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 tropical depression

17 – 33m/s tropical storm

> 33m/s hurricane

typhoon

severe tropical cyclone

severe tropical storm

(Atlantic)

(Eastern Pacific)

(Western Pacific)

(Indian Ocean)

Affected regions

Hurricane

Hurricane Tropical cyclone

Tropical cyclone

Typhoon

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 daysCore warming - 2 days

Max intensity – 3 daysDecay - 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

storm surge – rise of water level associated with low pressure system

•Eye•Eyewall•Rainbands

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:

Wind speed is max. sustained speed

Max. at right front quadrant (in Northern hemisphere)

Global warming may be increasing intensity

Category Wind speed (m/s)

Storm surge (m)

1 33-42 1.0-1.7

2 43-49 1.8-2.6

3 50-58 2.7-3.8

4 59-69 3.9-5.6

5 70+ 5.7+

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 devilDust devil – heating from ground

not associated with a supercell thunderstormtypically lower intensity than tornados

Dust Devil on Mars

Dust devil on Mars, photographed by the Mars rover Spirit

Martian dust devils can be fifty times as wide and ten times as high as Earth dust devils

Mars - dust storm• Direction of winds

changes with seasons– Extreme summer in

southern hemisphere

• Winds initiate huge dust storms

Venus –polar vortex

• Polar Vortex– hurricane-like storms – four times larger than Earth. – Each vortex has two "eyes"

© The Planetary Society

South PoleVenus Express, 2006

Example exam questionsQ1. 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