Transcript Ch12: Tropical Storms and Hurricanes

Where Do the Hurricanes Come From?
Global Climate System
Spatial
Scale
Globe
Global warming
Multi-decadal Oscillation
Ice age
Glacial cycle
El Nino
Biennial Oscillation
Continent
Monsoon
Madden-Julian Oscillation
Tropical waves
Annular modes
Diurnal variation
Heat wave
Midlatitude cyclone
Tropical cyclone
State
City
Football
field
1 mm
1 m
10-4 m
Mesoscale convective system
Thunderstorm
Tornado
Shallow convection
Boundary layer turbulence
Vapor/Cloud/precipitation
Radiation
Composition
10-15sec 1sec 1min
1day
1mon
1yr
10yr
100yr
100,000yr
Time Scale
The most common atmospheric
circulation structure
H
L
Radiation
Cooling
or No
Heating
Convection
Heating
Latent/Sensible
Conduction
H
L
Imbalance of heating
 Imbalance of temperature
 Imbalance of pressure
 Wind
Land/Ocean/Ice
Feedback
Video: Hurricane Katrina
The 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
2.5
16
4.4
8.9
5.4
3.4
4.3
– Different names: hurricanes, typhoons, cyclones
– Genesis region generally in the tropics, especially between 5N-15N and 5S15S.
– Western Pacific has the highest average number of tropical cyclones per
Necessary environmental conditions for
tropical cyclone formation
1.
2.
3.
4.
5.
6.
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)
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
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
H
L
Vertical Structure:
Heating, temperature, pressure and wind
H
L
• 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
Vertical Structure: Feedbacks
Sea surface temperature
1. Convergence-Convection (CISK): positive
2. Surface flux-Convection (WISHE): positive
3. Air-sea interaction: negative
The most common atmospheric
circulation structure
H
L
Radiation
Cooling
or No
Heating
Convection
Heating
Latent/Sensible
Conduction
H
L
Imbalance of heating
 Imbalance of temperature
 Imbalance of pressure
 Wind
Land/Ocean/Ice
Feedback
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 Landfall
Tropical cyclone Destruction and Fatalities
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)
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