Chris Parkes Rm 455 Kelvin Building Planetary Atmospheres, the Environment and Life (ExCos2Y) Topic 5: Atmospheric Convection Chris Parkes Rm 455 Kelvin Building

Absorption spectrum of atmosphere Revision Radiation 4. Solar Radiation Wm-2μm-1 μm Sun: Incoming Absorption spectrum of atmosphere Spectrum of incoming & outgoing radiation Insolation – daily & annual variation Albedo Energy budget Greenhouse effect Earth: Outgoing

Convection in the Atmosphere What drives it? Hadley cell - a simple model A more realistic model of earth’s atmospheric convection

Archimedes’ Principle Upward buoyancy Archimedes’ Principle Objects in fluid experience an upward (buoyancy) force equal to the weight of the displaced volume of fluid Static balloon must have buoyancy equal to its weight Hot air is less dense than cold air weight

Hot air rises Air moves in “parcels” – like balloons but without the fabric A parcel hotter than surroundings will experience a greater buoyancy force than its weight – net force upward – it will rise. Upward buoyancy cooler surroundings warmer air parcel weight As rises: Temperature decreases Pressure decreases Volume Increases (see lecture topic 3) pV=T

Column of air being heated Pressure (mb) 200 Height Heating 500 800 Initially at same temperature as surroundings Heating at ground level Air volume expansion

Column of air being heated Pressure difference at the top leads to outflow Less weight in column Lower pressure at surface Inflow towards low pressure Convective flow of air Pressure (mb) H 200 Height 500 800 L Initially at same temperature as surroundings Heating near ground level Air volume expansion

Equator (low pressure) The Hadley Cell (1735) Tropopause B Height North South A Ground Equator (low pressure) Air column AB expands High pressure at B Low pressure at A (w.r.t. surroundings) Warm air rises. At B further convection is limited by temperature inversion at the tropopause

Equator (low pressure) The Hadley Cell (1735) Tropopause C B Height North South Convection Cell D A Ground (High pressure) Equator (low pressure) Air moves away from equator cools gradually becoming more dense (B to C) Air sinks back to surface (C to D) Movement of air from high to low pressure (D to A)

Equator (low pressure) The Hadley Cell (1735) Tropopause C B Height North South Convection Cell D A Ground (High pressure) Equator (low pressure) Vertical motion is on average ~10 cm/s (c.f. 10 m/s in cumulus cloud) – caused by change in density and pressure Horizontal motion is due to pressure difference. Changes are small ΔP = 50 mb (average P = 1000mb)  5% change

Equator (low pressure) The Hadley Cell (1735) Tropopause C B Height North South Convection Cell D A Ground (High pressure) Equator (low pressure) No air is created or lost Mass moved per unit time = speed × density Must be the same for surface and high level winds. Density lower at higher altitude  high altitude winds are fast

Pressure “systems” High pressure Air sinking, generally cooling Mostly over oceans Low pressure Air rising, generally being heated Mostly over land Here high and low pressure refer to surface pressure i.e. top of “low” pressure region has a higher pressure than surroundings

Differential heating on Earth Poles receive same amount of energy over larger area - less energy density on surface North Solar energy Solar energy Equator receives a quantity of solar energy over a small area Equator Rotating an unit area by 60º reduce incident radiation by half  at 60º latitudes only get half of sun’s energy

Hadley cells on Non-rotating planet All area heated, but - More solar heating at equator creates hotter region Air rises at equator (inter-tropical convergence zone, ITCZ) Cooler air from poles moves towards equator to region of lower pressure In reality on Earth: Rotation Day/night difference Annual variation Global air movement takes weeks Cold Hadley cell Hot Equator Hadley cell Cold

Venus as Hadley Cell Mars Venus: Mars: Slow rotation (Venus day is 243 Earth days) weak rotation effect – works like Hadley cell Dense atmosphere Efficient transport of heat – equator and poles similar temperature, despite incident radiation angle effect Mars: Thin atmosphere Very little heat transported, poles much colder than equator Mars

Rotation - The Coriolis effect Apparent deflection of objects from a straight path when viewed in a rotating frame Apparent “force” pushing outward going bodies to the right and inward going bodies to the left Rotation of earth means: Apparent movement to the right while moving on northern hemisphere and, to the left in the southern hemisphere

Coriolis Force Merry-go-round Balls path deviates to the right Ball rolled inwards Or ball rolled outwards Would be reversed if anti-clockwise coriolis force opposite in south / north hemispheres

The Coriolis effect

The Coriolis effect

The Coriolis effect In Hadley cell in northern hemisphere: upper air moves north  coriolis pushes it eastwards lower surface air moves south  coriolis pushes it westwards Simple Hadley circulation cell model breaks down

The Three-cell model of Earth’s atmosphere Direct (Hadley) cell - from equator to 30º Indirect (Ferrel) cell - from ~30º to 60º Polar cell Indirect cell driven by the other two

The Three-cell model of Earth’s atmosphere Surface and upper winds have east/west as well as north/south component East/west balanced such that the whole atmosphere rotates with the globe Better model needs to include: North/south & east/west movement Angular momentum Differential heating Effect of land mass Seasonal changes … easterlies jet streams westerlies trade winds

Smaller scale convection – Sea Breezes Land heats up quicker than sea Air above land begins to rise Sea air moves inland since rising air above land produces lower pressure Size of effect increases throughout the day Keep coastal regions cooler than inland Reverse at night

Example exam questions Q1. State what is a Hadley cell and explain how it works? Q2. How does differential heating arise on Earth? Q3. Sketch a diagram to explain the Coriolis effect. Q4. Explain how sea breezes keep the coastal region cooler than inland. Next lecture – wind

Current in fluid under gravitational field & differential heating Convection … advection – mechanism of heat transfer Current in fluid under gravitational field & differential heating