Where Do the Hurricanes Come From?

RadiationVapor/Cloud/precipitation

Shallow convection Boundary layer turbulence

Mesoscale convective system ThunderstormTornado

Heat waveMidlatitude cycloneTropical cyclone

Diurnal variation

Madden-Julian Oscillation Tropical wavesAnnular modes

100,000yr100yr10yr1yr1mon1day1min1sec10-15sec

Global Climate System

Globe

Continent

State

City

Football field

1 mm

1 m

Spatial Scale

Time Scale

10-4 m Composition

Monsoon

El NinoBiennial Oscillation

Global warmingMulti-decadal Oscillation

Ice ageGlacial cycle

The most common atmospheric circulation structure

L

H

H

L

HeatingCoolingor No Heating

Imbalance of heating Imbalance of temperature Imbalance of pressure Wind

Radiation Convection

Latent/Sensible

Conduction

Land/Ocean/Ice Feedback

Video: Hurricane KatrinaThe Storm that Drowned a City

https://www.youtube.com/watch?v=974O47UmrII

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.– Western Pacific has the highest average number of tropical cyclones per

year

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 (do not form between 5N-5S where Coriolis force is too weak)

6. Pre-existing low-level rotating circulations (tropical waves and other disturbances)

Necessary environmental conditions for tropical cyclone formation

Four stages of tropical cyclone formation

Organized convection >200 km, >24 hours

Vortex w/ max wind <38 mph

Vortex w/ max wind 39-73 mph

Vortex w/ max wind >74 mph

• 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: Wind flow

L

H

Vertical Structure: Heating, temperature, pressure and wind

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

• Maximum rainfall occur in the region of eye wall

• Minimum surface pressure occurs in the region of central eye

• Maximum winds occur in the region of eye wall

L

H

Vertical Structure: Feedbacks

1. Convergence-Convection (CISK): positive

2. Surface flux-Convection (WISHE): positive

3. Air-sea interaction: negative

Sea surface temperature

The most common atmospheric circulation structure

L

H

H

L

HeatingCoolingor No Heating

Imbalance of heating Imbalance of temperature Imbalance of pressure Wind

Radiation Convection

Latent/Sensible

Conduction

Land/Ocean/Ice Feedback

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

- central pressures (ended in 1990s)- maximum sustained wind speeds (used

now)

Hurricane Intensity Scale

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 Landfall

Destruction caused by:• Hurricane-scale winds (>74 mph)• Rainfall (10 in/day)• 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

Trends and variability in Hurricane Activity

• Destructive hurricane seasons result in public awareness and general concern that hurricane activity is related to global warming.

• There is evidence for an increase trend in major Atlantic hurricanes.

• The Atlantic Multidecadal Oscillation (AMO) is a 60yr oscillation in water temperatures and is a major factor in the increase in Atlantic hurricane activity.

Summary

• Tropical cyclone genesis: Western Pacific has the highest averaged number per year. 6 necessary conditions. 4 stages.

• Tropical cyclone structure: 3 major components, rotation direction of inflow and outflow, location of maximum wind and rainfall, 3 feedbacks

• Tropical cyclone intensity scale. Category 1: 74mph, category 5: 155mph

• Tropical cyclone destruction: 4 reasons? Which side has the most intense destruction?

• Trends and variability in tropical cyclone activity