17ATOC - Lecture notes 17 PDF

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Lecture 17...

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Tropical Cyclone Introduction: -Tropical cyclone: Low pressure systems formed over warm tropical waters with sustained winds of 63 km/h or greater and gusts in excess of 90 km/h near the centre -Atlantic tropical cyclone paths mean they affect more than just the tropics  30% global insurance losses related to tropical cyclones  Since 1900, tropical cyclones have claimed ~600 lives in Canada or her waters  The Perfect Storm 1991  Luis 1995  Gabrielle 2001  Gustav 2002  Fabian 2003  Juan 2003 -Amongst the most powerful and destructive meteorological systems on Earth -80-100 develop globally, over 7 large tropical ocean basins each year -Many make landfall and can cause considerable damage due to high winds, surges and heavy rain  Galveston 1900  Camille 1969  Katrina 2005  royal palm impaled with 1x4 board located near inner edge of eyewall with winds more than 220 km/h Tropical Cyclones and Hurricanes: -“Hurricane”, “Cyclone”, “Baguios”, “Typhoon” all signify a strong tropical cyclone (winds > 120 km/h) -Non-frontal, synoptic (large) scale low-pressure system (cyclone) originating over (sub)tropical waters with:  Organized convection (strong ascending air)  Cyclonic surface wind circulation (counter clockwise) -Tropical cyclone regions and seasons  Late summer and early autumn for most basins  Most of the year for the North-West Pacific Hurricanes: -From the Carib Indian word “huracan” -Derived from a Mayan god who created the world with his breath, blowing on the oceans to create dry land -Hurricane season:  Atlantic basin  June to November, sharply peaking from late August through September  Eastern pacific basin  May to November, Naming Tropical Cyclones: -West-Indies: Saint’s Names (San Felipe 1876) -Australian forecaster early 19th century -During WWII Pacific cyclones named after girlfriends and wives of US army air Corp and navy -North Atlantic has a 6 year repeat cycle -Retired names  such as Hazel, Camille, Tomas Hurricanes and Canada: -Hurricanes often end up over Canadian land or waters -Usually have lost most of their strength -May reach eastern Canada still with hurricane-strength winds -Hurricanes Juan and Isabel occurred in September 2003  Hurricane Juan heavily damaged parts of Atlantic Canada

 Hurricane Isabel was the costliest, deadliest and strongest hurricane in the 2003 Atlantic basin -Statistics over the past 110 years (1886-1995):  Average number tropical cyclones affecting Canada each year:  Average percentage all Atlantic tropical cyclones that affect Canada:  1995 Hurricane Luis was a category 3 storm, affected Avalon Peninsula of Newfoundland  Sep. 1996 Hortense near Cape Breton, Nova Scotia  120 km/h winds, $3 million in damage (flooding, wind damage and power outages) -Hurricane Hazel 1954  Storm of a lifetime for Ontariens  Significant damage in central Canada  Hazel formed in early October 1954, crossed the Caribbean and the eastern U.S. before entering southern Ontario in mid-October  Hazel left over 1,000 dead in Haiti, 6 in the Bahamas and another 95 in the U.S.  81 people killed and $100 million in damage in Ontario  Technically no longer a hurricane when it entered Canada  downgraded to just a tropical depression  Most of damage in Ontario from severe rainfall  200 mm in less than 24 hours Necessary Conditions for Hurricane Development: -Warm ocean waters (> 26.5° C over 50m) -Unstable atmosphere -Pre-existing near surface disturbance with sufficient vorticity (rotation) and convergence -Moist air in lower and mid-troposphere -Minimum distance of at least 500 km from equator (coriolis force) -Low (< 10 m/s) vertical wind shear between surface and upper troposphere  Wind shear: variation in wind velocity occurring along a direction at right angles to the wind’s direction and tending to exert a turning force -Upper tropospheric anticyclone Tropical Systems: Easterly Wave: -Easterly waves: atmospheric trough, an elongated area of relatively low air pressure -Approximate location, amplitude and wavelength of easterly waves:  Moving 18-35 km/hr  1000-1500 km distance between trough axes -Around 2 EWs/week from Africa to North America -Only 9% of EW form tropical storms or hurricanes -Around 60% of all Atlantic tropical storms and minor hurricanes originate from EWs -Around 85% of the intense (or major) hurricanes originate as EWs  Cape Verde Hurricanes are Atlantic and happen because of tropical waves that passed over Cape Verde islands after exiting the coast of West Africa Stages of Development: -Categories vary from region to region -Atlantic and Eastern Pacific regions:  Tropical disturbance  Tropical depression  Tropical storm  Tropical cyclone (hurricane) Wind (km/h) 30-39

Stage NA (disturbance)

40-60 Tropical Depression 61-120 Tropical Storm >120 Cyclone, Hurricane, Typhoon, etc. -Sustained winds are defined as a 1 min average wind measured at about 10 m above the surface Hurricanes: -Intense tropical system of strong thunderstorms, well-defined surface circulation and maximum sustained winds of 120 km/h or higher -Destructive  Violent winds  Flooding due to high tides and heavy rainfall  Cyclones Chapala and Megh hit Yemen -Characteristics:  Diameter: around 500 km  Life cycle: a few days  Central pressure: 950 hPa or less  Winds often > 180 kt  Cloud tops > 12 km  Movement 15-25 km/h westwards  Curving away from equator  Torrential rains Hurricane Structure: -Outer region  Low levels: cyclonic winds, increasing towards centre, weak convection  High levels: anticyclonic flow, directed outward, Cirrus and Cirrostratus clouds -Rain bands  Spiral belts of clouds  Hurricane-force winds, showers -Eye wall  Ring-shaped inner region  Strongest winds, heaviest rain -Eye  D = 25-50 km  Light winds (< 10 m/s), release of latent heat  No rain -Typhoon Haiyan  one of the most intense tropical cyclones on record, Philippines Formation Heat Engine Theory: -Heat taken at high T, converted to work, then taken out at cold T -Small swirling eddies transfer sensible and latent heat into the storm from ocean -The warmer the water, the greater the wind speed  great transfer of heat -Amount of work done proportional to the difference in T between input and output  Max. achievable strength of a hurricane is proportional to the difference in air temperature between Tropopause and surface -The warmer the water, the greater the wind speed  great transfer of heat -Amount of work done proportional to the difference in T between input and output -Max. achievable strength of a hurricane is proportional to the difference in air temperature between Tropopause and surface

Factors Maintaining Hurricanes: -Supply of moisture and heat  Energy from latent heat release  300-400 billion kWh/day  10-20 billion tons/day of rain  Isothermal expansion  Divergence aloft -These factors explain:  Rapid weakening over land and cold water  Tropical cyclone seasons and regions...