02 Wind Systems

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Deutsches Windenergie - Institut GmbH http://www.dewi.de

Global and LocalWind Systems

An Overview

Peter Busche

Deutsches Windenergie-Institut GmbH,DEWI Wilhelmshaven


Main factors influencing wind systems

Explaining real wind systems using models

Global wind systems

Local wind systems

Wind Profile

Global and local wind systems

Contents:


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Winds: An Introduction

Classification TIME-DISTANCE SCALE EXAMPLES

Macroscale:1. Planetary2. Synoptic

Weeks to years (1000-40,000km)Days to weeks (100-5000km)

Westerlies & cyclonesanticyclones & hurricanes, jet streams

Mesoscale Minutes to days (1-100 km) Land-sea breezes, thunderstorms &

tornadoes

Microscale Seconds to minutes (< 1 km) Turbulence, dust devils and gustsTable 1.0: Scale of atmospheric wind systems. (adapted from Lutgens & Tarback, 1998)

Global wind systems- MacroscaleLocal wind systems - Mesoscale


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Winds: Driving Force

Wind mainly develop due to spatial atmospheric differencescaused by net solar radiation at different latitudes

Figure showing development of wind due to net solar radiation


Reasons for the poleward flow at high altitudes

1. Convergence of hot air in the tropics (thermally driven)

2. Vertical Pressure Gradient

g = earth's acceleration, note:

= air density

R = gas constant

T = temperature

r = Earths radius

Standard Atmosphere: ISO 2533 -2 to 32 kmgeometrical- and geopotential altitude

= gdz

dp

TR

p

=

2

+=

hr

rgg n


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Main factors influencing wind systems

1. Pressure gradient force

p = the pressure difference between two points

= air density = 1.225 kg/m at standard conditions

d = distance (meters) between two points

2. Coriolis acceleration (force)

= angular rotation rate of the earth (7.27 E -5 1/s)u = geostrophic wind velocity

= the latitude (no Coriolis force at the Equator!,lattitude = 0)

3. Centripetal accelerationActs on air flowing around centres of circulation

4. Frictional decelerationActs in a direction opposite to the wind flow.

d

pPGF

=

1

Fc = 2**u*sinForce acting onmoving objectsdue to earths

rotation


Coriolis forces- winds will be deflected to the right

in the northern hemisphere

- winds will be deflected to the left

in the southern hemisphere

- wind will rotate clockwise around

high pressure zone on northern hemisphere

- wind will rotate counter clockwise around

low pressure zone on northern hemisphere

- wind will rotate counter clockwise around

high pressure zone on southern hemisphere

- wind will rotate clockwise around

low pressure zone on southern hemisphere

as seen by an observer on the earth sitting

in the high pressure zone

NP

SP

EW

H

H

L earthrotation

L

H

L


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Geostrophic Winds / Surface Winds

Figure 1: Geostrophic wind Figure 2: Ageostrophic/real wind


Explaining real wind systems using cell models

Single cell

model (Hadley) 3 cell model


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Global wind systems

(a) Imaginary uniform earth with idealised pressure belts and

(b) the real earth with zonal patterns disrupted by large landmasses,which cause semi-permanent high and low pressure cells.


Actual weather map (The Westerlies)

Rossby Waves

Jet Streams


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ITCZ (Intertropical convergence zone)


Meteorology

Wind from sea

Thermals

Upwin

ds

Sea Beach Grass Trees Hills Woods Buildings Cities Valleys Mountains

Affecting the wind:

Temperature

Roughness

Obstacles

Orography

Sun = Source of Wind


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Local wind systems (1)

1. Land-Sea breeze

2. Anabatic-Katabatic winds

3. Mountain-Valley winds

4. Bora-Foehn

Local (Minor) wind systems are generally of interest only in

description of weather and local climate.However, the main ones occur quite often.


Land breeze / Sea breeze

Sea breeze (daytime)

Land breeze (night)

(Thermally induced)


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Anabatic / Catabatic Wind

Anabatic winds

Catabatic winds

*Mountain-Valley windsare formed the same way

but are characterisedby flow along the valleys

(Thermally induced)


Formed by deformation of air currents crossingmountain ranges

Fhn / Bora

(Orographically induced)


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Local wind systems (2)

Further (minor) local wind systems include:

1. Snow breeze

2. Cloud breeze

3. Land-river breeze

4. Local winds at urban heat islands

5. Eclipse winds


Wind systems in the Mediterranean Sea region


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Layers of the lower atmosphere

1000 m

100 m

5 mm laminal boundary layer

Surface layer

Prandtl layer

Ekman layer

free atmosphere

turbulent boundary layer

height


0

20

40

60

80

100

120

140

1980 1985 1990 1995 2000 2005 2010

Year

serial production

prototypes

Rotordiameter

,m

Development of size

600 kW

500 kW

300 kW

50 kW

2,500 kW

1,500 kW

4,500 kW


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0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6

Windspeed [m/s]

Height[m]

Water

Grass

City

Geostrophic Wind1000

BoundaryLayer

SurfaceLayer

Geostrophic windinduced by large-scalepressure distribution

Wind speed increasingwith height

Inside the surface layer:logarithmic wind profile

Vertical wind profile


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02 Wind Systems