Where Do the Hurricanes Come From?. Radiation Vapor/Cloud/precipitation Shallow convection Boundary...
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Transcript of Where Do the Hurricanes Come From?. Radiation Vapor/Cloud/precipitation Shallow convection Boundary...
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
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: 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.
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