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/stropical depression 17 – 33m/stropical storm > 33m/shurricane typhoon severe tropical cyclone severe tropical storm (Atlantic) (Eastern Pacific) (Western Pacific) (Indian Ocean)

Affected regions 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 days Core warming - 2 days Max intensity – 3 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 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 CategoryWind speed (m/s) Storm surge (m)

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 / 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 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 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