Transcript Title here - University of Manchester

Planetary Atmospheres, the Environment and Life
(ExCos2Y)
Topic 8: Storm Systems
Chris Parkes
Rm 455 Kelvin Building
7. Water
The effects of Waters
Heat Capacity of water
Wind – water interaction
Ocean Circulation
El Nino
Revision
Storm Systems
* Tropical cyclones * Tornadoes * Dust storms
Hurricanes
Tropical Cyclones
classification according to wind
speed
< 17m/s
17 – 33m/s
> 33m/s
tropical depression
tropical storm
hurricane (Atlantic)
typhoon (Eastern Pacific)
severe tropical cyclone (Western Pacific)
severe tropical storm (Indian Ocean)
Affected regions
Hurricane
Hurricane
Typhoon
Tropical
cyclone
Tropical
cyclone
Hurricane Formation Conditions
• Warm Ocean (> 26.5ºC) to 50m depth
– Provides energy for convection
• Coriolis Force
– Formation of vortex, at > 5º latitude
• High Humidity
– Heat of condensation powers hurricane
• Suitable Upper Winds
– Winds in different directions at different heights would disrupt
vortex formation
• Unstable Air
– Cools fast with height
• Pre-existing disturbed weather
– Allows cyclone to start
Typical radius 450 Km
Necessary conditions for Hurricane Formation
Hurricane formation and Atmospheric “waves”
Wave – disturbance in lower troposphere (0-5km)
Due to African easterly jet
Period 3~4 days; wavelength 2~2.5k km; speed ~20km/h
~60 per year; these waves start ~85% intense hurricane
Hurricane life cycle
Formation – 3 days
Max intensity – 3 days
Core warming - 2 days Decay - 2 days
Hurricane Life-Cycle
• Formation
– “Seeded” in tropical disturbance
– Thunderstorm activity – liberation of heat stored in warm
waters
– Heat transfer warm water to humid air
– Release of latent heat when vapour condenses
– Energy drives convection
• Decay
– moves inland: friction lessens winds, source of moisture
removed
– or over cooler water
– System stops moving, sea surface cooled
– move to higher latitudes – cooler water
– upper wind patterns disrupt vortex
Hurricane Structure
• Central Eye:
• ~30-60 km diameter
– relatively calm
– Slowly sinking air
• Eyewall
– Circle of thunderstorms, strongest winds
• Outer spiral rain bands
Hurricane structure
•Eye
•Eyewall
•Rainbands
storm surge – rise of water
level associated with low
pressure system
Hurricane air circulation
Release of “latent heat” of water vapour
 increase convection
Massive outflow at troposphere  fall in surface pressure
Low surface pressure (950mb)  large inflow at surface
Coriolis force causes inflow to rotate
Hurricane classification
Saffir-Simpson scale:
Category
Storm
surge (m)
1
2
Wind
speed
(m/s)
33-42
43-49
3
4
50-58
59-69
2.7-3.8
3.9-5.6
5
70+
5.7+
1.0-1.7
1.8-2.6
Wind speed is max. sustained speed
Max. at right front quadrant (in Northern hemisphere)
Global warming may be increasing intensity
Hurricane Rotation Direction
• Coriolis Force driving wind directions
Tornado
• Also known as Twisters
• Not the same as hurricanes !
– Only 75m across
– Only last minutes to hours
– Form on ground
• Very Strong Wind speeds
– 100 Km/hr
• Formed by
“super-cell” thunderstorm
• Hurricanes can spawn Tornados
Tornado alley
Supercell structure
Supercell structure
Supercell / Tornado formation
Other storm features – Water Spouts
• Tornado-like over water
• Wind speeds lower than typical tornado
• Funnel is condensation
– Does not ‘suck’ up water
Other storm features – dust devil
Dust devil – heating from ground
not associated with a supercell thunderstorm
typically lower intensity than tornados
Dust Devil on Mars
Martian dust devils can be
fifty times as wide
and ten times as high as
Earth dust devils
Dust devil on Mars, photographed by the Mars rover Spirit
Mars - dust storm
• Direction of winds
changes with seasons
– Extreme summer in
southern hemisphere
• Winds initiate huge
dust storms
Venus –polar vortex
© The Planetary Society
• Polar Vortex
– hurricane-like storms
– four times larger than Earth.
– Each vortex has two "eyes"
South Pole
Venus Express, 2006
Example exam questions
Q1. List the conditions necessary for the
formation of hurricanes.
Q2. What are the main features of a tropical
cyclone?
Q3. Sketch a diagram to explain the supercell
thunderstorm structure.
Q4. Why are tropical cyclones not observed in
equatorial regions?
Next topic – Climate Change