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************************************************* FAQ: HURRICANES, TYPHOONS, AND TROPICAL CYCLONES ************************************************* Part I: ------- A : BASIC DEFINITIONS B : TROPICAL CYCLONE NAMES C : TROPICAL CYCLONE MYTHS D : TROPICAL CYCLONE WINDS E : TROPICAL CYCLONE RECORDS F : TROPICAL CYCLONE FORECASTING G : TROPICAL CYCLONE CLIMATOLOGY H : TROPICAL CYCLONE OBSERVATION By Christopher W. Landsea NOAA AOML/Hurricane Research Division 4301 Rickenbacker Causeway Miami, Florida 33149 landsea@aoml.noaa.gov 18 July, 1997 *********************** New for this month..... ....................... How do tropical cyclones form? (Subject A10) What names have been retired in the Atlantic basin? (Subject B3 - Revised) What are the most and least tropical cyclones occurring in the Atlantic basin and striking the USA? (Subject E9 - Revised - Table of individual years added) What refereed articles were written in recent years about tropical cyclones ? (Subject J4 - Revised) ....................... New for this month..... *********************** This is currently a two-part FAQ (Frequently Asked Questions report) that is in its second full incarnation (version 2.4). However, there may be some errors or discrepancies that have not yet been found. If you do see an item that needs correction, please contact me directly. This file (Part I) contains various definitions, answers for questions about names, myths, winds, records, forecasting, climatology and observation of tropical cyclones. Part II provides sites that you can access both real-time information about tropical cyclones, what is available on-line for historical storms, as well as good books to read and various references for tropical cyclones. Keep in mind that this FAQ is not considered a reviewed paper to reference. Its main purpose is to provide quick answers for (naturally) frequently asked questions as well as to be a pointer to various sources of information. I'd like to thank various people for helping to put together this FAQ: Sim Aberson, Jack Beven, Gary Padgett, Tom Berg, Julian Heming, Neal Dorst, Gary Gray, Stephen Caparotta, Steven Young, D. Walston all provided substantial bits to this FAQ. Also thanks to the many people who provided additional questions and information for this FAQ: Ilana Stern, Dave Pace, Dave Blanchard, Ken Fung, James (I R A Aggie) Stricherz, Mike Dettinger, Jan Schloerer, Eric Blake, Jeff Kepert, Frank Woodcock, Roger Edson, Bill Cherepy, Stephen Jascourt, Kelly Dean, Malcolm ???, Jon Gill, Ken Waters, Derek West, Gert van Dijken, George Gumbert III, Edward Reid, Tim Trice, Michael Scott, Kerry Emanuel, George Sambataro, James Lewis Free, Sam Biller, David Faciane, Eric Gross, Jeff Hawkins, Mike Fiorino and Madeleine Hall. Many thanks also to Jan Null for providing the first .html version of the FAQ. If I didn't get to all the suggested FAQs, I'll try to include them in future versions. Where can I get the latest version of this document????? -------------------------------------------------------- ASCII VERSION: An ascii edition of the two portions for this FAQ are posted monthly on sci.geo.meteorology and on sci.environment usually early in each month. One can also ftp to retrieve the latest files at: hrd-type42.nhc.noaa.gov. Login as 'anonymous' and password as your email address. The files are available at that directory (TCfaqI and TCfaqII). If you do not have ftp access, you can request copies from me directly via email. FANCY VERSION: Neal Dorst has created a much enhanced World Wide Web version that is starting to include in helpful pictures as well. This user friendly site is available via your favorite web server at: http://www.aoml.noaa.gov/hrd/tcfaq/tcfaqHED.html *************************************************************************** OUTLINE ------- A : BASIC DEFINITIONS A1) What is a hurricane, typhoon, or tropical cyclone? A2) What are "Cape Verde"-type hurricanes? A3) What is a super-typhoon? A4) Where do these easterly waves come from and what causes them? A5) What is a sub-tropical cyclone? A6) How are tropical cyclones different from mid-latitude storms? A7) How are tropical cyclones different from tornadoes? A8) What does the acronym "CDO" in a discussion of tropical cyclones mean? A9) What is a TUTT? A10) How do tropical cyclones form? B : TROPICAL CYCLONE NAMES B1) Why are tropical cyclones named? B2) What are the tropical cyclone names through 2001? B3) What names have been retired in the Atlantic basin? B4) What is the origin of the name "hurricane"? C : TROPICAL CYCLONE MYTHS C1) Doesn't the low pressure in the tropical cyclone center cause the storm surge? C2) Doesn't the friction over land kill tropical cyclones? C3) Aren't big tropical cyclones also intense tropical cyclones? C4) Why don't we try to destroy tropical cyclones by: pick one or more - a) seeding them with silver iodide. b) placing a substance on the ocean surface. c) nuking them d) etc. ? C5) During a hurricane are you supposed to have the windows and doors on the storm side closed and the windows and doors on the lee side open? D : TROPICAL CYCLONE WINDS D1) How are Atlantic hurricanes ranked? D2) How are Australian tropical cyclones ranked? D3) Why do tropical cyclones' winds rotate counter-clockwise (clockwise) in the Northern (Southern) Hemisphere? D4) How do I convert from mph to knots (to m/s) and from inches of mercury to mb (to hPa)? D5) How does the damage that hurricanes cause increase as a function of wind speed? E : TROPICAL CYCLONE RECORDS E1) Which is the most intense tropical cyclone on record? E2) Which tropical cyclone intensified the fastest? E3) Which tropical cyclone has produced the highest storm surge? E4) What are the largest rainfalls associated with tropical cyclones? E5) Which are the largest and smallest tropical cyclones on record? E6) Which tropical cyclone lasted the longest? E7) Which tropical cyclones have caused the most deaths and most damage? E8) What are the average, most, and least tropical cyclones occurring in each basin? E9) What are the most and least tropical cyclones occurring in the Atlantic basin and striking the USA? E10) For the U.S., what are the 10 most intense, 10 costliest, and 10 highest death toll hurricanes on record? E11) What tropical storms and hurricanes have moved from the Atlantic to the Northeast Pacific or vice versa? F : TROPICAL CYCLONE FORECASTING F1) What regions around the globe have tropical cyclones and who is responsible for forecasting there? F2) What is Prof. Gray's seasonal hurricane forecast for this year and what are the predictive factors? F3) How has Dr. Gray done in previous years of forecasting hurricanes? F4) What are those track and intensity models that the Atlantic forecasters are talking about in the tropical storm and hurricane Discussions? G : TROPICAL CYCLONE CLIMATOLOGY G1) What is the annual cycle of occurrence seen in each basin? G2) How does El Nino-Southern Oscillation affect tropical cyclone activity around the globe? G3) What may happen with tropical cyclone activity in a 2xCO2 world? G4) Are we getting stronger and more frequent hurricanes, typhoons, and tropical cyclones in the last several years? G5) Why do tropical cyclones occur primarily in the summer and autumn? G6) What determines the movement of tropical cyclones? G7) Why doesn't the South Atlantic Ocean experience tropical cyclones? G8) Does an active June and July mean the rest of the season will be busy too? G9) Why do hurricanes hit the East coast of the U.S., but never the West coast? G10) How much lightning occurs in tropical cyclones? H : TROPICAL CYCLONE OBSERVATION H1) What is the Dvorak technique and how is it used? H2) Who are the "Hurricane Hunters" and what are they looking for? *************************************************************************** Subject: A1) What is a hurricane, typhoon, or tropical cyclone? The terms "hurricane" and "typhoon" are regionally specific names for a strong "tropical cyclone". A tropical cyclone is the generic term for a non-frontal synoptic scale low-pressure system over tropical or sub- tropical waters with organized convection (i.e. thunderstorm activity) and definite cyclonic surface wind circulation (Holland 1993). Tropical cyclones with maximum sustained surface winds (see note below) of less than 17 m/s (34 kt) are called "tropical depressions". (This is not to be confused with the condition mid-latitude people get during a long, cold and grey winter wishing they could be closer to the equator ;-) Once the tropical cyclone reaches winds of at least 17 m/s they are typically called a "tropical storm" and assigned a name. If winds reach 33 m/s (64 kt), then they are called: a "hurricane" (the North Atlantic Ocean, the Northeast Pacific Ocean east of the dateline, or the South Pacific Ocean east of 160E); a "typhoon" (the Northwest Pacific Ocean west of the dateline); a "severe tropical cyclone" (the Southwest Pacific Ocean west of 160E or Southeast Indian Ocean east of 90E); a "severe cyclonic storm" (the North Indian Ocean); and a "tropical cyclone" (the Southwest Indian Ocean) (Neumann 1993). Note that just the definition of "maximum sustained surface winds" depends upon who is taking the measurements. The World Meteorology Organization guidelines suggest utilizing a 10 min average to get a sustained measurement. Most countries utilize this as the standard. However the National Hurricane Center (NHC) and the Joint Typhoon Warning Center (JTWC) of the USA use a 1 min averaging period to get sustained winds. This difference may provide complications in comparing the statistics from one basin to another as using a smaller averaging period may slightly raise the number of occurrences (Neumann 1993). *************************************************************************** Subject: A2) What are "Cape Verde"-type hurricanes? Cape Verde-type hurricanes are those Atlantic basin tropical cyclones that develop into tropical storms fairly close (<1000km or so) of the Cape Verde Islands and then become hurricanes before reaching the Caribbean. (That would be my definition, there may be others.) Typically, this may occur in August and September, but in rare years (like 1995) there may be some in late July and/or early October. The numbers range from none up to around five per year - with an average of around 2. *************************************************************************** Subject: A3) What is a super-typhoon? A "super-typhoon" is a term utilized by the U.S. Joint Typhoon Warning Center in Guam for typhoons that reach maximum sustained 1-minute surface winds of at least 130 kt (240 km/h). This is the equivalent of a strong Saffir-Simpson category 4 or category 5 hurricane in the Atlantic basin or a category 5 severe tropical cyclone in the Australian basin. *************************************************************************** Subject: A4) Where do these easterly waves come from and what causes them? It has been recognized since at least the 1930s (Dunn 1940) that lower tropospheric (from the ocean surface to about 5 km with a maximum at 3 km) westward traveling disturbances often serve as the "seedling" circulations for a large proportion of tropical cyclones over the North Atlantic Ocean. Riehl (1945) helped to substantiate that these disturbances, now known as African easterly waves, had their origins over North Africa. While a variety of mechanisms for the origins of these waves were proposed in the next few decades, it was Burpee (1972) who documented that the waves were being generated by an instability of the African easterly jet. (This instability - known as baroclinic-barotropic instability - is where the value of the potential vorticity begins to decrease toward the north.) The jet arises as a result of the reversed lower-tropospheric temperature gradient over western and central North Africa due to extremely warm temperatures over the Saharan Desert in contrast with substantially cooler temperatures along the Gulf of Guinea coast. The waves move generally toward the west in the lower tropospheric tradewind flow across the Atlantic Ocean. They are first seen usually in April or May and continue until October or November. The waves have a period of about 3 or 4 days and a wavelength of 2000 to 2500 km, typically (Burpee 1974). One should keep in mind that the "waves" can be more correctly thought of as the convectively active troughs along an extended wave train. On average, about 60 waves are generated over North Africa each year, but it appears that the number that is formed has no relationship to how much tropical cyclone activity there is over the Atlantic each year. While only about 60% of the Atlantic tropical storms and minor hurricanes (Saffir-Simpson Scale categories 1 and 2) originate from easterly waves, nearly 85% of the intense (or major) hurricanes have their origins as easterly waves (Landsea 1993). It is suggested, though, that nearly all of the tropical cyclones that occur in the Eastern Pacific Ocean can also be traced back to Africa (Avila and Pasch 1995). It is currently completely unknown how easterly waves change from year to year in both intensity and location and how these might relate to the activity in the Atlantic (and East Pacific). *************************************************************************** Subject: A5) What is a sub-tropical cyclone? A sub-tropical cyclone is a low-pressure system existing in the tropical or subtropical latitudes (anywhere from the equator to about 50N) that has characteristics of both tropical cyclones and mid-latitude (or extratropical) cyclones. Therefore, many of these cyclones exist in a weak to moderate horizontal temperature gradient region (like mid-latitude cyclones), but also receive much of their energy from convective clouds (like tropical cyclones). Often, these storms have a radius of maximum winds which is farther out (on the order of 60-125 miles [100-200 km] from the center) than what is observed for purely "tropical" systems. Additionally, the maximum sustained winds for sub-tropical cyclones have not been observed to be stronger than about 64 kt (33 m/s). Many times these subtropical storms transform into true tropical cyclones. A recent example is the Atlantic basin's Hurricane Florence in November 1994 which began as a subtropical cyclone before becoming fully tropical. Note there has been at least one occurrence of tropical cyclones transforming into a subtropical storm (e.g. Atlantic basin storm 8 in 1973). Subtropical cyclones in the Atlantic basin are classified by the maximum sustained surface winds: less than 34 kt (18 m/s) - "subtropical depression", greater than or equal to 34 kt (18 m/s) - "subtropical storm". Note that while these are not given names, they are warned on and forecasted for by the National Hurricane Center similar to the treatment received by tropical cyclones in the region. *************************************************************************** Subject: A6) How are tropical cyclones different from mid-latitude storms? The tropical cyclone is a low-pressure system which derives its energy primarily from evaporation from the sea in the presence of high winds and lowered surface pressure and the associated condensation in convective clouds concentrated near its center (Holland 1993). Mid-latitude storms (low pressure systems with associated cold fronts, warm fronts, and occluded fronts) primarily get their energy from the horizontal temperature gradients that exist in the atmosphere. Structurally, tropical cyclones have their strongest winds near the earth's surface (a consequence of being "warm-core" in the troposphere), while mid-latitude storms have their strongest winds near the tropopause (a consequence of being "warm-core" in the stratosphere and "cold-core" in the troposphere). "Warm-core" refers to being relatively warmer than the environment at the same pressure surface ("pressure surfaces" are simply another way to measure height or altitude). *************************************************************************** Subject: A7) How are tropical cyclones different from tornadoes? While both tropical cyclones and tornadoes are atmospheric vortices, they have little in common. Tornadoes have diameters on the scale of 100s of meters and are produced from a single convective storm (i.e. a thunderstorm or cumulonimbus). A tropical cyclone, however, has a diameter on the scale of 100s of *kilometers* and is comprised of several to dozens of convective storms. Additionally, while tornadoes require substantial vertical shear of the horizontal winds (i.e. change of wind speed and/or direction with height) to provide ideal conditions for tornado genesis, tropical cyclones require very low values (less than 10 m/s or 20 kt) of tropospheric vertical shear in order to form and grow. These vertical shear values are indicative of the horizontal temperature fields for each phenomenon: tornadoes are produced in regions of large temperature gradient, while tropical cyclones are generated in regions of near zero horizontal temperature gradient. Tornadoes are primarily an over-land phenomena as solar heating of the land surface usually contributes toward the development of the thunderstorm that spawns the vortex (though over-water tornadoes have occurred). In contrast, tropical cyclones are purely an oceanic phenomena - they die out over-land due to a loss of a moisture source. Lastly, tropical cyclones have a lifetime that is measured in days, while tornadoes typically last on the scale of minutes. An interesting side note is that tropical cyclones at landfall often provide the conditions necessary for tornado formation. As the tropical cyclone makes landfall and begins decaying, the winds at the surface die off quicker than the winds at, say, 850 mb. This sets up a fairly strong vertical wind shear that allows for the development of tornadoes, especially on the tropical cyclone's right side (with respect to the forward motion of the tropical cyclone). For the southern hemisphere, this would be a concern on the tropical cyclone's left side - due to the reverse spin of southern hemisphere storms. (Novlan and Gray 1974) *************************************************************************** Subject: A8) What does the acronym "CDO" in a discussion of tropical cyclones mean? "CDO" is an acronym that stands for "central dense overcast". This is the cirrus cloud shield that results from the thunderstorms in the eyewall of a tropical cyclone and its rainbands. Before the tropical cyclone reaches hurricane strength (64 kt or 33 m/s), typically the CDO is uniformly showing the cold cloud tops of the cirrus with no eye apparent. Once the storm reaches the hurricane strength threshold, usually an eye can be seen in either the infrared or visible channels of the satellites. Tropical cyclones that have nearly circular CDO's are indicative of favorable, low vertical shear environments. *************************************************************************** Subject: A9) What is a "TUTT"? A "TUTT" is a Tropical Upper Tropospheric Trough. A TUTT low is a TUTT that has completely cut-off. TUTT lows are more commonly known in the Western Hemisphere as an "upper cold low". TUTTs are different than mid- latitude troughs in that they are maintained by subsidence warming near the tropopause which balances radiational cooling. TUTTs are important for tropical cyclone forecasting as they can force large amounts of harmful vertical wind shear over tropical disturbances and tropical cyclones. There are also suggestions that TUTTs can assist tropical cyclone genesis and intensification by providing additional forced ascent near the storm center and/or by allowing for an efficient outflow channel in the upper troposphere. For a more detailed discussion on TUTTs see the article by Fitzpatrick et al. (1995). *************************************************************************** Subject: A10) How do tropical cyclones form? To undergo tropical cyclogenesis, there are several favorable precursor environmental conditions that must be in place (Gray 1968, 1979): 1. Warm ocean waters (of at least 26.5 C [80 F]) throughout a sufficient depth (unknown how deep, but at least on the order of 50 m [150 ft]). Warm waters are necessary to fuel the heat engine of the tropical cyclone. 2. An atmosphere which cools fast enough with height such that it is potentially unstable to moist convection. It is the thunderstorm activity which allows the heat stored in the ocean waters to be liberated for the tropical cyclone development. 3. Relatively moist layers near the mid-troposphere (5 km [3 mi]). Dry mid levels are not conducive for allowing the continuing development of widespread thunderstorm activity. 4. A minimum distance of at least 500 km [300 mi] from the equator. For tropical cyclogenesis to occur, there is a requirement for non-negligible amounts of the Coriolis force to provide for near gradient wind balance to occur. Without the Coriolis force, the low pressure of the disturbance cannot be maintained. 5. A pre-existing near-surface disturbance with sufficient vorticity and convergence. Tropical cyclones cannot be generated spontaneously. To develop, they require a weakly organized system with sizable spin and low level inflow. 6. Low values (less than about 10 m/s [20 mph]) of vertical wind shear between the surface and the upper troposphere. Vertical wind shear is the magnitude of wind change with height. Large values of vertical wind shear disrupt the incipient tropical cyclone and can prevent genesis, or, if a tropical cyclone has already formed, large vertical shear can weaken or destroy the tropical cyclone by interfering with the organization of deep convection around the cyclone center. Having these conditions met is necessary, but not sufficient as many disturbances that appear to have favorable conditions do not develop. Recent work (Velasco and Fritsch 1987, Chen and Frank 1993, Emanuel 1993) has identified that large thunderstorm systems (called mesoscale convective complexes [MCC]) often produce an inertially stable, warm core vortex in the trailing altostratus decks of the MCC. These mesovortices have a horizontal scale of approximately 100 to 200 km [75 to 150 mi], are strongest in the mid-troposphere (5 km [3 mi]) and have no appreciable signature at the surface. Zehr (1992) hypothesizes that genesis of the tropical cyclones occurs in two stages: stage 1 occurs when the MCC produces a mesoscale vortex and stage 2 occurs when a second blow up of convection at the mesoscale vortex initiates the intensification process of lowering central pressure and increasing swirling winds. *************************************************************************** Subject: B1) Why are tropical cyclones named? Tropical cyclones are named to provide ease of communication between forecasters and the general public regarding forecasts, watches, and warnings. Since the storms can often last a week or longer and that more than one can be occurring in the same basin at the same time, names can reduce the confusion about what storm is being described. According to Dunn and Miller (1960), the first use of a proper name for a tropical cyclone was by an Australian forecaster early in this century. He gave tropical cyclone names "after political figures whom he disliked. By properly naming a hurricane, the weatherman could publicly describe a politician (who perhaps was not too generous with weather-bureau appropriations) as 'causing great distress' or 'wandering aimlessly about the Pacific.'" (Perhaps this should be brought back into use ;-) During World War II, tropical cyclones were informally given women's names by USA Air Force and Navy meteorologists (after their girlfriends or wives) who were monitoring and forecasting tropical cyclones over the Pacific. From 1950 to 1952, tropical cyclones of the North Atlantic Ocean were identified by the phonetic alphabet (Able-Baker-Charlie-etc.), but in 1953 the USA Weather Bureau switched to women's names. In 1979, the WMO and the USA National Weather Service (NWS) switched to a list of names that also included men's names. The Northeast Pacific basin tropical cyclones were named using women's names starting in 1959 for storms near Hawaii and in 1960 for the remainder of the Northeast Pacific basin. In 1978, both men's and women's names were utilized. The Northwest Pacific basin tropical cyclones were given women's names officially starting in 1945 and men's names were also included beginning in 1979. The North Indian Ocean region tropical cyclones are not named. The Southwest Indian Ocean tropical cyclones were first named during the 1960/1961 season. The Australian and South Pacific region (east of 90E, south of the equator) started giving women's names to the storms in 1964 and both men's and women's names in 1974/1975. *************************************************************************** Subject: B2) What are the tropical cyclone names through 2001? NORTHERN HEMISPHERE TROPICAL CYCLONE NAMES (Courtesy of Gary Padgett, Jack Beven and James Lewis Free) Atlantic, Gulf of Mexico, Caribbean Sea --------------------------------------- 1996 1997 1998 1999 2000 2001 1. Arthur Ana Alex Arlene Alberto Allison 2. Bertha Bill Bonnie Bret Beryl Barry 3. Cesar Claudette Charley Cindy Chris Chantal 4. Dolly Danny Danielle Dennis Debby Dean 5. Edouard Erika Earl Emily Ernesto Erin 6. Fran Fabian Frances Floyd Florence Felix 7. Gustav Grace Georges Gert Gordon Gabrielle 8. Hortense Henri Hermine Harvey Helene Humberto 9. Isidore Isabel Ivan Irene Isaac Iris 10. Josephine Juan Jeanne Jose Joyce Jerry 11. Kyle Kate Karl Katrina Keith Karen 12. Lili Larry Lisa Lenny Leslie Lorenzo 13. Marco Mindy Mitch Maria Michael Michelle 14. Nana Nicholas Nicole Nate Nadine Noel 15. Omar Odette Otto Ophelia Oscar Olga 16. Paloma Peter Paula PhilippePatty Pablo 17. Rene Rose Richard Rita Rafael Rebekah 18. Sally Sam Shary Stan Sandy Sebastien 19. Teddy Teresa Tomas Tammy Tony Tanya 20. Vicky Victor Virginie Vince Valerie Van Eastern North Pacific (east of 140W) --------------------- 1993 1994 1995 1996 1997 1998 1. Adrian Aletta Adolph Alma Andres Agatha 2. Beatriz Bud Barbara Boris Blanca Blas 3. Calvin Carlotta Cosme Cristina Carlos Celia 4. Dora Daniel Dalila Douglas Dolores Darby 5. Eugene Emilia Erick Elida Enrique Estelle 6. Fernanda Fabio Flossie Fausto Felicia Frank 7. Greg Gilma Gil Genevieve Guillermo Georgette 8. Hilary Hector Henriette Hernan Hilda Howard 9. Irwin Ileana Ismael Iselle Ignacio Isis 10. Jova John Juliette Julio Jimena Javier 11. Kenneth Kristy Kiko Kenna Kevin Kay 12. Lidia Lane Lorena Lowell Linda Lester 13. Max Miriam Manuel Marie Marty Madeline 14. Norma Norman Narda Norbert Nora Newton 15. Otis Olivia Octave Odile Olaf Orlene 16. Pilar Paul Priscilla Polo Pauline Paine 17. Ramon Rosa Raymond Rachel Rick Roslyn 18. Selma Sergio Sonia Simon Sandra Seymour 19. Todd Tara Tico Trudy Terry Tina 20. Veronica Vicente Velma Vance Vivian Virgil 21. Wiley Willa Wallis Winnie Waldo Winifred 22. Xina Xavier Xina Xavier Xina Xavier 23. York Yolanda York Yolanda York Yolanda 24. Zelda Zeke Zelda Zeke Zelda Zeke (The 1999 names will be identical to the list for 1993.) Central North Pacific (from the dateline to 140W) --------------------- Akoni Aka Alika Ana Ema Ekeka Ele Ela Hana Hali Huko Halola Io Iolana Ioke Iune Keli Keoni Kika Kimo Lala Li Lana Loke Moke Mele Maka Malia Nele Nona Neki Niala Oka Oliwa Oleka Oko Peke Paka Peni Pali Uleki Upana Ulia Ulika Wila Wene Wali Walaka Each year the next name is just the one following the last from the previous year. Once through a list the next name will be off of the top of the next list. Western North Pacific (west of the dateline) --------------------- Ann Abel Amber Alex Bart Beth Bing Babs Cam Carlo Cass Chip Dan Dale David Dawn Eve Ernie Ella Elvis Frankie Fern Fritz Faith Gloria Greg Ginger Gil Herb Hannah Hank Hilda Ian Isa Ivan Iris Joy Jimmy Joan Jacob Kirk Kelly Keith Kate Lisa Levi Linda Leo Marty Marie Mort Maggie Niki Nestor Nichole Neil Orson Opal Otto Olga Piper Peter Penny Paul Rick Rosie Rex Rachel Sally Scott Stella Sam Tom Tina Todd Tanya Violet Victor Vicki Virgil Willie Winnie Waldo Wendy Yates Yule Yanni York Zane Zita Zeb Zia Each year the next name is just the one following the last from the previous year. Once through a list the next name will be off of the top of the next list. North Indian Ocean ------------------ Tropical cyclones in this region are not named. SOUTHERN HEMISPHERE TROPICAL CYCLONE NAMES (Thanks to Julian Heming, Jack Beven, Gary Padgett, Frank Woodcock and Jon Gill.) Southwest Indian (west of 90E) ---------------- 1996-1997 1997-1998 1998-1999 1999-2000 ANTOINETTE AIMAY ALDA ASTRIDE BORDELLA BIBIANNE BIRENDA BABIOLA CHANTELLE CINDY CHIKITA CONNIE DANIELLA DONALINE DAVINA DAMIENNE ELVINA ELSIE EVRINA ELINE FABRIOLA FIONA FRANCINE FELICIA GRETELLE GEMMA GENILA GLORIA HELINDA HILLARY HELVETIA HUDAH ILETTA IRELAND IRINA INNOCENTE JOSIE JUDITH JOCYNTHA JONNA KARLETTE KIMMY KRISTINA KENETHA LISETTE LYNN LINA LISANNE MARYSE MONIQUE MARSIA MAIZY NELDA NICOLE NAOMIE NELLA OCLINE OLIVETTE ORACE ORTENSIA PHYLLIS PRISCA PATRICIA PRISCILLA ROLINA RENETTE RITA REBECCA SHERYL SARAH SHIRLEY SOPHIA THELMA TANIA TINA TERRENCE VENYDA VALENCIA VERONIQUE VICTORINE WILTINA WANICKY WILVENIA WILNA YOLETTE YANDAH YASTRIDE YANSELMA [The other areas have lists which they continually rotate through - i.e. don't start again from 'A' each year] Western Australian region (90E to 125E) ------------------------- ADELINE, BERTIE, CLARE, DARYL, EMMA, FLOYD, GLENDA, HUBERT, ISOBEL, JACOB, KARA, LEE, MELANIE, NICHOLAS, OPHELIA, PANCHO, RHONDA, SELWYN, TIFFANY, VICTOR, ZELIA, ALISON, BILLY, CATHY, DAMIEN, ELLE, FREDERIC, GWENDA, HAMISH, ILSA, JOHN, KIRRILY, LEON, MARCIA, NORMAN, OLGA, PAUL, ROSITA, SAM, TARYN, VINCENT, WALTER, ALEX, BESSI, CHRIS, DIANNE, ERROL, FIONA, GRAHAM, HARRIET, INIGO, JANA, KEN, LINDA, MONTY, NICKY, OSCAR, PHOEBE, SALLY, TIM, VIVIENNE, WILLY Northern Australian region (125E to 137E) -------------------------- AMELIA, BRUNO, CORAL, DOMINIC, ESTHER, FERDINAND, GRETEL, HECTOR, IRMA, JASON, KAY, LAURENCE, MARIAN, NEVILLE, OLWYN, PHIL, RACHEL, SID, THELMA, VANCE, WINSOME, ALISTAIR, BONNIE, CRAIG, DEBBIE, EVAN, FAY, GEORGE, HELEN, IRA, JASMINE, KIM, LAURA, MATT, NARELLE, OSWALD, PENNY, RUSSELL, SANDRA, TREVOR, VALERIE, WARWICK Eastern Australian region (137E to 160E, south of ~10S) ------------------------- ALFRED, BLANCH, CHARLES, DENISE, ERNIE, FRANCES, GREG, HILDA, IVAN, JOYCE, KELVIN, LISA, MARCUS, NORA, OWEN, POLLY, RICHARD, SADIE, THEODORE, VERITY, WALLACE, ANN, BRUCE, CECILY, DENNIS, EDNA, FERGUS, GILLIAN, HAROLD, ITA, JUSTIN, KATRINA, LES, MAY, NATHAN, OLINDA, PETE, RONA, SANDY, TESSI, VAUGHAN, WYLVA, ABIGAIL, BERNIE, CLAUDIA, DES, ERICA, FRITZ, GRACE, HARVEY, INGRID, JIM, KATE, LARRY, MONICA, NELSON, ODETTE, PIERRE, REBECCA, STEVE, TANIA, VERNON, WENDY Fiji Area next 10 names (160E to 120W) ----------------------- Yasi, Zaka, Atu, Beti, Cyril, Drena, Evan, Freda, Gavin, Hina Papua New Guinea (140E to 160E, north of ~10S) ---------------- Adel, Epi, Guba, Ila, Kamo, Tako, Upia *************************************************************************** Subject: B3) What names have been retired in the Atlantic basin? In the Atlantic basin, tropical cyclone names are "retired" (that is, not to be used again for a new storm) if it is deemed to be quite noteworthy because of the damage and/or deaths it caused. This is to prevent confusion with a historically well-known cyclone with a current one in the Atlantic basin. The following list gives the names that have been retired through the year 1996 and the year of the storm in question. (Kindly provided by Gary Padgett, Jack Beven and James Lewis Free). Agnes 1972, Alicia 1983, Allen 1980, Andrew 1992, Anita 1977, Audrey 1957 Betsy 1965, Beulah 1967, Bob 1991 Camille 1969, Carla 1961, Carmen 1974, Carol 1965, Celia 1970, Cesar 1996, Cleo 1964, Connie 1955 David 1979, Diana 1990, Diane 1955, Donna 1960, Dora 1964 Edna 1968, Elena 1985, Eloise 1975 Fifi 1974, Flora 1963, Fran 1996, Frederic 1979 Gilbert 1988, Gloria 1985, Gracie 1959 Hattie 1961, Hazel 1954, Hilda 1964, Hortense 1996, Hugo 1989 Inez 1966, Ione 1955 Janet 1955, Joan 1988 Klaus 1990 Luis 1995 Marilyn 1995 Opal 1995 Roxanne 1995 *************************************************************************** B4) What is the origin of the name "hurricane"? "HURRICANE...derived from 'hurican', the Carib god of evil... alternative spellings: foracan, foracane, furacana, furacane, furicane, furicano, haracana, harauncana, haraucane, haroucana, harrycain, hauracane, haurachana, herican, hericane, hericano, herocane, herricao, herycano, heuricane, hiracano, hirecano, hurac[s]n, huracano, hurican, hurleblast, hurlecan, hurlecano, hurlicano, hurrican, hurricano, hyrracano, urycan, hyrricano, jimmycane, oraucan, uracan, uracano" From the _Glossary of Meteorology_ *************************************************************************** Subject: C1) Doesn't the low pressure in the tropical cyclone center cause the storm surge? No. Many people assume that the partial vacuum at the center of a tropical cyclone allows the ocean so rise up in response, thus causing the destructive storm surges as the cyclone makes landfall. However, this effect would be, for example, with a 900 mb central pressure tropical cyclone, only 1.0 m (3 ft). The total storm surge for a tropical cyclone of this intensity can be from 6 to 10 m (19 to 33 ft), or more. Most (>85%) of the storm surge is caused by winds pushing the ocean surface ahead of the storm on the right side of the track (left side of the track in the Southern Hemisphere). Since the surface pressure gradient (from the tropical cyclone center to the environmental conditions) determines the wind strength, the central pressure indirectly does indicate the height of the storm surge, but not directly. Note also that individual storm surges are dependent upon the coastal topography, angle of incidence of landfall, speed of tropical cyclone motion as well as the wind strength. *************************************************************************** Subject: C2) Doesn't the friction over land kill tropical cyclones? (Parts of this section are written by Sim Aberson.) No. During landfall, the increased friction over land acts - somewhat contradictory - to both decrease the sustained winds and also to increase the gusts felt at the surface (Powell and Houston 1996). The sustained (1 min or longer average) winds are reduced because of the dampening effect of larger roughness over land (i.e. bushes, trees and houses over land versus a relatively smooth ocean). The gusts are stronger because turbulence increases and acts to bring faster winds down to the surface in short (a few seconds) bursts. However, after just a few hours, a tropical cyclone over land will begin to weaken rapidly - not because of friction - but because the storm lacks the the moisture and heat sources that the ocean provided. This depletion of moisture and heat hurts the tropical cyclone's ability to produce thunderstorms near the storm center. Without this convection, the storm rapidly fills. An early numerical simulation (Tuleya and Kurihara 1978) had shown that a hurricane making landfall over a very moist region (i.e. mainly swamp) so that surface evaporation is unchanged, intensification may result. However, a more recent study (Tuleya 1994) that has a more realistic treatment of surface conditions found that even over a swampy area a hurricane would weaken because of limited heat sources. Indeed, nature conducted this experiment during Andrew as the hurricane traversed the very wet Everglades, Big Cypress and Corkscrew Swamp areas of southwest Florida. Andrew weakened dramatically: peak winds decreased about 33% and the sea level pressure in the eye filled 19 mb (Powell and Houston 1996). *************************************************************************** Subject: C3) Aren't big tropical cyclones also intense tropical cyclones? No. There is very little association between intensity (either measured by maximum sustained winds or by central pressure) and size (either measured by radius of 15 m/s [gale force] winds or the radius of the outer closed isobar) (Weatherford and Gray 1988). Hurricane Andrew is a good example of a very intense tropical cyclone (922 mb central pressure and 64 m/s (125 kt) sustained winds at landfall in Florida) that was also relatively small (15 m/s winds extended out only about 150 km from the center). Weatherford and Gray (1988) also showed that changes of both intensity and size are essentially independent of one another. *************************************************************************** Subject: C4) Why don't we try to destroy tropical cyclones by: pick one or more - a) seeding them with silver iodide, b) nuking them, c) placing a substance on the ocean surface, d) etc. ? Actually for a couple decades NOAA and its predecessor tried to weaken hurricanes by dropping silver iodide - a substance that serves as a effective ice nuclei - into the rainbands of the storms. The idea was that the silver iodide would enhance the thunderstorms of the rainband by causing the supercooled water to freeze, thus liberating the latent heat of fusion and helping the rainband to grow at the expense of the eyewall. With a weakened convergence to the eyewall, the strong inner core winds would also weaken quite a bit. Neat idea, but it, in the end, had a fatal flaw: there just isn't much supercooled water available in hurricane convection - the buoyancy is fairly small and the updrafts correspondingly small compared to the type one would observe in mid-latitude continental super or multicells. The few times that they did seed and saw a reduction in intensity was undoubtedly due to what is now called "concentric eyewall cycles". Concentric eyewall cycles naturally occur in intense tropical cyclones (wind > 50 m/s or 100 kt). As tropical cyclones reach this threshold of intensity, they usually - but not always - have an eyewall and radius of maximum winds that contracts to a very small size, around 10 to 25 km. At this point, some of the outer rainbands may organize into an outer ring of thunderstorms that slowly moves inward and robs the inner eyewall of its needed moisture and momentum. During this phase, the tropical cyclone is weakening (i.e. the maximum winds die off a bit and the central pressure goes up). Eventually the outer eyewall replaces the inner one completely and the storm can be the same intensity as it was previously or, in some cases, even stronger. A concentric eyewall cycle occurred in Hurricane Andrew (1992) before landfall near Miami: a strong intensity was reached, an outer eyewall formed, this contracted in concert with a pronounced weakening of the storm, and as the outer eyewall completely replaced the original one the hurricane reintensified. Thus nature accomplishes what NOAA had hoped to do artificially. No wonder that the first few experiments were thought to be successes. To learn about the STORMFURY project as it was called, read Willoughby et al. (1985). To learn more about concentric eyewall cycles, read Willoughby et al. (1982) and Willoughby (1990). As for the other ideas, there has been some experimental work in trying to develop a liquid that when placed over the ocean surface would prevent evaporation from occurring. If this worked in the tropical cyclone environment, it would probably have a detrimental effect on the intensity of the storm as it needs huge amounts of oceanic evaporation to continue to maintain its intensity (Simpson and Simpson 1966). However, finding a substance that would be able to stay together in the rough seas of a tropical cyclone proved to be the downfall of this idea. There was also suggested about 20 years ago (Gray et al. 1976) that the use of carbon black (or soot) might be a good way to modify tropical cyclones. The idea was that one could burn a large quantity of a heavy petroleum to produce vast numbers of carbon black particles that would be released on the edges of the tropical cyclone in the boundary layer. These carbon black aerosols would produce a tremendous heat source simply by absorbing the solar radiation and transferring the heat directly to the atmosphere. This would provide for the initiation of thunderstorm activity outside of the tropical cyclone core and, similarly to STORMFURY, weaken the eyewall convection. This suggestion has never been carried out in real- life. Lastly, there always appears ideas during the hurricane season that one should simply use nuclear weapons to try and destroy the storms. Apart from the concern that this might not even alter the storm, this approach neglects the problem that the released radiation would fairly quickly move with the tradewinds to over land. Needless to say, this is not a good idea. < Start Soap Box > Perhaps the best solution is not to try to alter or destroy the tropical cyclones, but just learn to co-exist better with them. Since we know that coastal regions are vulnerable to the storms, enforce building codes that can have houses stand up to the force of the tropical cyclones. Also the people that choose to live in these locations should willing to shoulder a fair portion of the costs in terms of property insurance - not exorbitant rates, but ones which truly reflect the risk of living in a vulnerable region. < End Soap Box > *************************************************************************** Subject: C5) During a hurricane are you supposed to have the windows and doors on the storm side closed and the windows and doors on the lee side open? No! All of the doors and windows should be closed (and shuttered) throughout the duration of the hurricane. The pressure differences between inside your house and outside in the storm do not build up enough to cause any damaging explosions. (No house is built airtight.) The winds in a hurricane are highly turbulent and an open window or door - even if in the lee side of the house - can be an open target to flying debris. All exterior windows should be boarded up with either wooden or metal shutters. *************************************************************************** Subject: D1) How are Atlantic hurricanes ranked? The USA utilizes the Saffir-Simpson hurricane intensity scale (Simpson and Riehl 1981) for the Atlantic and Northeast Pacific basins to give an estimate of the potential flooding and damage to property given a hurricane's estimated intensity: Saffir-Simpson Maximum sustained Minimum surface Storm surge Category wind speed (m/s,kt) pressure (mb) (m,ft) -------------- ------------------- --------------- --------------- 1 33-42 m/s [64-83 kt] >= 980mb 1.0-1.7 m [3-5 ft] 2 43-49 [84-96] 979-965 1.8-2.6 [6-8] 3 50-58 [97-113] 964-945 2.7-3.8 [9-12] 4 59-69 [114-135] 944-920 3.9-5.6 [13-18] 5 > 69 [> 135] < 920 > 5.6 [> 18] 1: MINIMAL: Damage primarily to shrubbery, trees, foliage, and unanchored homes. No real damage to other structures. Some damage to poorly constructed signs. Low-lying coastal roads inundated, minor pier damage, some small craft in exposed anchorage torn from moorings. Example: Hurricane Jerry (1989) 2: MODERATE: Considerable damage to shrubbery and tree foliage; some trees blown down. Major damage to exposed mobile homes. Extensive damage to poorly constructed signs. Some damage to roofing materials of buildings; some window and door damage. No major damage to buildings. Coast roads and low-lying escape routes inland cut by rising water 2 to 4 hours before arrival of hurricane center. Considerable damage to piers. Marinas flooded. Small craft in unprotected anchorages torn from moorings. Evacuation of some shoreline residences and low-lying areas required. Example: Hurricane Bob (1991) 3: EXTENSIVE: Foliage torn from trees; large trees blown down. Practically all poorly constructed signs blown down. Some damage to roofing materials of buildings; some wind and door damage. Some structural damage to small buildings. Mobile homes destroyed. Serious flooding at coast and many smaller structures near coast destroyed; larger structures near coast damaged by battering waves and floating debris. Low-lying escape routes inland cut by rising water 3 to 5 hours before hurricane center arrives. Flat terrain 5 feet of less above sea level flooded inland 8 miles or more. Evacuation of low- lying residences within several blocks of shoreline possibly required. Example: Hurricane Gloria (1985) 4: EXTREME: Shrubs and trees blown down; all signs down. Extensive damage to roofing materials, windows and doors. Complete failures of roofs on many small residences. Complete destruction of mobile homes. Flat terrain 10 feet of less above sea level flooded inland as far as 6 miles. Major damage to lower floors of structures near shore due to flooding and battering by waves and floating debris. Low-lying escape routes inland cut by rising water 3 to 5 hours before hurricane center arrives. Major erosion of beaches. Massive evacuation of all residences within 500 yards of shore possibly required, and of single- story residences within 2 miles of shore. Example: Hurricane Andrew (1992) 5: CATASTROPHIC: Shrubs and trees blown down; considerable damage to roofs of buildings; all signs down. Very severe and extensive damage to windows and doors. Complete failure of roofs on many residences and industrial buildings. Extensive shattering of glass in windows and doors. Some complete building failures. Small buildings overturned or blown away. Complete destruction of mobile homes. Major damage to lower floors of all structures less than 15 feet above sea level within 500 yards of shore. Low-lying escape routes inland cut by rising water 3 to 5 hours before hurricane center arrives. Massive evacuation of residential areas on low ground within 5 to 10 miles of shore possibly required. Example: Hurricane Camille (1969) Note that tropical storms are not on this scale, but can produce extensive damage with rainfall-produced flooding. Note also that category 3, 4, and 5 hurricanes are collectively referred to as intense (or major) hurricanes. These intense hurricanes cause over 70% of the damage in the USA even though they account for only 20% of tropical cyclone landfalls (Landsea 1993). Note that in comparison with the Australian scale (subject D2), Australian 1 and and most of Australian 2 are within the tropical storm categorization (i.e. would not be on the Saffir-Simpson scale). An Australian 3 would be approximately equal to either a Saffir-Simpson category 1 or 2 hurricane. An Australian 4 would be about the same as a Saffir-Simpson category 3 or 4 hurricane. An Australian 5 would be about the same as a Saffir-Simpson category 5 hurricane. *************************************************************************** Subject: D2) How are Australian tropical cyclones ranked? The Australian forecasters have developed a scale for tropical cyclone intensity for storms in their area of responsibility - 90 to 160E (Holland 1993). Note that the sustained winds are based upon a 10 min averaging period instead of the USA 1 minute period. Australian Scale Sustained Winds (km/hr) 1 63-90 km/hr 2 91-125 3 126-165 4 166-225 5 > 225 There are further comments on this scale in subject D1). *************************************************************************** Subject: D3) Why do tropical cyclones' winds rotate counter-clockwise (clockwise) in the Northern (Southern) Hemisphere? The reason is that the earth's rotation sets up an apparent force (called the Coriolis force) that pulls the winds to the right in the Northern Hemisphere (and to the left in the Southern Hemisphere). So when a low pressure starts to form north of the equator, the surface winds will flow inward trying to fill in the low and will be deflected to the right and a counter-clockwise rotation will be initiated. The opposite (a deflection to the left and a clockwise rotation) will occur south of the equator. NOTE: This force is too tiny to effect rotation in, for example, water that is going down the drains of sinks and toilets. The rotation in those will be determined by the geometry of the container and the original motion of the water. Thus one can find both clockwise and counter- clockwise flowing drains no matter what hemisphere you are located. If you don't believe this, test it out for yourself. *************************************************************************** Subject: D4) How do I convert from mph to knots (to m/s) and from inches of mercury to mb (to hPa)? For winds: 1 mile per hour (mph) = 0.864 knots (kt) 1 mph = 1.609 kilometers per hour (kph) 1 mph = 0.4470 meters per second (m/s) 1 kt = 1.853 kph 1 kt = 0.5148 m/s 1 m/s = 3.600 kph For pressures: 1 inch of mercury = 33.86 mb = 33.86 hPa For distances: 1 ft = 0.3048 m *************************************************************************** Subject: 41) How does the damage that hurricanes cause increase as a function of wind speed? Or to rephrase the question: Would a minimal 74 mph hurricane cause one half of the damage that a major hurricane with 148 mph winds? No, the amount of damage (at least experienced along the U.S. mainland) does not increase linearly with the wind speed. Instead, the damage produced increases exponentially with the winds. The 148 mph hurricane (a category 4 on the Saffir-Simpson Scale) may produce - on average - up to 100 times the damage of a minimal category 1 hurricane! Landsea (1993) analyzed the damage caused by various categories of tropical storms and hurricanes after normalizing by both the inflation rate and population changes. Tropical cyclones from 1944 through 1990 were tabulated in terms of 1990 U.S. dollars. The following table summarizes the findings: Intensity (cases) Median Damage "Potential Damage" Tropical/Subtropical Storm (75) <$1,000,000 0 Hurricane Cat. 1 (34) $24,000,000 1 Hurricane Cat. 2 (14) $218,000,000 10 Hurricane Cat. 3 (24) $1,108,000,000 50 Hurricane Cat. 4 (6) $2,274,000,000 100 Hurricane Cat. 5 (1) $5,933,000,000 250 The "Potential Damage" values just provide a reference value if one assigns the median damage caused by a category 1 hurricane to be "1". The rapid increase in damage as the categories go up is apparent. Note that this study was done in mid-1992 (i.e. before Andrew) and thus the median and potential damage values for the category 4 and 5 hurricanes may be on the conservative side. Other interesting findings: * Mean annual damage in mainland US is $1,857,000,000. (Again, this value is pre-Andrew.) * The damage is nearly evenly divided between that caused on the US Gulf Coast (Florida panhandle to Texas) and the US East Coast (Florida peninsula to Maine). * Even though the intense hurricanes (the category 3, 4 and 5 storms) comprise only 20% of all US landfalling tropical cyclones, they account for 71% of all of the damage. (Again, the figure is pre-Andrew. With Andrew included, the damage percentage is likely 75 to 80%.) *************************************************************************** Subject: E1) Which is the most intense tropical cyclone on record? Typhoon Tip in the Northwest Pacific Ocean on 12 October 1979 was measured to have a central pressure of 870 mb and estimated surface sustained winds of 85 m/s (165 kt) (Dunnavan and Diercks 1980). Typhoon Nancy on 12 September, 1961 is listed in the best track data for the Northwest Pacific region as having an estimated maximum sustained winds of 185 kt with a central pressure of 888 mb. However, it is now recognized (Black 1992) that the maximum sustained winds estimated for typhoons during the 1940s to 1960s were too strong and that the 185 kt (and numerous 160 kt to 180 kt reports) is somewhat too high. Note that Hurricane Gilbert's estimated 888 mb lowest pressure in mid- September 1988 is the most intense [as measured by lowest sea level pressure] for the Atlantic basin (Willoughby et al 1989), it is almost 20 mb weaker (higher) than the above Typhoon Tip of the Northwest Pacific Ocean. While the central pressures for the Northwest Pacific typhoons are the lowest globally, the North Atlantic hurricanes have provided sustained wind speeds possibly comparable to the Northwest Pacific. From the best track database, both Hurricane Camille (1969) and Hurricane Allen (1980) have winds that are estimated to be 165 kt. Measurements of such winds are inherently going to be suspect as instruments often are completely destroyed or damaged at these speeds. *************************************************************************** Subject: E2) Which tropical cyclone intensified the fastest? Typhoon Forrest in September 1983 in the Northwest Pacific Ocean deepened by 100 mb (976 to 876 mb) in just under 24 hr (Roger Edson, personal communication). Estimated surface sustained winds increased a maximum of 30 kt in 6 hr and 85 kt in one day (from 65 to 150 kt). *************************************************************************** Subject: E3) Which tropical cyclone has produced the highest storm surge? The Bathurst Bay Hurricane produced a 13 m (about 42 ft) surge in Bathurst Bay, Australia in 1899 (Whittingham 1958). *************************************************************************** Subject: E4) What are the largest rainfalls associated with tropical cyclones? 12 hr: 1144 mm (45.0") at Foc-Foc, La Reunion Island in Tropical Cyclone Denise, 7-8 January, 1966. 24 hr: 1825 mm (71.8") at Foc-Foc, La Reunion Island in Tropical Cyclone Denise, 7-8 January, 1966. 48 hr: 2467 mm (97.1") at Aurere, La Reunion Island 8-10 April, 1958. 72 hr: 3240 mm (127.6") at Grand-Ilet, La Reunion Island in Tropical Cyclone Hyacinthe, 24-27 January, 1980. 10 d: 5678 mm (223.5") at Commerson, La Reunion Island in Tropical Cyclone Hyacinthe, 18-27 January, 1980. (Holland 1993) *************************************************************************** Subject: E5) Which are the largest and smallest tropical cyclones on record? Typhoon Tip had gale force winds (15 m/s) which extended out for 1100 km in radius in the Northwest Pacific on 12 October, 1979 (Dunnavan and Diercks 1980). Tropical Cyclone Tracy had gale force winds that only extended 50 km radius when it struck Darwin, Australia, on 24 December, 1974 (Bureau of Meteorology 1977). *************************************************************************** Subject: E6) Which tropical cyclone lasted the longest? Hurricane/Typhoon John lasted 31 days as it traveled both the Northeast and Northwest Pacific basins during August and September 1994. (It formed in the Northeast Pacific, reached hurricane force there, moved across the dateline and was renamed Typhoon John, and then finally recurved back across the dateline and renamed Hurricane John again.) Hurricane Ginger was a tropical cyclone for 28 days in the North Atlantic Ocean back in 1971. *************************************************************************** Subject: E7) Which tropical cyclones have caused the most deaths and most damage? "The death toll in the infamous Bangladesh Cyclone of 1970 has had several estimates, some wildly speculative, but it seems certain that at least 300,000 people died from the associated storm tide [surge] in the low-lying deltas." (Holland 1993) The largest damage caused by a tropical cyclone as estimated by monetary amounts has been Hurricane Andrew (1992) as it struck the Bahamas, Florida and Louisiana, USA: US $30 *Billion* (R. Sheets - personal communication 1996). Most of this figure was due to destruction in southeast Florida. *************************************************************************** Subject: E8) What are the average, most, and least tropical cyclones occurring in each basin? Based on data from 1968-1989 (1968/69 to 1989/90 for the Southern Hemisphere): Tropical Storm or stronger Hurricane/Typhoon/Severe Tropical Cyclone (>17 m/s sustained winds) (>33 m/s sustained winds) -------------------------------------------------------------------------- Basin Most/Least Average Most/Least Average Atlantic 18/4 9.7 12/2 5.4 NE Pacific 23/8 16.5 14/4 8.9 NW Pacific 35/19 25.7 24/11 16.0 N Indian 10/1 5.4 6/0 2.5 SW Indian 15/6 10.4 10/0 4.4 SE Indian/Aus 11/1 6.9 7/0 3.4 Aus/SW Pacific 16/2 9.0 11/2 4.3 Globally 103/75 83.7 65/34 44.9 Note that the data includes subtropical storms in the Atlantic basin numbers. (Neumann 1993) Starting in 1944, systematic aircraft reconnaissance was commenced for monitoring both tropical cyclones and disturbances that had the potential to develop into tropica cyclones. This is why both Neumann et al. (1993) and Landsea (1993) recommend utilizing data since 1944 for computing climatological statistics. However, for tropical cyclones striking the USA East and Gulf coasts - because of highly populated coast lines, data with good reliability extends back to around 1899. Thus, the following records hold for the entire Atlantic basin (from 1944-1995) and for the USA coastline (1899-1995): Maximum Minimum Tropical storms/hurricanes: 19*(1995) 4 (1983) Hurricanes: 12 (1969) 2 (1982) Intense Hurricanes: 7 (1950) 0 (many times,1994 last) USA landfalling storms/hurricanes: 8 (1916) 1 (many,1991) USA landfalling hurricanes: 6 (1916,1985) 0 (many,1994) USA landfalling intense hurricanes: 3 (1909,33,54) 0 (many,1994) (*) As a footnote, 1933 is recorded as being the most active of any Atlantic basin season on record (reliable or otherwise) with 21 tropical storms and hurricanes. For the Northeast Pacific, the records stand at maximums of 27 tropical storms/hurricanes in 1992 and 16 hurricanes in 1990. Reliable records go back in this basin to around 1966 when geostationary satellite coverage began. For the Northwest Pacific, the peak year stands at 1964 with 39 tropical storms, 26 of which became typhoons. Reliable records for this basin begin around 1960. *************************************************************************** Subject: E9) What are the most and least tropical cyclones occurring in the Atlantic basin and striking the USA? Starting in 1944, systematic aircraft reconnaissance was commenced for monitoring both tropical cyclones and disturbances that had the potential to develop into tropical cyclones. This is why both Neumann et al. (1993) and Landsea (1993) recommend utilizing data since 1944 for computing climatological statistics. However, for tropical cyclones striking the USA East and Gulf coasts - because of highly populated coast lines, data with good reliability extends back to around 1899. Thus, the following records hold for the entire Atlantic basin (from 1944-1996) and for the USA coastline (1899-1996): Maximum Minimum Tropical storms/hurricanes: 19*(1995) 4 (1983) Hurricanes: 12 (1969) 2 (1982) Intense Hurricanes: 7 (1950) 0 (many times,1994 last) USA landfalling storms/hurricanes: 8 (1916) 1 (many,1991) USA landfalling hurricanes: 6 (1916,1985) 0 (many,1994) USA landfalling intense hurricanes: 3 (1909,33,54) 0 (many,1994) (*) As a footnote, 1933 is recorded as being the most active of any Atlantic basin season on record (reliable or otherwise) with 21 tropical storms and hurricanes. Below is a table with individual years for the numbers of named storms (tropical storms and hurricanes) - NS, named and subtropical storms - NS&Sub, hurricanes - H, hurricane days - HD, and intense hurricanes - IH: Atlantic basin tropical cyclone data: Year NS NS&Sub H HD IH 1944 11.00 11.00 7.00 27.00 3.00 1945 11.00 11.00 5.00 14.00 2.00 1946 6.00 6.00 3.00 6.00 1.00 1947 9.00 9.00 5.00 28.00 2.00 1948 9.00 9.00 6.00 29.00 4.00 1949 13.00 13.00 7.00 22.00 3.00 1950 13.00 13.00 11.00 60.00 7.00 1951 10.00 10.00 8.00 36.00 2.00 1952 7.00 7.00 6.00 23.00 3.00 1953 14.00 14.00 6.00 18.00 3.00 1954 11.00 11.00 8.00 32.00 2.00 1955 12.00 12.00 9.00 47.00 5.00 1956 8.00 8.00 4.00 13.00 2.00 1957 8.00 8.00 3.00 21.00 2.00 1958 10.00 10.00 7.00 30.00 4.00 1959 11.00 11.00 7.00 22.00 2.00 1960 7.00 7.00 4.00 18.00 2.00 1961 11.00 11.00 8.00 48.00 6.00 1962 5.00 5.00 3.00 11.00 0.00 1963 9.00 9.00 7.00 37.00 2.00 1964 12.00 12.00 6.00 43.00 5.00 1965 6.00 6.00 4.00 27.00 1.00 1966 11.00 11.00 7.00 42.00 3.00 1967 8.00 8.00 6.00 36.00 1.00 1968 7.00 8.00 4.00 10.00 0.00 1969 17.00 18.00 12.00 40.00 3.00 1970 10.00 10.00 5.00 7.00 2.00 1971 13.00 13.00 6.00 29.00 1.00 1972 4.00 7.00 3.00 6.00 0.00 1973 7.00 8.00 4.00 10.00 1.00 1974 7.00 11.00 4.00 14.00 2.00 1975 8.00 9.00 6.00 21.00 3.00 1976 8.00 10.00 6.00 26.00 2.00 1977 6.00 6.00 5.00 7.00 1.00 1978 11.00 12.00 5.00 14.00 2.00 1979 8.00 9.00 5.00 22.00 2.00 1980 11.00 11.00 9.00 38.00 2.00 1981 11.00 12.00 7.00 23.00 3.00 1982 5.00 6.00 2.00 6.00 1.00 1983 4.00 4.00 3.00 4.00 1.00 1984 12.00 13.00 5.00 18.00 1.00 1985 11.00 11.00 7.00 21.00 3.00 1986 6.00 6.00 4.00 11.00 0.00 1987 7.00 7.00 3.00 5.00 1.00 1988 12.00 12.00 5.00 21.00 3.00 1989 11.00 11.00 7.00 32.00 2.00 1990 14.00 14.00 8.00 27.00 1.00 1991 8.00 8.00 4.00 8.00 2.00 1992 6.00 7.00 4.00 16.00 1.00 1993 8.00 8.00 4.00 10.00 1.00 1996 13.00 13.00 9.00 45.00 6.00 Mean from 1950 - 1990 9.34 9.78 5.83 23.69 2.17 Standard Deviation 4.24 4.51 3.15 17.37 1.81 *************************************************************************** Subject: E10) For the U.S., what are the 10 most intense, 10 costliest, and 10 highest death toll hurricanes on record? Updated from Hebert et al. (1992): 10 Most Intense USA (continental) hurricanes from 1900-1994: (at time of landfall with landfall area) ------------------------------------------------------------ HURRICANE YEAR CATEGORY CENTRAL PRESSURE 1. "Labor Day" - FL Keys 1935 5 892 mb 2. Camille - LA/MS 1969 5 909 3. Andrew - SE FL 1992 4 922 4. Unnamed - FL Keys/S TX 1919 4 927 5. Unnamed - Lake Okeechobee, FL 1928 4 929 6. DONNA - FL Keys 1960 4 930 7. Unnamed - Galveston, TX 1900 4 931 8. Unnamed - Grand Isle, LA 1909 4 931 9. Unnamed - New Orleans, LA 1915 4 931 10. Carla - C TX 1961 4 931 Note that Hurricane Gilbert's estimated 888 mb lowest pressure in mid- September 1988 is the most intense [as measured by lowest sea level pressure] for the Atlantic basin, but it affected the USA only as a weakening tropical depression (Neumann et al 1993). 10 Costliest USA (continental) hurricanes from 1900-1994: (adjusted to 1990 dollars - except for Andrew) --------------------------------------------------------- HURRICANE YEAR CATEGORY DAMAGE (USA) 1. Andrew - SE FL/LA 1992 4 ~$30,000,000,000 2. Hugo - SC 1989 4 7,155,120,000 3. Betsy - FL/LA 1965 3 6,461,303,000 4. Agnes - NE U.S. 1972 1 6,418,143,000 5. Camille - LA/MS 1969 5 5,242,380,000 6. Diane - NE U.S. 1955 1 4,199,645,000 7. "New England" 1938 3 3,593,853,000 8. Frederic - AL/MS 1979 3 3,502,942,000 9. Alicia - N TX 1983 3 2,391,854,000 10. Carol - NE U.S. 1954 3 2,370,215,000 Note that this does not take into account the massive coastal population increases and structural buildup that have occurred along the US East and especially the Gulf coasts during the past few decades. Intense hurricanes will continue to inflict massive destruction along the USA coastlines, even with perfect forecasts of their track and intensity. 10 Deadliest USA (continental) hurricanes from 1900-1994: --------------------------------------------------------- HURRICANE YEAR CATEGORY DEATHS 1. Unnamed - Galveston, TX 1900 4 6000+ 2. Unnamed - Lake Okeechobee, FL 1928 4 1836+ 3. Unnamed - Fl Keys/S TX 1919 4 600-900 4. "New England" 1938 3 600 5. "Labor Day" - FL Keys 1935 5 408 6. Audrey - SW LA/N TX 1957 4 390 7. Unnamed - NE U.S. 1944 3 390 8. Unnamed - Grand Isle, LA 1909 4 350 9. Unnamed - New Orleans, LA 1915 4 275 10. Unnamed - Galveston, TX 1915 4 275 + (These values are estimate and may be conservative of the true numbers of fatalities.) ADDENDUM: Unnamed - LA - 1893 - 2000 Unnamed - SC/GA - 1893 - 1000-2000 Unnamed - GA/SC - 1881 - 700 One can take some comfort in the fact that even with the massive damage amounts reported with hurricanes in the last couple decades, none of those hurricanes caused huge numbers of deaths in the USA. This is because of the increasingly skillful forecasts of hurricane tracks, the ability to communicate warnings to the public via radio and television, and the infrastructure that allows for evacuations to proceed safely for those in the hurricane's path (Sheets 1990). However, if people chose to ignore warnings or if evacuations are not able to remove people from danger (because of too many people overcrowding limited escape routes - the Florida Keys and US 1 is a good example), then the potential remains for disasters similar to what was seen decades ago. *************************************************************************** Subject: E11) What tropical storms and hurricanes have moved from the Atlantic to the Northeast Pacific or vice versa? (Stephen Caparotta, D. Walston, Steven Young and Gary Padgett compiled this list.) Here is a list of tropical cyclones that have crossed from the Atlantic basin to the Northeast Pacific and vice versa. The tropical cyclone must have been of at least tropical storm strength in both basins (i.e. sustained winds of at least 34 kt, or 18 m/s). This record only goes back to 1949. Before the advent of geostationary satellite pictures in the mid-1960s, the number of Northeast Pacific tropical cyclones was undercounted by a factor of 2 or 3. Thus the lack of many of these events during the 1960s and earlier is mainly due to simply missing the Northeast Pacific TCs. There has not been a recorded case where the same tropical cyclone crossed into the Northeast Pacific then crossed back into the Atlantic. Atlantic Hurricane Cesar (July 1996) became Northeast Pacific Hurricane Douglas. Atlantic Tropical Storm Bret (August 1993) became Hurricane Greg in the Northeast Pacific. Northeast Pacific Hurricane Cosme became Atlantic Tropical Storm Allison (June 1989). Atlantic Hurricane Joan (October 1988) became Northeast Pacific Hurricane Miriam. Atlantic Hurricane Greta (September 1978) became Northeast Pacific Hurricane Olivia. Atlantic Hurricane Fifi (September 1974) became Northeast Pacific Tropical Storm Orlene. Atlantic Hurricane Irene (September 1971) became Northeast Pacific Tropical Storm Olivia. Atlantic Hurricane Hattie (October-November 1961) became Northeast Pacific Tropical Storm Simone. A Northeast Pacific Tropical Storm (September-October 1949) became an Atlantic Hurricane (Storm #10) and made landfall in TX. *************************************************************************** Subject: F1) What regions around the globe have tropical cyclones and who is responsible for forecasting there? There are seven tropical cyclone "basins" where storms occur on a regular basis: --- Atlantic basin (including the North Atlantic Ocean, the Gulf of Mexico, and the Caribbean Sea) --- Northeast Pacific basin (from Mexico to about the dateline) --- Northwest Pacific basin (from the dateline to Asia including the South China Sea) --- North Indian basin (including the Bay of Bengal and the Arabian Sea) --- Southwest Indian basin (from Africa to about 100E) --- Southeast Indian/Australian basin (100E to 142E) --- Australian/Southwest Pacific basin (142E to about 120W) The National Hurricane Center in Miami, Florida, USA has responsibil- ities for monitoring and forecasting tropical cyclones in the Atlantic and Northeast Pacific basin east of 140W. The Central Pacific Hurricane Center has responsibilities for the remainder of the Northeast Pacific basin to the dateline. The Northwest Pacific basin is shared in forecasting duties by China, Thailand, Korea, Japan, the Philippines, and Hong Kong. The North Indian basin tropical cyclones are forecasted by India, Thailand, Pakistan, Bangladesh, Burma, and Sri Lanka. Reunion Island, Madagascar, Mozambique, Mauritius, and Kenya provide forecasts for the Southwest Indian basin. Australia and Indonesia forecast tropical cyclone activity in the Southeast Indian/Australian basin. Lastly, for the Australian/Southwest Pacific basin Australia, Papua New Guinea, Fiji, and New Zealand forecast tropical cyclones. Note also that the USA Joint Typhoon Warning Center (JTWC) issues warnings for tropical cyclones in the Northwest Pacific, the North Indian, the Southwest Indian, the Southeast Indian/Australian, and the Australian/Southwest Pacific basins, though they are not specifically tasked to do so by the WMO. The USA Naval Western Oceanography Center in Pearl Harbor, Honolulu does the same for the Pacific Ocean east of 180E. (Neumann 1993) Note that on rare occasions, tropical cyclones (or storms that appear to be similar in structure to tropical cyclones) can develop in the Mediterranean Sea. These have been noted to occur in September 1947, September 1969, January 1982, September 1983, and, most recently, during 13 to 17 January, 1995. Some study of these storms has been reported on by Mayengon (1984) and Ernest and Matson (1983), though it has not been demonstrated fully that these storms are the same as those found over tropical waters. It may be that these Mediterranean tropical cyclones are more similar in nature to polar lows. The following are the addresses of tropical cyclone centers listed above that are responsible for issuing advisories and/or warnings on tropical cyclones (thanks to Jack Beven for these): National Hurricane Center Mail: 11691 SW 17th St. Miami, FL 33165-2149 USA WWW: http://www.nhc.noaa.gov/index.html Central Pacific Hurricane Center Mail: National Weather Service Forecast Office University of Hawaii at Manoa Department of Meteorology 2525 Correa Rd. (HIG) Honolulu, HI 96822 USA Naval Pacific Meteorological and Oceanographic Center Mail: NPMOC/AJTWC Box 113 Pearl Harbor, HI 96860 USA Joint Typhoon Warning Center - Guam Mail: NPMOCW/JTWC PCS 486, Box 17 FPO AP 96536-0051 USA WWW: http://www.npmocw.navy.mil/npmocw/prods/jtwc.html Regional Specialized Meteorological Center Tokyo, Japan - Typhoon Center Mail: Japanese Meteorological Agency 1-3-4 Ote-machi, Chiyoda-ku Tokyo Japan Royal Observatory - Hong Kong Mail: 134A Nathan Road Kowloon Hong Kong Bangkok Tropical Cyclone Warning Center - Thailand Mail: Director Meteorological Department 4353 Sukumvit Rd. Bangkok 10260 Thailand Fiji Tropical Cyclone Warning Center Mail: Director Fiji Meteorological Services Private Mail Bag Nadi Airport Fiji New Zealand Meteorological Service Mail: Director Met Service PO Box 722 Wellington New Zealand Port Moresby Tropical Cyclone Warning Center Mail: Director National Weather Service PO Box 1240 Boroko, NCD Paupa New Guinea Brisbane Tropical Cyclone Warning Center Mail: Regional Director Bureau of Meteorology GPO Box 413 Brisbane 4001 Australia Darwin Tropical Cyclone Warning Center Mail: Regional Director Bureau of Meteorology GPO Box 735 Darwin 5790 Australia Perth Tropical Cyclone Warning Center Mail: Regional Director Bureau of Meteorology GPO Box 6080 Perth 9001 Australia Jakarta, Indonesia Mail: Director Analysis and Processing Centre Jalan Arief Rakhman Hakim 3 Jakarta Indonesia Regional Tropical Cyclone Advisory Centre - Reunion Mail: Director of Meteorological Services PO Box 4 97490 Sainte Clotilde Reunion Sub-Regional Tropical Cyclone Warning Center - Mauritius Mail: Director of Meteorological Service Vacoas Mauritius Sub-Regional Tropical Cyclone Warning Center - Madagascar Mail: Director of Meteorological Service PO Box 1254 Antananarivo 101 Madagascar Nairobi, Kenya Mail: Director of Meteorological Services PO Box 30259 Nairobi Kenya Maputo, Mozambique Mail: Director of Meteorology PO Box 256 Maputo Mozambique The following cities are also mentioned as tropical cyclone warning centers, though I don't have the addresses for them. Philippines: Manila China: Beijing Dalian Shanghai Guangzhou Korea: Seoul Vietnam: Hanoi India: New Delhi Calcutta Bombay Bangladesh: Dhaka Burma: Rangoon Sri Lanka: Colombo Maldive Islands: Male *************************************************************************** Subject: F2) What is Prof. Gray's seasonal hurricane forecast for this year and what are the predictive factors? Prof. Bill Gray at Colorado State University in Fort Collins, Colorado (USA) has issued seasonal hurricane forecasts for the Atlantic basin since 1984. Details of his forecasting technique can be found in Gray (1984a,b) and Gray et al. (1992, 1993, 1994). Landsea et al. (1994) also provides verifications of the first 10 years of forecasting. A quick summary of the components follows: * El Nino/Southern Oscillation (ENSO) - During El Nino events (ENSO warm phase), tropospheric vertical shear is increased inhibiting tropical cyclone genesis and intensification. La Nina events (ENSO cold phase) enhances activity. * African West Sahel rainfall - In years of West Sahel drought conditions, the Atlantic hurricane activity is much reduced - especially the intense hurricane activity (Landsea and Gray 1992). Wet West Sahel years mean a higher chance of low-latitude "Cape Verde" type hurricanes. This is also due to higher tropospheric vertical shear in the drought years, though there may also be changes in the structure of African easterly waves as well to make them less likely to go through tropical cyclogenesis. * Stratospheric quasi-biennial oscillation (QBO) - During the 12 to 15 months when the equatorial stratosphere has the winds blowing from the east (east phase QBO), Atlantic basin tropical cyclone activity is reduced. The east phase is followed by 13 to 16 months of westerly winds in the equatorial stratosphere where the Atlantic activity is increased. It is believed (but not demonstrated) that the reduced activity in east years is due to increased lower stratospheric to upper tropospheric vertical shear which may disrupt the tropical cyclone structure. * Caribbean sea level pressure anomalies (SLPA) - During seasons of lower than average surface pressure around the Caribbean Sea, the Atlantic hurricane activity is enhanced. When it is higher than average, the tropical cyclone activity is diminished. Higher pressure indicates either a weaker Inter-tropical Convergence Zone (ITCZ) or a more equatorward position of the ITCZ or both. * Caribbean 200 mb zonal wind anomalies (ZWA) - The 200 mb winds around the Caribbean are often reflective of the ENSO or West Sahelian rainfall conditions (i.e. westerly ZWA corresponds to El Ninos and West Sahel drought conditions). However, the winds also provide some independent measure of the tropospheric vertical shear, especially in years of neutral ENSO and West Sahel rainfall. Dr. Gray and his forecast team issues seasonal forecasts in late November, early June, and early August of each year with a verification of the forecasts given in late November. To obtain these forecasts, surf to: http://tropical.atmos.colostate.edu/forecasts/index.html Also available (via unix machines) a finger command to get a table with the latest forecast info and what the observations have been of the season so far. Available via: finger forecast@typhoon.atmos.colostate.edu *************************************************************************** Subject: F3) How has Dr. Gray done in previous years of forecasting hurricanes? Here are the numbers that Dr. Gray has issued for his real-time Atlantic tropical cyclone seasonal forecasting: Year Early December Early June Early August Observed Forecast Forecast Forecast Named Storms: 1950 to 1990 Mean = 9.3 1984 --- 10 10 12 1985 --- 11 10 11 1986 --- 8 7 6 1987 --- 8 7 7 1988 --- 11 11 12 1989 --- 7 9 11 1990 --- 11 11 14 1991 --- 8 7 8 1992 8 8 8 6 1993 11 11 10 8 1994 10 9 7 7 1995 12 12 16 19 1996 8 10 11 13 Hurricanes: 1950 to 1990 Mean = 5.8 1984 --- 7 7 5 1985 --- 8 7 7 1986 --- 4 4 4 1987 --- 5 4 3 1988 --- 7 7 5 1989 --- 4 4 7 1990 --- 7 6 8 1991 --- 4 3 4 1992 4 4 4 4 1993 6 7 6 4 1994 6 5 4 3 1995 8 8 9 11 1996 5 6 7 9 Intense Hurricanes: 1950 to 1990 Mean = 2.3 1990 --- 3 2 1 1991 --- 1 0 2 1992 1 1 1 1 1993 3 2 2 1 1994 2 1 1 0 1995 3 3 3 5 1996 2 2 3 6 *************************************************************************** Subject: F4) What are those track and intensity models that the Atlantic forecasters are talking about in the tropical storm and hurricane Discussions? (Track model information contributed by Sim Aberson) A variety of hurricane track forecast models are run operationally for the Atlantic hurricane basin: (1) The basic model that is used as a "no-skill" forecast to compare other models against is CLIPER (CLImatology and PERsistence), a multiple regression model that best utilizes the persistence of the motion and also incorporates climatological track information (Neumann 1972, Merrill 1980). Surprisingly, CLIPER was difficult to beat with numerical model forecasts until the 1980s. (2) A statistical-dynamical model, NHC90 (McAdie 1991), uses geopotential height predictors from the Aviation model to produce a track forecast four times per day. The primary synoptic time NHC90 forecasts (00 and 12 UTC) are based upon 12 h old Aviation runs. A special version of NHC90, NHC90-LATE, is run at primary synoptic times with the current Aviation run, and is available a number of hours after NHC90. Both versions of NHC90 have been run operationally since 1990. (3) The Beta and Advection Model, BAM, follows a trajectory in the pressure-weighted vertically-averaged horizontal wind from the Aviation model beginning at the current storm location, with a correction that accounts for the beta effect (Marks 1992). Three versions of this model, one with a shallow-layer (BAMS), one with a medium-layer (BAMM), and one with a deep-layer (BAMD), are run. BAMS runs using the 850-700 mb layer, BAMM with the 850-400 mb layer, and BAMD with the 850-200 mb layer. The deep-layer version was run operationally for primary synoptic times in 1989; all three versions have been run four times per day since 1990. (4) A nested barotropic hurricane track forecast model (VICBAR) has been run four times daily since 1989. The 0000 and 1200 UTC runs are based upon current NCEP analyses, the others upon six hour old data (Aberson and DeMaria 1994). Another barotropic model, LBAR, for Limited-Area Barotropic Model, is also being run operationally every 6 h based upon six hour old data, so is available for earlier use by the NHC forecasters. (5) A triply-nested movable mesh primitive equation model developed at the Geophysical Fluid Dynamics Laboratory (Bender et al 1993), known as the GFDL model, has provided forecasts since the 1992 hurricane season. (6) The NCEP Aviation and MRF models (Lord 1993) have been used for track forecasting since the 1992 hurricane season. These are global models. (7) The United Kingdom Meterological Office's global model (UKMET) is utilized for forecasting the track of tropical cyclones around the world (Radford 1994). The National Hurricane Center starting receiving these operationally during 1996. (8) The United States Navy Operational Global Atmospheric Prediction Systems (NOGAPS) is also a global numerical model that shows skill in forecasting tropical cyclone track (Fiorino et al. 1993). This model was also first received operationally at the National Hurricane Center during 1996. Despite the variety of hurricane track forecast models, there are only a few models that forecast intensity change for the Atlantic basin: (1) Similar to the CLIPER track model, SHIFOR (Statistical Hurricane Intensity Forecast model) is used as a "no-skill" intensity change forecast. It is a multiple regression statistical model that best utilizes the persistence of the intensity trends and also incorporates climatological intensity change information (Jarvinen and Neumann 1979). Surprisingly, no other intensity models provide better forecasts on average than SHIFOR. (2) A statistical-synoptic model, SHIPS (Statistical Hurricane Intensity Prediction Scheme), has been available the National Hurricane Center since the mid-1990s (DeMaria and Kaplan 1994). It takes current information on the synoptic scale on the sea surface temperatures, vertical shear, etc. with an optimal combination of the trends in the cyclone intensity. For the first time in 1996, SHIPS outperformed SHIFOR (by having lower absolute wind speed errors) from the 24 hour to 72 hour forecasts, though the differences were small. (3) The GFDL model, described above in the track forecasting models, also issues forecasts of intensity change for the National Hurricane Center. However, to date, these have yet to show any skill (i.e. GFDL errors are larger than those from SHIFOR). *************************************************************************** Subject: G1) What is the annual cycle of occurrence seen in each basin? While the Atlantic hurricane season is "officially" from 1 June to 30 November, the Atlantic basin shows a very peaked season with 78% of the tropical storm days, 87% of the minor (Saffir-Simpson Scale categories 1 and 2 - see subject D1) hurricane days, and 96% of the intense (Saffir- Simpson categories 3, 4 and 5) hurricane days occuring in August through October (Landsea 1993). Peak activity is in early to mid September. Once in a few years there may be a tropical cyclone occurring "out of season" - primarily in May or December. The Northeast Pacific basin has a broader peak with activity beginning in late May or early June and going until late October or early November with a peak in storminess in late August/early September. The Northwest Pacific basin has tropical cyclones occurring all year round regularly though there is a distinct minimum in February and the first half of March. The main season goes from July to November with a peak in late August/early September. The North Indian basin has a double peak of activity in May and November though tropical cyclones are seen from April to December. The severe cyclonic storms (>33 m/s winds) occur almost exclusively from April to June and late September to early December. The Southwest Indian and Australian/Southeast Indian basins have very similar annual cycles with tropical cyclones beginning in late October/ early November, reaching a double peak in activity - one in mid-January and one in mid-February to early March, and then ending in May. The Australian/Southeast Indian basin February lull in activity is a bit more pronounced than the Southwest Indian basin's lull. The Australian/Southwest Pacific basin begin with tropical cyclone activity in late October/early November, reaches a single peak in late February/early March, and then fades out in early May. Globally, September is the most active month and May is the least active month. (Neumann 1993) *************************************************************************** Subject: G2) How does El Nino-Southern Oscillation affect tropical cyclone activity around the globe? The effect of El Nino-Southern Oscillation (ENSO) on Atlantic tropical cyclones is described in subject F2). The Australian/Southwest Pacific shows a pronounced shift back and forth of tropical cyclone activity with fewer tropical cyclones between 145 and 165E and more from 165E eastward across the South Pacific during El Nino (warm ENSO) events. There is also a smaller tendency to have the tropical cyclones originate a bit closer to the equator. The opposite would be true in La Nina (cold ENSO) events. See papers by Nicholls (1979), Revell and Goulter (1986), Dong (1988), and Nicholls (1992). The western portion of the Northeast Pacific basin (140W to the dateline) has been suggested to experience more tropical cyclone genesis during the El Nino year and more tropical cyclones tracking into the sub-region in the year following an El Nino (Schroeder and Yu 1995), but this has not been completely documented yet. The Northwest Pacific basin, similar to the Australian/Southwest Pacific basin, experiences a change in location of tropical cyclones without a total change in frequency. Pan (1981), Chan (1985), and Lander (1994) detailed that west of 160E there were reduced numbers of tropical cyclone genesis with increased formations from 160E to the dateline during El Nino events. The opposite occurred during La Nina events. Again there is also the tendency for the tropical cyclones to also form closer to the equator during El Nino events than average. The eastern portion of the Northeast Pacific, the Southwest Indian, the Southeast Indian/Australian, and the North Indian basins have either shown little or a conflicting ENSO relationship and/or have not been looked at yet in sufficient detail. *************************************************************************** Subject: G3) What may happen with tropical cyclone activity in a 2xCO2 world? Two impacts of anthropogenic climate change due to increasing amounts of "greenhouse" gases that may occur (Houghton et al., 1990, 1992) are increased tropical sea surface temperatures (moderate confidence) and increased tropical rainfall associated with a slightly stronger inter- tropical convergence zone (ITCZ) (moderate/low confidence). Because of these possible changes, there have been many suggestions based upon global circulation and theoretical modeling studies that increases may occur in the frequency (AMS Council and UCAR Board of Trustees, 1988; Houghton et al., 1990; Broccoli and Manabe, 1990; Ryan et al., 1992; Haarsma et al., 1993), area of occurrence (Houghton et al., 1990; Ryan et al., 1992), mean intensity (AMS Council and UCAR Board of Trustees, 1988; Haarsma et al., 1993), and maximum intensity (Emanuel, 1987; AMS Council and UCAR Board of Trustees, 1988; Houghton et al., 1990; Haarsma et al., 1993; Bengtsson et al., 1994) of tropical cyclones. In contrast, there have been some conclusions that decreases in frequency may result (Broccoli and Manabe 1990; Bengtsson et al., 1994). One report (Leggett, 1994) has suggested that increased tropical cyclone incidence and severity have already taken place, but provided no quantitative evidence. Any changes in tropical cyclone activity are intrinsically tied in with large-scale changes in the tropical atmosphere. One key feature that has been focused upon has been possible changes in sea surface temperatures (SSTs). But SSTs by themselves cannot be considered without corresponding information regarding the moisture and stability in the tropical troposphere. What has been identified in the current climate as being necessary for genesis and maintenance for tropical cyclones (e.g. SSTs of at least 80F or 26.5C) might change in a 2xCO2 world because of possible changes in the moisture and/or stability. Additionally, besides the thermodynamic variables, changes in the tropical dynamics will also play a big role in determining changes in tropical cyclone activity. For example, if the vertical wind shear over the tropical North Atlantic decreased (increased) during the hurricane season in a 2xCO2 world, then we would see a significant increase (decrease) in activity. Another large unknown is how the monsoonal circulations may change. If the monsoons became more active, then it may be possible that more tropical cyclones in the oceanic monsoon regions might result. One last final wild card in all of this is how the El Nino-Southern Oscillation (ENSO) may change in a 2xCO2 world, as ENSO is the largest single factor controlling year-to-year variability of tropical cyclones globally - see sections G2) and F2). If the warm phase of ENSO (the "El Nino" events) occurred more often and/or with more intensity, then the inhabitants along the Atlantic basin and Australia would have fewer tropical cyclones to worry about. But people living in Hawaii and in the South Central Pacific would have more storms to deal with. The reverse would be true if the cold phase (or "La Nina") became more prevalent. Overall, it is difficult to assess globally how changes of tropical cyclone intensities (both the mean and the maximum), frequencies, and area of occurrence may change in a 2xCO2 world. It may very well turn out that changes around the globe may not be consistent, with some regions receiving more activity while others getting less. Certainly, this is an area of research that needs to continue until more definitive answers are found. *************************************************************************** Subject: G4) Are we getting stronger and more frequent hurricanes, typhoons, and tropical cyclones in the last several years? Globally, probably not. For the Atlantic basin, definitely not. In fact, as documented in Landsea (1993), the number of intense hurricanes (those hurricanes reaching Saffir-Simpson scale 3, 4, and 5 - defined in subject D1) has actually gone *down* during the 1970s and the 1980s, both in all basin intense hurricanes as well as those making landfall along the U.S. coastline. "With Andrew in 1992 and the busy 1995 hurricane season, have things changed during the 1990s?" No. Even taking into account Andrew, the period 1991 to 1994 was the *quietest* four years on record - using reliable data going back to 1944 (Landsea et al. 1996). Of course, with a very active Atlantic hurricane season (19 tropical storms and hurricanes, 11 hurricanes, and 5 intense hurricanes), it is quite possible that we may be moving to a regime of more tropical cyclone activity - but one year does not a trend make. Some more interesting tidbits about Atlantic tropical cyclones (from Landsea et al. 1996): * no significant change in total frequency of tropical storms and hurricanes over 52 years (1944-1995), * a strong *DECREASE* in numbers of intense hurricanes, * no change in the strongest hurricanes observed each year, * A moderate *DECREASE* in the max intensity reached by all storms over a season, * no hurricanes have been observed over the Caribbean Sea during the years 1990-1994 - the longest period of lack of hurricanes in the area since 1899. This was followed up by 3 hurricanes in just one year - 1995 - to affect the region, * 1991-1994 is the quietest (in terms of frequency of total storms - 7.5 per year, hurricanes - 3.8, and intense hurricanes - 1.0) four year period on record, since 1944. As for the other basins, Black (1992) has identified a moderately severe bias in the Northwest Pacific reported maximum sustained winds during the 1940s to the 1960s that makes interpretation of trends difficult for that region. Nicholls (1992) has shown that the numbers of tropical cyclones around Australia (105-165E) has decreased rather dramatically since the mid-1980s. Some of this reduction is undoubtedly due to having more El Nino events since that time (i.e. 1986-87, 1991-2, 1993, 1994-95). However, even taking into account the El Nino effect, there is still a reduction that is unexplained and may be due to changes in tropical cyclone monitoring. The other basins have not been examined for trends, partly because the data will likely not be trustworthy before the advent of the geo- stationary satellites in the mid-1960s. IMHO, I would suspect though that the western portion of the Northeast Pacific, the eastern portion of the Northwest Pacific, and the South Pacific east of 165E would have a real upward trend of tropical cyclone occurrences because of the more frequent El Nino events in the last decade or so (see section G2 for more information on El Nino effects). *************************************************************************** Subject: G5) Why do tropical cyclones occur primarily in the summer and autumn? As described in subject G1), the primary time of year for getting tropical cyclones is during the summer and autumn: July-October for the Northern Hemisphere and December-March for the Southern Hemisphere (though there are differences from basin to basin). The peak in summer/autumn is due to having all of the necessary ingredients become most favorable during this time of year: warm ocean waters (at least 26C or 80F), a tropical atmosphere that can quite easily kick off convection (i.e. thunderstorms), low vertical shear in the troposphere, and a substantial amount of large- scale spin available (either through the monsoon trough or easterly waves - see subject A4)). While one would intuitively expect tropical cyclones to peak right at the time of maximum solar radiation (late June for the tropical Northern Hemisphere and late December for the tropical Southern Hemisphere), it takes several more weeks for the oceans to reach their warmest temperatures. The atmospheric circulation in the tropics also reaches its most pronounced (and favorable for tropical cyclones) at the same time. This time lag of the tropical ocean and atmospheric circulation is analogous to the daily cycle of surface air temperatures - they are warmest in mid-afternoon, yet the sun's incident radiation peaks at noon. *************************************************************************** Subject: G6) What determines the movement of tropical cyclones? Tropical cyclones - to a first approximation - can be thought of as being steered by the surrounding environmental flow throughout the depth of the troposphere (from the surface to about 12 km or 8 mi). Dr. Neil Frank, former director of the U.S. National Hurricane Center, used the analogy that the movement of hurricanes is like a leaf being steered by the currents in the stream, except that for a hurricane the stream has no set boundaries. In the tropical latitudes (typically equatorward of 20-25 N or S), tropical cyclones usually move toward the west with a slight poleward component. This is because there exists an axis of high pressure called the subtropical ridge that extends east-west poleward of the storm. On the equatorward side of the subtropical ridge, general easterly winds prevail. However, if the subtropical ridge is weak - oftentimes due to a trough in the jet stream - the tropical cyclone may turn poleward and then recurve back toward the east. On the poleward side of the subtropical ridge, westerly winds prevail thus steering the tropical cyclone back to the east. These westerly winds are the same ones that typically bring extratropical cyclones with their cold and warm fronts from west to east. Many times it is difficult to tell whether a trough will allow the tropical cyclone to recurve back out to sea (for those folks on the eastern edges of continents) or whether the tropical cyclone will continue straight ahead and make landfall. For more non-technical information on the movement of tropical cyclones, see Pielke's _The Hurricane_. For a more detailed, technical summary on the controls on tropical cyclone motion, see Elsberry's chapter in _Global Perspectives on Tropical Cyclones_. Both books are detailed in Part II of the FAQ. *************************************************************************** Subject: G7) Why doesn't the South Atlantic Ocean experience tropical cyclones? Though many people might speculate that the sea surface temperatures are too cold, the primary reasons that the South Atlantic Ocean gets no tropical cyclones are that the tropospheric (near surface to 200mb) vertical wind shear is much too strong and there is typically no inter-tropical convergence zone (ITCZ) over the ocean (Gray 1968). Without an ITCZ to provide synoptic vorticity and convergence (i.e. large scale spin and thunderstorm activity) as well as having strong wind shear, it becomes very difficult to nearly impossible to have genesis of tropical cyclones. However, in rare occasions it may be possible to have tropical cyclones form in the South Atlantic. In McAdie and Rappaport (1991), the USA National Hurricane Center documented the occurrence of a strong tropical depression/weak tropical storm that formed off the coast of Congo in mid-April 1991. The storm lasted about five days and drifted toward the west-southwest into the central South Atlantic. So far, there has not been a systematic study as to the conditions that accompanied this rare event. *************************************************************************** Subject: G8) Does an active June and July mean the rest of the season will be busy too? No. The number of named storms (hurricanes) occurring in June and July correlates at an insignificant r = +0.13 (+0.02) versus the whole season activity. Actually, there is a slight _negative_ association of early season storms (hurricanes) versus late season - August through November - r = -0.28 (-0.35). Thus, early season activity, be it very active or quite calm, has little bearing on the season as a whole. These correlations are based on the years 1944-1994. *************************************************************************** Subject: G9) Why do hurricanes hit the East coast of the U.S., but never the West coast? Hurricanes form both in the Atlantic basin (i.e. the Atlantic Ocean, Gulf of Mexico and Caribbean Sea) to the east of the continental U.S. and in the Northeast Pacific basin to the west of the U.S. However, the ones in the Northeast Pacific almost never hit the U.S., while the ones in the Atlantic basin strike the U.S. mainland just less than twice a year on average. There are two main reasons. The first is that hurricanes tend to move toward the west-northwest after they form in the tropical and subtropical latitudes. In the Atlantic, such a motion often brings the hurricane into the vicinity of the U.S. east coast. In the Northeast Pacific, a west-northwest track takes those hurricanes farther off-shore, well away from the U.S. west coast. In addition to the general track, a second factor is the difference in water temperatures along the U.S. east and west coasts. Along the U.S. east coast, the Gulf Stream provides a source of warm (> 80 F or 26.5 C) waters to help maintain the hurricane. However, along the U.S. west coast, the ocean temperatures rarely get above the lower 70s, even in the midst of summer. Such relatively cool temperatures are not energetic enough to sustain a hurricane's strength. So for the occasional Northeast Pacific hurricane that does track back toward the U.S. west coast, the cooler waters can quickly reduce the strength of the storm. *************************************************************************** Subject: G10) How much lightning occurs in tropical cyclones? Surprisingly, not much lightning occurs in the inner core (within about 100 km or 60 mi) of the tropical cyclone center. Only around a dozen or less cloud-to-ground strikes per hour occur around the eyewall of the storm, in strong contrast to an overland mid-latitude mesoscale convective complex which may be observed to have lightning flash rates of greater than 1000 per hour (!) maintained for several hours. Hurricane Andrew's eyewall had less than 10 strikes per hour from the time it was over the Bahamas until after it made landfall along Louisiana, with several hours with no cloud-to-ground lightning at all (Molinari et al. 1994). However, lightning can be more common in the outer cores of the storms (beyond around 100 km or 60 mi) with flash rates on the order of 100s per hour. This lack of inner core lightning is due to the relative weak nature of the eyewall thunderstorms. Because of the lack of surface heating over the ocean ocean and the "warm core" nature of the tropical cyclones, there is less buoyancy available to support the updrafts. Weaker updrafts lack the super-cooled water (e.g. water with a temperature less than 0 C or 32 F) that is crucial in charging up a thunderstorm by the interaction of ice crystals in the presence of liquid water (Black and Hallett 1986). The more common outer core lightning occurs in conjunction with the presence of convectively-active rainbands (Samsury and Orville 1994). One of the exciting possibilities that recent lightning studies have suggested is that changes in the inner core strikes - though the number of strikes is usually quite low - may provide a useful forecast tool for intensification of tropical cyclones. Black (1975) suggested that bursts of inner core convection which are accompanied by increases in electrical activity may indicate that the tropical cyclone will soon commence a deepening in intensity. Analyses of Hurricanes Diana (1984), Florence (1988) and Andrew (1992), as well as an unnamed tropical storm in 1987 indicate that this is often true (Lyons and Keen 1994 and Molinari et al. 1994). *************************************************************************** Subject: H1) What is the Dvorak technique and how is it used? The Dvorak technique is a methodology to get estimates of tropical cyclone intensity from satellite pictures. Vern Dvorak developed the scheme using a pattern recognition decision tree in the early 1970s (Dvorak 1975, 1984). Utilizing the current satellite picture of a tropical cyclone, one matches the image versus a number of possible pattern types: Curved band Pattern, Shear Pattern, Eye Pattern, Central Dense Overcast (CDO) Pattern, Embedded Center Pattern or Central Cold Cover Pattern. If infrared satellite imagery is available for Eye Patterns (generally the pattern seen for hurricanes, severe tropical cyclones and typhoons), then the scheme utilizes the difference between the temperature of the warm eye and the surrounding cold cloud tops. The larger the difference, the more intense the tropical cyclone is estimated to be. From this one gets a data "T-number" and a "Current Intensity (CI) Number". CI numbers have been calibrated against aircraft measurements of tropical cyclones in the Northwest Pacific and Atlantic basins. On average, the CI numbers correspond to the following intensities: CI Maximum Sustained Central Pressure Number One Minute Winds (mb) (kt) (Atlantic) (NW Pacific) 0.0 <25 ---- ---- 0.5 25 ---- ---- 1.0 25 ---- ---- 1.5 25 ---- ---- 2.0 30 1009 1000 2.5 35 1005 997 3.0 45 1000 991 3.5 55 994 984 4.0 65 987 976 4.5 77 979 966 5.0 90 970 954 5.5 102 960 941 6.0 115 948 927 6.5 127 935 914 7.0 140 921 898 7.5 155 906 879 8.0 170 890 858 Note that this estimation of both maximum winds and central pressure assumes that the winds and pressures are always consistent. However, since the winds are really determined by the pressure gradient, small tropical cyclones (like the Atlantic's Andrew in 1992, for example) can have stronger winds for a given central pressure than a larger tropical cyclone with the same central pressure. Thus caution is urged in not blindly forcing tropical cyclones to "fit" the above pressure- wind relationships. (The reason that lower pressures are given to the Northwest Pacific tropical cyclones in comparison to the higher pressures of the Atlantic basin tropical cyclones is because of the difference in the background climatology. The Northwest Pacific basin has a lower background sea level pressure field. Thus to sustain a given pressure gradient and thus the winds, the central pressure must accordingly be smaller in this basin.) The errors for using the above Dvorak technique in comparison to aircraft measurements taken in the Northwest Pacific average 10 mb with a standard deviation of 9 mb (Martin and Gray 1993). Atlantic tropical cyclone estimates likely have similar errors. Thus an Atlantic hurricane that is given a CI number of 4.5 (winds of 77 kt and pressure of 979 mb) could in reality be anywhere from winds of 60 to 90 kt and pressures of 989 to 969 mb. These would be typical ranges to be expected; errors could be worse. However, in the absence of other observations, the Dvorak technique does at least provide a consistent estimate of what the true intensity is. While the Dvorak technique was calibrated for the Atlantic and Northwest Pacific basin because of the aircraft reconnaissance data ground truth, the technique has also been quite useful in other basins that have limited observational platforms. However, at some point it would be preferable to re-derive the Dvorak technique to calibrate tropical cyclones with available data in the other basins. Lastly, while the Dvorak technique is primarily designed to provide estimates of the current intensity of the storm, a 24 h forecast of the intensity can be obtained also by extrapolating the trend of the CI number. Whether this methodology provides skillful forecasts is unknown. *************************************************************************** Subject: H2) Who are the "Hurricane Hunters" and what are they looking for? (Contributed by Neal Dorst.) In the Atlantic basin (Atlantic Ocean, Gulf of Mexico, and Caribbean Sea) hurricane reconnaissance is carried out by two government agencies, the U.S. Air Force Reserves' 53rd Weather Reconnaissance Squadron and NOAA's Aircraft Operations Center. The U.S. Navy stopped flying hurricanes in 1975. The 53rd WRS is based at Keesler AFB in Mississippi and maintains a fleet of ten WC-130 planes. These cargo airframes have been modified to carry weather instruments to measure wind, pressure, temperature and dew point as well as drop instrumented sondes and make other observations. AOC is presently based at MacDill AFB in Tampa, Florida and among its fleet of planes has two P-3 Orions, originally made as Navy sub hunters, but modified to include three radars as well as a suite of meteorological instruments and dropsonde capability. Starting in 1996 AOC has added to its fleet a Gulfstream IV jet that will be able to make hurricane observations from much higher altitudes (up to 45,000 feet). It has a suite of instruments similar to those on the P-3s. The USAF planes are the workhorses of the hurricane hunting effort. They are often deployed to a forward base, such as Antigua, and carry out most of the reconnaissance of developing waves and depressions. Their mission in these situations is to look for signs of a closed circulation and any strengthening or organizing that the storm might be showing. This information is relayed by radio to the National Hurricane Center for the hurricane specialists to evaluate. The NOAA planes are more highly instrumented and are generally reserved for when developed hurricanes are threatening landfall, especially landfall on U.S. territory. They are also used to conduct scientific research on storms. The planes carry between six to fifteen people, both the flight crew and the meteorologists. Flight crews consist of a pilot, co-pilot, flight engineer, navigator, and electrical technicians. The weather crew might consist of a flight meteorologist, lead project scientist, cloud physicist, radar specialist, and dropsonde operators. The primary purpose of reconnaissance is to track the center of circulation, these are the co-ordinates that the National Hurricane Center issues, and to measure the maximum winds. But the crews are also evaluating the storm's size, structure, and development and this information is also relayed to NHC via radio and satellite link. Most of this data, which is critical in determining the hurricane's threat, cannot be obtained from satellite. *************************************************************************** REFERENCES ---------- Aberson, S.D., and M. DeMaria (1994): Verification of a Nested Barotropic Hurricane Track Forecast Model (VICBAR). _Mon. 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