion issued by the Na­ band of thunderstorms encircled the eye wall (see Figure an t e avalResearch Laboratory) tional Hurricane Center on August 21 , 2008: 11.56b), Katrina weakened before making landfall at 1 lZ on August 29 as a strong Category-3, with maximum sus­ large ocean swells while it was a Category-5 hurricane. TROPICAL STORM FAY DISCUSSION NUMBER 24 tained winds of 110 kt (127 mph). Essentially, the band average height of the highest one-third of the waves) . NATIONAL HURRICANE CENTER MIAMI FL According to the National Hurricane Center , a buoy lo­ of thunderstorms that encircled Katrina 's core robbed Eleven hours later , the same buoy measured a signifi­ 1100 AM EDT THU AUG 21 2008 cated about 110 km (70 mi) south of Dauphin Island, moisture from the eye-wall thunderstorms, paving the cant wave height of 55 ft (16.8 m), the largest ever AL, reported a significant wave height of30 ft (9.1 m) THERE IS BASICALLY NOTHING NEW TO REPORT. way for Katrina's maximum sustained winds to weaken. measured by a buoy operated by the National Data at 00Z on August 29 (the significant wave height is the FAY HAS BEEN MEANDERING FOR THE PAST At face value, Katrina's weakening seems like it would Buoy Center. Figure 11.57 shows the impact of the 12 HOURS OR SO WITH LITTLE CHANGE IN I J have mitigated damage. But not so fast. With the spiral INTENSITY ... STEERING CURRENTS HAVE band of thunderstorms around the storm's eye wall REMAINED VERY LIGHT ... CONSEQUENTLY intercepting moisture destined for Katrina's core, the FIGURE11.51 Stormsurge pushed a FAY HAS BARELY MOVED SINCE YESTERDAY. largeship onshorein radiu~ of hurricane-force winds expanded outward, ex­ southPlaquemines Parish near the point Given Fay's slow advance across Florida during the tending at least 130 km (about 80 mi) to the east of Ka­ whereHurricane Katrina first madelandfall period August 20-23, rainfalls were prodigious (see Fig­ trina 's center . The radius of tropical-storm-force winds in Louisiana(courtesy of NOAA). ure 11.55). During Fay's siege, a whopping 27.65 inches extended nearly 370 km (230 mi) east of the storm's (70 cm) fell about eight miles northwest of Melbourne, center. In terms of size, Katrina was massive. ?n the east-c~ntral coast of Florida. According to prelim­ The large swath of strong onshore winds pushed the mary data f~~m the National Weather Service, Fay was sea toward land, causing water to pile up in coastal shal­ the third w~ test tropical cyclone on record in the state lows and leading to a devastating inland surge of water. ofF~o~?d the wettest on record for east-central Florida. Observations of high-water marks indicated that the 07 :he_past 30 years, inland flooding has accounted storm surge was 24-28 ft (7.3- 8.5 m) in a 32-km (20 mi) for nearly 60 percent of the fatalities caused by tropical swath along the Mississippi Coast, 10-15 ft (3.0-4.6 m) cyclones . The threat posed by hurricanes ( or tropical along the coast of western Alabama, and 6 ft (1.8 m) storms) doesn't end with heavy rain, of course. Storm along the western panhandle of Florida. surges and tornadoes are also part of a hurricane's for­ Large waves contributed to these high-water marks. midable arsenal. Let 's investigate. Within the 24 hours prior to landfall, Katrina generate d 498 CHAPTER 11 Tropical Weather, Part II: Hurricanes CHAPTER 11 Tropical Weather, Part II: Hurricanes 499 This surge caused some of the levees protecting New Orleans to fail, flooding many parts of the city which FIGURE11.60 Hurricane lie below sea level. Andrew(1992), In terms of the threat posed by storm surge, New Or­ the last hurricaneto makelandfall in the UnitedStates at Category- leans is arguably the most vulnerable city in the United 5 intensity,inflicted serious wind States. Worldwide, the low lands of , located damagein southernFlorida along the northern shores of the Bay of Bengal, are prob­ (courtesyof NOAA). ably the most vulnerable of all. In November 2007, Very Severe Cyclon ic Storm Sidr, with maximum sustained winds of 135 kt (155 mph), slammed into Bangladesh, causing a massive storm surge that killed approx imately 10,000 people (see Figure 11.59). In 1991, a 20-ft (6.1 m) storm surge produced by a strong tropical cyclone overwhelmed southeastern Bangladesh, killing 138,000 people and leaving an estimated ten million people homeless. FIGURE11.58 Northerlywinds to the west of the track of HurricaneKatrina produced a largestorm surge Obviously, a tropical cyclone's strong winds can inflict alongthe southernshores of LakePontchartrain, causing levees plenty of damage. Figure 11.60 shows some of the wind that protectedNew Orleans to fail. damage in southern Florida inflicted by Hurricane An­ drew as it made landfall around 5 a.m. (local time) on August 24, 1992 . We note that Andrew briefly intensi­ storm surge in south Plaquemines Parish near Katrina's fied around landfall as low-level convergence increased first landfall. in response to greater surface friction over land. When ground. In the case of Andrew , enhanced frictional con­ There was another very important storm surge to the most hurricanes make landfall , sustained winds typi­ vergence boosted updrafts in eye-wall thunderstorms . west of the track of Katrina (see Figure 11.58). Along the cally decrease, but wind gusts near the ground usually In the path of this energized eye wall, some 2000 streaks southern shores of Lake Pontchartrain, strong northerly increase as mechanical eddies energized by friction mix of damage were attributed to this temporarily intensi­ winds produced a storm surge of 10-14 ft (3.0-4.3 m). momentum from faster winds higher up toward the fied convection. After landfall , Andrew weakened over the Florida peninsula as its lifeline of moisture from warm seas was cut off. Near and after landfall of a tropical cyclone, tornadoes A color-enhanced FIGURE11.59 can also pose a danger, particularly along the Gulf and infraredsatellite Southeast Coasts. Routinely, the Storm Prediction Cen­ imageof VerySevere Cyclonic Storm ter issues tornadoes watches for areas in the right-front Sidr at 110ozon November15, quadrant of the storm (see Figure 11.61). Here , surface 2007.At the time, maximum winds are slowed by friction , while speedy winds aloft sustainedwinds were135 kt (155 are, for the most part, unencumbered. In effect, friction mph). Sidr'sstorm surgekilled an estimated10,000 people (courtesy ~reates a layer in the lower troposphere where wind speeds of the NavalResearch Laboratory). mcrease rapidly with altitude . In turn, a "horizontal roll" ~an form, much like a pencil held in one hand (represent­ mg slower winds) rolls when your other hand rubs over it (representing faster winds aloft). In tum, thunderstorm updrafts can tilt the roll into an upright position, possibly generating a tornado . The 2004 hurricane season was a reco~d-setting one for producing tornadoes, as landfalling tropical systems spawned around 300 twisters in the United States. The remnant circulation from Hurricane FIGURE11.61 Theradar refle ctivity shortly beforeHurricane Ivan alone produced 123 tornadoes during the period Sep­ fke madelandfall on September13, 2008. At the tember 15-17, 2004 , from Florida to Pennsylvania , the time, the Storm PredictionCenter in Norman,OK , had issueda most from any U.S. tropical cyclone on record. tornadowatch for the countiesoutlined in red,most of which lay in the right-frontquadrant of the hurricane.Preliminary data indi­ We ciose with some final thoughts about forecasting tropical cyclones. catethat Ike spawnedapproximately 25 tornadoes(courtesy of Storm PredictionCenter). 500 CHAPTER 11 Tropical Weather, Part II: Hurricanes CHAPTER 11 Tropical Weather , Part II: Hurricanes 50 1 THEROLE OF THE NATIONAL HURRICANE NHC Official Annua.1Av~rage Track ' Errors CENTER(NHC): ITS MISSIONAND VISION Atlantic Basin Tropical Storms and Hurricanes ropical Storm Katrina ugust 24, 2005 NHC's Mission: To save lives, mitigate property loss, PM EDT Wednesday NWS TPC/NationolHurricane Center Advisorys and improve economic efficiency by issuing the best 600 Current Center Location25 .6 N n 2 w watches, warnings.forecasts and analyses ofhaza:dous Max SustainedWind 4S mph Current MovementNW at 9 mph 500 @ Current Center location tropical weather, and by increasing understandzng of • Forecast CenterPositions H Sustained wind • 73 mph these hazards. ,...___ S Sustained 1vind39-73 mph ...... :::,,..Potential Day 1-3 TrackArea cr Potentiol Da.y45 TrackArea HurricaneWatch NH C's Vision: To be Americas calm, clear and trusted 300 - TropicolStorm Warning TroplcolStonn Watch vo~e. m. the eye oifthe storm ' and, with our partners, en- able communities to be safe from tropical weather threats. 200 Over the past few decades, a majority of the fatalities caused by hurricanes and tropical storms ':ere a conse­ quence of inland flooding from heavy_rams. _Unfortu­ 1975 1980 1985 1990 1995 2000 2005 Year nately, Hurricane Katrina served as a gnm remmder that TheSLOSH model's prediction for the storm FIGURE11.62 storm surge still poses the greatest threat from lan~­ surgeof HurricaneKatrina (generated by the FIGURE11.63 Annualaverage errors in 24-hour(red), 48-hour falling tropical cyclones. Although the final de~th_toll is 03Zmodel run on August29, 2005).The height of the stormsurge (green)and 72-hour(yellow) track forecasts for not certain, approximately 1500 people lost ~heir lives to is color-codedin feet (red indicates25 feetand higher).The black ·AtlanticBasin tropical storms and hurricanes, for the period1970 direct impacts from Hurricane Katrina. W1thou~ reser­ line indicatesthe NationalHurricane Center's predicted path of the to 2007,in nauticalmiles (1 nauticalmile equals1.15 miles) . Errors vation, storm surge likely caused most of the _estui:iat~d eyeof the hurricane(courtesy of NOAA). in 96-hourand 120-hourforecasts are shown from 2001to 2007 FIGURE11.64 NHC'sfive-day track forecast for thenTropical 1300 deaths in Louisiana and the 200 deaths m M1ss1~­ (courtesyof the NationalHurricane Center). StormKatrina, issued at 5 p.m. (EDT)on sippi (the fatalit ~es in Mississippi occurred largely m August24, 2005.The official forecast is the blackline, and the The National Hurricane Center alerts commu~itie~ to whiteswath represents the uncertaintyin the trackforecast. Note three coastal communities). , . . ical storms during the period 1970-2007 (it also in­ The National Hurricane Center in Miami, FL, has the threat of hurricanes with two levels of adv1sones . that this "coneof uncertainty"widens with time, indicatingthe cludes the average annual error in the 96- and 120-hour growinguncertainty in the track forecast.Indeed, Katrina's track worked closely with coastal communities to de'-j~lop NHC forecasters issue a hurricane watch for ~c~astal forecasts from 200 I to 2007). For one, two, and three­ couldhave fallen anywhere within this coneof uncertainty. Focus evacuation plans based on the predicted heights of storm area when hurricane conditions are possible :v1thm _the day track forecasts, the improvement averages between your attentionon the possiblelandfalls along the northernGulf surges. Of course, the height of the storm surg~ depends next 36 hours. The second, more urgent ad:-7isory,is a one and two percent per year. Two-day hurricane track Coast.Although the officialforecast took Katrinainto the Florida on the wind strength (category) of the hurncane and hurricane warning, which NHC forecasters iss_ue_whe n forecasts today are as accurate as one-day forecasts were panhandleon Monday,August 29, the possiblelandfalls spanned the location where the storm makes landfall. It also ~e­ they expect hurricane conditions to ar:ive_ w1thm the in 1970. Still, the average track error at 48 hours is about from easternLouisiana-including New Orleans-to South pends on the topography of the coastline and the adJa­ next 24 hours. When issuing these adv1s?n_es, the _Na­ 100 miles (160 km). Because of this uncertainty, NHC Carolina.Of course,the coneof uncertaintyrelated to Katrina's cent ocean floor. Indeed, if the slope of the ocean flo~r tional Hurricane Center must weigh confl1ctmg societal track forecasts always include a "cone of uncertainty" first landfallin southernFlorida was muchnarrower , indicating less uncertainty,given that the eventuallandfall was lessthan adjacent to the coast is relatively gentle,_ wate~ thats impacts . NHC must allow enough lead tim~ to_evacua te that widens with time (see Figure 11.64). pushed toward land by hurricane- for~e wmds piles up densely populated areas, especially on barner islands or 48 hoursaway (courtesy of NOAA). In contrast to track forecasts, NH C's forecasts of the more readily, setting the stage for a higher storm surge other locations where evacuation routes can be cut o~ by intensity of hurricanes and tropical storms have im­ compared to an ocean floor which has a steeper slope rising water. On the flip side, evacuations are expensive. proved only slightly since 1970. One of the primary rea­ cane Wilma at 07Z on October 19, 2005. For the record, away from land. . It costs time and money ~oboard wi~dows and_san db:~ sons that intensity forecast improvements have lagged the satellite sensor passively detected microwaves emit­ To help communities formulate evacuat10n plans, the homes and businesses m preparat10n for wmds a those of track forecasts is that complex processes such ted by precipitation in the eye wall and spiral bands of National Hurricane C , ter provides local emergency waves. Businesses lose money when people ev~c~ate as eye-wall replacement cycles are the primary con­ Wilma. The eye was a "pinhole" at this time, consistent management officials "th output from a computer shore areas. And it costs a lot of money just to f~c1ht~~ trollers of intensity changes, and these cycles are very with Wilma's Category-5 status, with maximum winds model called SLOSH (whi h stands for Sea, Lake and the evacuation. The estimated costs for pr~paratwn a d difficult to accurately model. For the record, an eye-wall of 150 kt ( 173 mph) . Note the spiral band starting to Overland Surges from Hu ricanes). Figure 11.62 shows evacuation average about $1 mi·11· wn per mi le ofw arne replacement cycle occurs in major hurricanes when a wrap around the eye wall, signaling the beginning of an the SLOSH model's pre iction for the storm surge of coastline. These preparat10n. costs mcrease · e achy . ear as spiral band completely encircles the existing eye wall. eye-wall replacement cycle. Figure 11.65b shows the Hurr icane Katrina, gen¢rated by the 03Z model r~n on coastal populations and coasta 1 prope rty va 1u es mcrease.. This spiral band robs the inner eye wall of moisture, so end result of the eye-wall replacement cycle, with the August 29, 2005. The 1black line is NH C's _predicted Advances in technology, researc h ' re mote. sensmkillg, spiral band having formed a new (outer) eye wall. At the the inner eye wall collapses and the coiling spiral band path of the eye. To get your bearings, the height of the and forecasting techmques. contmue· to impr_· ove t1he Hurri s - time, Wilma was too close to land (the Yucatan Penin­ forms a new outer eye wall. During the replacement storm surge is color-coded in feet. Note that the SLOSH and accuracy of forecasts issued by the Natwna . es sula) to regain ariy strength-maximum sustained winds cycle, the hurricane weakens, but, after the cycle is com­ model predicted a storm surge along the southern sho~e cane Center. Indeed, forecasts of the tracks ofhum caF~g- were 125 kt (144 mph). plete, the hurricane may become as strong (or stronger) of Lake Pontchartrain (which is where New Orleans 1s . . 1 ore accurate. I and tropical storms are mcreasmg y m . e 24- 36- as it was before the replacement cycle began . So what's our point here? Given that such comple x located) , and a monstrous 30-foot surge into Hancock processes cannot be accurately modeled, it's no wonder ure 11.63 shows the average annual err~r m th and ~op­ Figure 11.65a is a satellite image that shows deep con­ , MS . and 48-hour forecasts for Atlantic hurncanes why NH C's advance in intensity forecasting has lagged vection (yellow and red shades) associated with Hurri- behind improvements in track forecasting . 502 CHAPTER 11 Tropical Weather, Part II: Hurricanes CHAPTER 11 Tropical Weather, Part 11:Hurricanes 503 10/.19/05 0600Z 241 WILMA 10/.19/.05 0709Z ASUA-1 89H 101/191/05 0545Z G ES-12 IR

What a Difference a Day Makes It has been said that the most beautiful day to be seen edges of the hurricane (of course, some air also sinks is the one that precedes a hurricane. This tidbit of trop­ into the eye).This compensatingsubsidence often cre­ ical folklore is confirmed by satellite imagery of most ates a rather sharp transition from cloudy to clear con­ hurricanes-just check a few of the images in this ditions, with cumulus cloud development suppressed chapter. hundredsof kilometersahead of the storm. But, as care­ The old adage "What goes up, must come down" ful observersdating backto ChristopherColumbus have helps to explain why skies surrounding a hurricane are noted, the app~aranceof cirrus clouds (fanning off the relatively clear.Within a hurricane, rising currents of air tops of thunderstormswithin the hurricane),coupled with sustainthe thunderstormsthat drive the storm'sheat en­ rising swells overthe ocean surfaceand a steadilyfalling gine. Once updrafts reach the top of the storm, air barometric pressure, can belie the blue skies above (a) (b) spreadsout and diverges,eventually sinking on the outer foretelling the tempestuousapproach of a hurricane. ' (a) A satelliteimage showing the deepconvection (in yellowand red) associated with HurricaneWilma around 07Z on FIGURE11.65 October19, 2005. At thetime, Wilma was a Category-5hurricane, with maximumsustained winds of 150kt (173mph). Notethe spiralband starting to wraparound the eyewall , signalingthe start of an eye-wallreplacement cycle. The eye of Wilmawas only about2.3 mi (3.7 km) in diameter,and the satellitecould not resolve(detect) the "pinholeeye"; (b) A satelliteimage at 1845Zon October20 showsthe deepconvection associated with a new(outer) eye wall. As the spiralband encircled the innereye wall, it interceptedmoisture destinedfor the innereye wall. As a result, the innereye wall collapsedand the nowcircular spiral band took overas the new(outer) eye wall. Wilma'smaximum sustained speed at this time was125 kt (144mph) (courtesy of NASAand the NavalResearch Laboratory).

We end this section with some sobering thoughts . Coastal populations continue to increase at a rate of four More than 70 million people currently reside in the more to five percent per year, on average. Indeed, because the than 400 shore and near-coastal c~ ies abutting the lure of the ocean is so strong , people will always be at­ Atlantic Ocean and the Gulf of Mexico. T hese numbers tracted to live in harm 's way. Education and preparatio n swell dramatically during peak holiday dr vacations pe­ are the keys to preventing another disastrous loss ofli fe riods when millions more visit the/ ation 's shores. from a future tropical cyclone.