GLOBAL WIND SOURCES

Part A Module 1 – Wind Energy

TRADE WINDS

"Map prevailing winds on earth" by KVDP - Own work.

FLUID MOTION

• Fluid motion is driven by a pressure difference

SURFACE SOLAR HEATING • Sun heats the surface • Surface heats adjacent air • Density 𝜌 ~ 1

𝑇⁄ • As air density decreases, it rises • Air heating also reduces the local

pressure

SURFACE SOLAR HEATING IS NOT UNIFORM Solar energy received by a surface depends on: • The season • Latitude • Cloud cover • Surface properties • Energy emitted • Wind – convection heat

transfer

Solar energy incident on

surface

Net energy received by surface

SOLAR SEASONAL VARIATION • Earth’s axis of rotation

tilted 23.5o to the plane of its solar orbit

• Results in seasonally uneven heating of the northern and southern hemispheres

“Seasons”, by US National Weather Service

CLOUD COVER REDUCES INCIDENT SOLAR ENERGY

• Cloud cover not uniformly distributed

• Amount of solar energy reaching the earth’s surface reduced by: – Reflection – Absorption – Scattering

INFLUENCE OF SURFACE CONDITIONS Sun on grass

Sun on water

NON-UNIFORM SOLAR HEATING CREATES

PRESSURE DIFFERENCES • Surface in center receives

more energy than surroundings

• Heated air rises and creates a low pressure zone

• Rising air cools and spreads • Cooler, dense air sinks back

to surface creating high pressure region

EQUATORIAL WIND (W/0 ROTATION)

High & low pressure by NOAA Ocean Service Education

• Sun shining on equator

• Neglect earth’s rotation for the moment

• Solar driven wind currents – Hadley cell

L

H

H

CORIOLIS EFFECT • Earth rotates counterclockwise

underneath atmosphere • Red arrows represent wind

direction viewed from space • Blue arrows represent north-

south viewed from earth • Wind appears to move to right

viewed from earth (northern hemisphere)

• Effect is zero at equator and increases with latitude Modified “Globe blue lines”

UPPER EQUATORIAL WIND WITH ROTATION

Coriolis effect by NOAA Ocean Service Education

• As a result of Coriolis effect, solar-generated high level north-south equatorial wind moves right in northern hemisphere (left in southern hemisphere)

• Cooling air sinks at about 30oN, 30oS

TRADE WINDS • Hadley cell – 3-D

circulation pattern in the equatorial region

• Sinking air at 30oN (Tropic of Cancer), 30oS (Tropic of Capricorn) becomes the trade winds

• Some air continues to move north or south, where it becomes part of the westerlies

Trade winds by NOAA Ocean Service education

TRADE WINDS

"Map prevailing winds on earth" by KVDP - Own work.

POLAR AND FERREL CELLS

Two other sets of 3-D solar driven wind structures • Ferrel cells between 30oN and 60oN (30oS and 60oS) • Polar cells between 60oN and N Pole (60oS and S Pole) • Smaller cells towards poles – reduced solar input

Center for Multiscale Modeling of Atmospheric Processes (CMMAP)

3-D VIEW OF GLOBAL CIRCULATION • Hadley cells (1) generate

trade winds • Ferrel cells (2) generate

westerlies • Polar cells (3) generate

polar easterlies

Global Circulation by US National Weather Service

ROSSBY WAVES • Rossby waves are a type

of inertia wave – Associated with pressure

systems and the jet stream – Causes polar and

subtropical jets to meander

“Jetstream 2” by US National Weather Service

GLOBAL WIND SOURCES SUMMARY

• The sun is the ultimate source of energy for global winds • The three main global winds each result from a thermally

driven large scale circulation loop – Trade winds generated by the Hadley cell – Westerlies generated by the Ferrel cell – Polar easterlies generated by the polar cell

• Boundaries of these winds show considerable variability

PHOTO CREDITS • "Map prevailing winds on earth" by KVDP - Own work. Licensed under Public domain via Wikimedia Commons -

http://commons.wikimedia.org/wiki/File:Map_prevailing_winds_on_earth.png#mediaviewer/File:Map_prevailing_winds_on_earth.png • Seasons, by National weather service; JetSteam – Online School for Weather,

http://www.srh.noaa.gov/jetstream/global/images/seasons.jpg • “High and low pressure,” by NOAA Ocean Service Education,

http://oceanservice.noaa.gov/education/kits/currents/media/high_n_low_pressure.gif • Globe blue lines, a public domain image, http://www.wpclipart.com/education/supplies/globes/globes_2/globe_blue_lines.png.html • “Coriolis effect”, by NOAA Ocean Service Education, http://oceanservice.noaa.gov/education/kits/currents/media/coriolis_effect.gif • Trade winds, by NOAA Ocean Service Education, http://oceanservice.noaa.gov/education/kits/currents/media/trade_winds.gif • “All Cells”, Center for Multiscale Modeling of Atmospheric Processes CMMAP,

http://www.cmmap.org/images/learn/climate/allCells.jpg • Circulation, by National weather service; JetSteam – Online School for Weather,

http://www.srh.noaa.gov/jetstream/global/images/seasons.jpg • Jetstream 2, by National weather service; JetSteam – Online School for Weather,

http://www.srh.noaa.gov/jetstream/global/images/jetstream2.jpg