ATMOSPHERE Air Circulation

ATMOSPHERE Air Circulation UNIT 7 Meteorology and Climate

Atmosphere-Ocean Coupling • Why study atmospheric circulation? • Atmosphere and ocean processes are intertwined • Atmosphere-ocean interaction moderates surface temperatures, weather and climate • Weather: local atmospheric conditions • Climate: regional long-term weather • Atmosphere drives most ocean surface waves and currents

Composition of the Atmosphere • Dry Air: 78% Nitrogen, 21% Oxygen • BUT it is never completely dry • Typically contains about 1% water vapor • Chemical residence time of water vapor in the air is about 10 days • Warm airholds much more water vapor than cold air http://www.nature.com/scitable/knowledge/library/the-global-climate-system-74649049

Density of Air • Typical air density ~ 1 mg/cm3 • Temperature and pressure affect the density of air • Temperature: Hot air is less dense than cold air • Pressure: Air expands with elevation above sea level http://www.physicalgeography.net/fundamentals/7d.html

Density and Temperature • Rising air expands and cools • Vapor condenses into clouds and precipitation • Sinking air is compressed and warms • Clear air

Expanding Air Cools and Condenses • Like opening a pressurized bottle of soda • Air expands and cools • Water vapor condenses – cloud formation

Solar Heating of the Earth • Solar energy absorbed unevenly over Earth’s surface – why is air rising? • Energy absorbed / unit surface area varies with: • Angle of the Sun • Reflectivity of the surface (i.e., ice versus ocean) • Transparency of the atmosphere (i.e. clouds)

Solar Insolation Variations with Latitude

Solar Heating of the Earth • Sunlight heats the ground more intensely in the tropics than near poles July January

Solar Heating and Seasons • Seasons are caused by Earth’s 23.5° tilt http://astro.unl.edu/naap/motion1/animations/seasons_ecliptic.html

Solar Heat Energy • Equator absorbs more heat from the sun than it radiates away • Polar regions radiates much more heat than they absorb from the sun • Energy in at equator and heat out at poles • Heat transfer from • E.g. Equator isn’t that hot – Poles aren’t that cold • Evidence that the atmosphere (2/3) and oceans (1/3) redistribute heat (wind and ocean currents) • Convective heat transfer moderates Earth climate http://oceanmotion.org/html/resources/solar.htm#vishead

Convective Heat • Convective heat transfer model’s Earth climate • Heated air expands and rises, then cools and sinks Equator Poles

Atmospheric Circulation Cold, more dense air sinks near the Poles Warm, less dense air rises near the Equator Cold, more dense air sinks near the Poles Wind from the north

Actual Atmospheric Circulation • Air rises and sinks • More than one convection cell • Earth spins once per day that amounts to a speed for us at the surface of Earth of 100’s of miles/ hour • Coriolis Effect

Coriolis Effect Movies http://earthsciweb.org/GeoMod/index.php?title=Coriolis http://science.nasa.gov/science-news/science-at-nasa/2004/23jul_spin/ http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/fw/gifs/coriolis.mov

The Coriolis Effect on Earth • Surface velocity increases from poles to equator • Points on the equator must move faster than points near the poles to go around once a day • Latitude velocity differences land to curving paths • Northern hemisphere deflected to right

The Coriolis Effect • Strength of Deflection varies with latitude: • Maximum at the poles • Zero(!) at equator • Faster a planet rotates, the stronger the Coriolis effects • The larger the planet, the stronger the Coriolis effects (Jupiter spins once every 10 hours)

Hurricanes • A storm with lots of clouds has rising air – thus low-pressure at the surface! • Converging air sets up counter-rotation (cyclonic) Spinning clockwise Bending to the right Spinning counter clockwise Bending to the left

Hurricanes – Low Pressure • Hurricane is rising and already has moisture in it • Low pressure system at the surface – air rising so that means air is being sucked in at the base • Arrows get defected by Coriolis to the right • Set up a counter clockwise circulation in the northern hemisphere L

High Pressure • Air sinks and compresses and it gets warmer and dry • See clear air not clouds • Pushes air away • Rotate clockwise in northern hemisphere H

Atmospheric Circulation • Sinking air at 30° – deserts • Easterly winds – trade winds • Westerly winds at 30° and 60°

Atmospheric Circulation • 3 convection cells in each hemisphere • Each cell: ~30° latitudinal width • Veritcal Motions • Rising Air: 0° and 60° Latitude • Sinking Air: 30° and 90° Latitude • Horizontal Motions • Zonal winds flow nearly along latitude lines • Zonal winds within each cell band • DUE to DEFLECTIONS BY CORIOLIS!

Sea Breeze • Land warms fastest during the day. Air during the day expands and rises • Ocean surface temperature changes slowly. Air cools and becomes more dense, sinks then begins to rise over the land. • Result – wind from sea towards land

Land Breeze • Land cools fastest at night. Low heat capacity. Air contracts and sinks • Ocean surface temperature changes slowly. Air is pushed away and up by cooler denser land air. • Result – wind from land towards sea

Marine Layer • Cold waters, warm air: think cloud layer on ocean surface • Subtropics: H pressure, regional subsidence • Cloud layer flows onto land at night • Evaporates over land by day LAND OCEAN

ATMOSPHERE Air Circulation