Weather Systems I: Tropical Cyclones

• Forces acting on a cloud/rain droplet. Terminal velocity. How does it change with cloud drop radius?

• Growth mechanisms for rain and snow

Review of last lecture

Summary of Precipitation processes:Condensation

Collision-coalescence

Bergeron Process

Rain Snow(can change to rain, sleet, or any other type of precipitation depending on underlying atmosphere

Riming/Aggregation

Observation of tropical cyclones: Aircrafts (The hurricane hunters)

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

Genesis and tracks of tropical cyclones

– Different names: hurricanes, typhoons, cyclones– Genesis region generally in the tropics, especially between 5N-15N and

5S-15S.

164.4

8.95.4

2.5

3.4

4.3

1. SST > 27 oC (Poleward of about 20o SST too cold for formation. Highest frequency in late summer to early autumn when water is warmest.)

2. Warm ocean mixed layer is thick enough to supply energy (this is why they weaken quickly upon landfall)

3. Unstable atmosphere with a moist lower/middle troposphere (central and western ocean basins)

4. Low vertical windshear (Otherwise upward transfer of latent heat disrupted)5. Coriolis force (hurricanes do not form between 5N-5S)6. Pre-existing low-level rotating circulations (tropical waves and other

disturbances)

Necessary environmental conditions for tropical cyclone formation

Tropical waves often cause tropical cyclone formation (e.g. African easterly waves,

Madden-Julian Oscillation)

Four stages of tropical cyclone formation

Tracks of Tropical Cyclones

Global distribution of the paths of all tropical cyclones (1985-2005)

Locations of hurricane landfall in U.S.

Hurricane names2007 2008 2009 2010 2011 2012Andrea Arthur Ana Alex Arlene AlbertoBarry Bertha Bill Bonnie Bret BerylChantal Cristobal Claudette Colin Cindy ChrisDean Dolly Danny Danielle Don DebbyErin Edouard Erika Earl Emily ErnestoFelix Fay Fred Fiona Franklin FlorenceGabrielle Gustav Grace Gaston Gert GordonHumberto Hanna Henri Hermine Harvey HeleneIngrid Ike Ida Igor Irene IsaacJerry Josephine Joaquin Julia Jose JoyceKaren Kyle Kate Karl Katia KirkLorenzo Laura Larry Lisa Lee LeslieMelissa Marco Mindy Matthew Maria MichaelNoel Nana Nicholas Nicole Nate NadineOlga Omar Odette Otto Ophelia OscarPablo Paloma Peter Paula Philippe PattyRebekah Rene Rose Richard Rina RafaelSebastien Sally Sam Shary Sean SandyTanya Teddy Teresa Tomas Tammy TonyVan Vicky Victor Virginie Vince ValerieWendy Wilfred Wanda Walter Whitney William

• 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

Horizontal Structure of tropical cyclones

Vertical Structure of tropical cyclones

Vertical cross-section: Structure

• Warm core - fueled by thermal energy - core warmer than outside at upper levels

Vertical cross-section: Feedbacks

1. Convergence-Convection (CISK): positive

2. Surface flux-Convection (WISHE): positive

3. Air-sea interaction: negative

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)

Tropical cyclone Destruction and Fatalities

Saffir-Simpson scale classifies hurricanes into five categories based on:

- central pressures- maximum sustained wind speeds- storm surge

Hurricane Intensity Scale

Hurricane-spawned tornadoes

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, 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?