Tropical Cyclones Storms passing within 65 miles of my home in Greenville, NC in my lifetime.
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Tropical Cyclones Storms passing within 65 miles of my home in Greenville, NC in my lifetime Backing way up … Today • Brief review: “The Tropics” – Conditions for development • Classical stages of development • Storm dangers Tomorrow • Forecast process • Case study: 2005 season (Katrina, Rita, Wilma, etc.) Analyzing the Tropics • • • • Located 23 ½ ° N to 23 ½ ° S Large amount of solar radiation Little seasonal variation Warm water = tremendous latent heat source • High relative humidity promotes cumulus cloud development • Easterly winds at the surface Necessary – but not sufficient – conditions for TC development (Gray 1968) 1. Strong moisture convergence into the vortex caused by frictionally-forced low level convergence (Ekman turning), 2. Accompanying upper tropospheric divergence that leads to deep cumulus convection, 3. Slightly more net divergence than convergence in the vortex column, 4. Horizontal wind shear present in the lower troposphere but minimal vertical shear, 5. Sea-surface and deeper ocean temperatures at or exceeding 26.5 °C, 6. Poleward latitude of at least 5 degrees to invoke Coriolis turning, and 7. A pre-existing low-level vorticity disturbance Classical factors governing TC motion (track) • • • • • Synoptic steering flow Coriolis (beta) effect Convective asymmetries Sea-surface temperature gradients Landmass interaction Frequency of Occurrence July 17 June formative regions July formative regions August formative regions September formative regions October formative regions November formative regions How to define a TC The phrase “tropical cyclone” (TC) is a general term for a cyclone originating over the tropical oceans (AMS Glossary 2nd edition) that is “driven principally by heat transfer with the ocean” (Emanuel 2003). TCs with wind speeds of at least 39 mph but not more than 73 mph are known as tropical storms. TCs with wind speeds at or over 74 mph (64 kts, or 33 m s-1) are known as hurricanes in the North Atlantic and Eastern North Pacific Oceans, typhoons in the Northwest Pacific Ocean, severe tropical cyclones in the Southwest Pacific and Southeast Indian oceans, and severe cyclonic storms in the North Indian Ocean (Neumann 1993). The term “hurricane” was derived from the various West Indian words for “monstrous gods” (Dunn and Miller 1960). Formative Mechanisms • Tropics characterized by strong tropical inversion 500-1000 m above ocean – Must have a lifting mechanism to overcome this very strong and very persistent inversion • Inter-Tropical Convergence Zone – NE trade winds north of zone collide with SE trade winds south of zone – Persistent thunderstorm formation • Stalled cold fronts over very warm waters – Enhanced convective activity • *Easterly waves – Forces surface convergence ( lifting) *Tropical easterly waves are the classical mechanism Stages of Development • • • • • Tropical (“Easterly”) Wave Tropical Disturbance Tropical Depression (given a #) Tropical Storm (given a name) Hurricane (categories 1 thru 5) Increasing Organization Tropical (“Easterly”) Wave Tropical (“Easterly”) Wave • Troughs of low pressure – Found in the easterly trade winds (hence named “easterly” – a wave in the easterlies) • Migrate westward at low latitudes – (i.e., they move from east to west) • Surface wind convergence – On the east side of the trough axis Tropical (“Easterly”) Wave • Tropical trough – “Upside down” – Notice rain is behind trough Subsidence (sinking) and clear air Rising air & Thunderstorms Tropical (“Easterly”) Wave Tropical (“Easterly”) Wave Tropical Disturbance • Broad area of disorganized clusters of thunderstorms • Commonly exist only 24 to 48 hours • Key question: can thunderstorm convection maintaining itself? – Critical to future intensification! Organization • • • • • Convergence Rising motion / thunderstorms Thunderstorms Intense latent heat release Latent heat release Enhances convection Convection Lower surface pressure Low surface pressure Wind convergence • POSITIVE FEEDBACK! Tropical Depression • A tropical disturbance that has closed isobars and a circulation center • Winds typically range 20mph to 30mph • Given a “number” by National Hurricane Center – subsequently tracked Tropical Storm • Now we’re talking! • Definition: a cyclonic circulation originating over tropical oceans with sustained surface winds of at least 39mph (17m/s or 34kts) • Named at this point Facts about naming of TCs • Each ocean basin has its own list of names – Atlantic, Northeast Pacific (off Mexico), Northwest Pacific (near China / Taiwan / Philippines), Northeast Indian, Northwest Indian, Southwest Pacific (off Australia) • World Meteorological Organization (a subgroup of the UN) approves names – Countries bounding each ocean basin submit names to WMO for approval • Thus names in Atlantic are of English, Spanish, French, Creole, and West Indian origin Facts about naming, ctd. • Atlantic basin has 6 lists of names (thus, this year’s names were also used in 2000, 1994, etc.) – 21 letters (no Q, U, X, Y, or Z), Alphabetical, alternate male/female – If exhaust the list (only happened in 2005), use Greek letters (Alpha, Beta, Gamma, Delta, Epsilon, Zeta, etc etc) • Other basins have only one list – Simply start with the next name, no matter if only one day or 100 days between storms Atlantic Tropical Cyclone Names 2006 2007 2008 2009 2010 2011 Alberto Beryl Chris Debby Ernesto Florence Gordon Helene Isaac Joyce Kirk Leslie Michael Nadine Oscar Patty Rafael Sandy Tony Valerie William Andrea Barry Chantal Dean Erin Felix Gabrielle Humberto Ingrid Jerry Karen Lorenzo Melissa Noel Olga Pablo Rebekah Sebastien Tanya Van Wendy Arthur Bertha Cristobal Dolly Edouard Fay Gustav Hanna Isidore Josephine Kyle Lili Marco Nana Omar Paloma Rene Sally Teddy Vicky Wilfred Ana Bill Claudette Danny Erika Fabian Grace Henri Ida Joaquin Kate Larry Mindy Nicholas Odette Peter Rose Sam Teresa Victor Wanda Alex Bonnie Colin Danielle Earl Fiona Gaston Hermine Igor Julia Karl Lisa Matthew Nicole Otto Paula Richard Shary Tomas Virginie Walter Arlene Bret Cindy Don Emily Franklin Gert Harvey Irene Jose Katia Lee Maria Nate Ophelia Philippe Rina Sean Tammy Vince Whitney Retired Hurricane Names (by year) 1954 Carol Hazel 1955 Connie Diane Ione Janet 1956 1957 Audre y 1958 1959 1960 Donna 1961 Carla Hattie 1962 1963 Flora 1964 Cleo Dora Hilda 1965 Betsy 1966 Inez 1967 Beula h 1968 Edna 1969 Camille 1970 Celia 1971 1972 Agnes 1973 1974 Carmen Fifi 1975 Eloise 1976 1977 Anita 1978 1979 David Frederic 1980 Allen 1981 1982 1983 Alicia 1984 1985 Elena Gloria 1986 1987 1988 Gilbert Joan 1989 Hugo 1990 Diana Klaus 1991 Bob 1992 Andrew 1993 1994 1995 Luis Marilyn Opal Roxanne 1996 Cesar Fran Hortense 1997 1998 Georges Mitch 1999 Floyd Lenny 2000 Keith 2001 Allison Iris Michelle 2002 Isidore Lili 2003 Fabian Isabel Juan 2004 Charley Frances Ivan Jeanne 2005 Dennis Katrina Rita Stan Wilma Tropical Storm KYLE Hurricane • Maximum sustained winds of at least 74mph (64 knots, 33m/s) • Most organized tropical cyclone – Well-defined parts • Positive feedback mechanism at its finest – Can last upwards of 20 days – Travel over 1000km • Categorized further based on intensity – Categories 1 thru 5 A two-dimensional depiction of hurricane structure: - Vertical motion (arrows) - Cloud matter (shading, both deep cumulus towers and cirrus shield [the central dense overcast]) Source: Frank (1977) Hurricane cross-sections Sources: Frank (1977) Temperature perturbation (degrees C). Notice the warm core is especially pronounced around 300 hPa Relative humidity distribution. Notice RH increases radially-inward West-east distribution of tangential velocity. Positive Vθ represents cyclonic motion. Notice the wind maxima is concentrated between 900 and 500 hPa. This feature (vertical location of strongest hurricane winds) will be revisited! Note also the reversal of wind direction above 300 hPa, from cyclonic to anticyclonic. This allows for mass to be “evacuated” from the inner-core updrafts, and encourages stronger updrafts. (It also is responsible for creating the central dense overcast). Source: Frank (1977) Radial view of equivalent potential temperature. Notice the warm-core structure of the eye. Hurricane Classification Classification in more detail Saffir-Simpson scale: 5 categories based on intensity Hurricane “Parts” • • • • Eye Central Dense Overcast Eye wall Spiral Rain Bands Tropical Depression #4 Hurricane Alberto Category 1 Hurricane Danielle Category 2 Hurricane Rita Category 3 Hurricane Fran Category 3 Hurricane Gert Hurricane Floyd Category 2 Category 4 Hurricane Mitch Category 5