Where Do the Hurricanes Come From?

Review of last lecture Forces acting on a cloud/rain droplet. Terminal velocity. How does it change with cloud drop radius? Growth mechanisms for rain and snow

Summary of Precipitation processes: Condensation Warm clouds Cool/cold clouds Collision- coalescence Bergeron Process Riming/ Aggregation Rain Snow (can change to rain, sleet, or any other type of precipitation depending on underlying atmosphere

Observation of tropical cyclones: Aircrafts (The hurricane hunters)

Observation of tropical cyclones: Satellite (1960s-Now)

Genesis and tracks of tropical cyclones 5.4 2.5 8.9 16 4.4 3.4 4.3 Different names: hurricanes, typhoons, cyclones Genesis region generally in the tropics, especially between 5N-15N and 5S-15S.

Necessary environmental conditions for tropical cyclone formation SST > 27 oC (Poleward of about 20o SST too cold for formation. Highest frequency in late summer to early autumn when water is warmest.) Warm ocean mixed layer is thick enough to supply energy (this is why they weaken quickly upon landfall) Unstable atmosphere with a moist lower/middle troposphere (central and western ocean basins) Low vertical windshear (Otherwise upward transfer of latent heat disrupted) Coriolis force (do not form between 5N-5S where Coriolis force is too weak) Pre-existing low-level rotating circulations (tropical waves and other disturbances)

Tropical waves often cause tropical cyclone formation (e. g Tropical waves often cause tropical cyclone formation (e.g. African easterly waves, Madden-Julian Oscillation)

Four stages of tropical cyclone formation Vortex w/ max wind >74 mph Vortex w/ max wind 39-73 mph Vortex w/ max wind <38 mph Organized convection >200 km, >24 hours

Hurricane Intensity Scale Saffir-Simpson scale classifies hurricanes into five categories based on: - central pressures (ended in 1990s) - maximum sustained wind speeds (used now) Categories 3, 4, 5 are collectively called major hurricanes. They account for 21% of hurricane landfall in U.S., but cause 83% of the damage.

Hurricane names Named alphabetically 6 lists in rotation When there is a hurricane that is too devastating, the name will retire and be replaced by a new name 2007 2008 2009 2010 2011 2012 Andrea Arthur Ana Alex Arlene Alberto Barry Bertha Bill Bonnie Bret Beryl Chantal Cristobal Claudette Colin Cindy Chris Dean Dolly Danny Danielle Don Debby Erin Edouard Erika Earl Emily Ernesto Felix Fay Fred Fiona Franklin Florence Gabrielle Gustav Grace Gaston Gert Gordon Humberto Hanna Henri Hermine Harvey Helene Ingrid Ike Ida Igor Irene Isaac Jerry Josephine Joaquin Julia Jose Joyce Karen Kyle Kate Karl Katia Kirk Lorenzo Laura Larry Lisa Lee Leslie Melissa Marco Mindy Matthew Maria Michael Noel Nana Nicholas Nicole Nate Nadine Olga Omar Odette Otto Ophelia Oscar Pablo Paloma Peter Paula Philippe Patty Rebekah Rene Rose Richard Rina Rafael Sebastien Sally Sam Shary Sean Sandy Tanya Teddy Teresa Tomas Tammy Tony Van Vicky Victor Virginie Vince Valerie Wendy Wilfred Wanda Walter Whitney William

Movie time! How do the meteorologists study and predict hurricanes?

Horizontal Structure of tropical cyclones Total diameter: about 600km Lifetime: up to a week or more Make up: many thunderstorms arranged in pinwheel formation Three components: 1. Central eye - clear skies, light winds (25 km diameter) 2. Eye wall - strongest winds (can be 200 mph), max precip. (up to 2500 mm/day) 3. Spiral rainbands Cylonic inflow, anticyclonic outflow. Outflow creates cirrus shield that often obscures pinwheel structure

Vertical Structure: Wind flow

Vertical Structure: Thermal Warm core - fueled by thermal energy - core warmer than outside at upper levels Minimum surface pressure occurs in the region of central eye Maximum winds and rainfall occur in the region of eye wall

Vertical Structure: Feedbacks Sea surface temperature Convergence-Convection (CISK): positive Surface flux-Convection (WISHE): positive Air-sea interaction: negative

Tropical cyclone Destruction and Fatalities Destruction caused by: Hurricane-scale winds Rainfall Storm surge (winds blowing coast-ward + lower atmosphere pressure) Fine-scale Tornadoes Destruction most intense on right side of cyclone (wind + storm speed)

Hurricane-spawned tornadoes More than half of the landfall hurricanes produce at least one tornado Hurricane Buelah (1967) produced 141 tornadoes Most likely occur in the right-front quadrant Generally less intense than those that occur in the great plains

Tropical cyclone forecast model By Tim Marchok of NOAA/GFDL

Tropical cyclone forecast skill: Track

Tropical cyclone forecast skill: Intensity

Summary Tropical cyclone genesis: 6 necessary conditions, 4 stages Tropical cyclone tracks Tropical cyclone structure: 3 major components, rotation direction of inflow and outflow, location of maximum wind and rainfall, 3 feedbacks Tropical cyclone destruction: 4 reasons? Which side has the most intense destruction? Tropical cyclone forecast: track and intensity Currently which skill is better?