wind systems - indiana university

G109: Weather and Climate

Wind Systems

4. Macroscale Winds1. Global Circulation

1. Single-cell Model2. Three-cell Model3. Zonal Precipitation

Patterns4. Semi-Permanent

Pressure Cells2. Asian Monsoon3. Jet Stream4. Rossby Waves

5. El Niño – Southern Oscillation

ReadingsA&B: Ch.8 (p. 213-247)CD Tutorial: El Niño – Southern Oscillation

Topics1. Concepts

1. Scale2. Wind Direction3. Differential Heating

2. Microscale Winds3. Mesoscale Winds

1. Land & sea breezes2. Mountain or valley winds3. Chinook4. Santa Ana winds5. Katabatic winds

G109: Weather and Climate 11: Wind Systems

Concepts

• Three major divisions

Days100 – 1000 kmSynopticMacro

Days – Weeks>1000 km (global)Planetary

Seconds – HoursKilometersMeso

Seconds – MinutesMetersMicro

TimeSpaceScale

Scale

Wind Direction• Based on where the wind is

Sea breeze: air coming from the sea

Northwest wind: wind blowing from the northwest


Concepts

• Spatially - get differences in surface heating Some areas are warmer than others

Occurs across the range of scales

e.g. Micro: grass - concrete (Lab 5)Meso: land - lakeMacro: equator - poles

• Heating rate and T differences →

• → winds

Differential Heating


Microscale Winds

• ExamplesTurbulent eddies

• Small whirls of air

• Dust devils

• Gusts


Mesoscale Winds: Land-Sea Breeze• Land-Sea (or Land-Lake) Breeze

Daily T differences between land and sea• Daytime: land heated

more intensely than waterAir above land heats more, expands verticallyAir aloft starts to flow

Near Surface: ••

Pressure Gradient Force•

Cool air blown onto land


Mesoscale Winds: Land-Sea Breeze

• Nighttime: reverseLand cooled more rapidly than waterWarmer over the waterAir blown from the land to the ocean

• Sea breeze – can have a significant modifying effect on the temperature in coastal areas

E.g., Chicago lake breeze

• Size of breeze


Mesoscale Winds: Mountain/Valley Wind

• DaytimeSlopes of mountains get more intense heating than air at the same elevation over the valley floor

May see cumulus clouds over peaks ⇒thunderstorms in the afternoons

→Most common in


Mesoscale Winds: Mountain/Valley Wind

• Sunset & NighttimeRapid cooling of slopes

Cool air drainage

→Most common in .

Lowest areas are first to experience radiation fog, frost damage

• Note: seasonal preference

Valley breezes are most common in

Mountain breezes are most common in .


Mesoscale Winds: Chinook Winds

• Chinook / FoehnDifferent names in different places

• Chinook – Rockies (Montana, Wyoming, Alberta)• Foehn - Alps, N.Z.

Low pressure system on the of a mountain barrier – pulls the air across

as it comes down mountainT can rise by 20oC Usually occur


Mesoscale Winds: Santa Ana Winds

• Santa Ana Winds – California High pressure system over the Rocky MountainsAir flows away from high, down western slopes

as it comes down mountainT can rise by 30oC Usually occur

Often contributes to spread of forest fires in CA


Mesoscale Winds: Katabatic Winds

• Katabatic Winds Cold downslope wind –Cold air sinks because more dense –

but still than lower elevation air it displacesIf channeled into narrow valleys → high velocitiesFrequently occur at edges of Greenland and Antarctic ice sheetsDifferent names in different places

• Bora: Balkans → Adriatic sea

• Mistral: Alps → France


Macroscale Winds: Global Circulation

• Synoptic and planetary (macroscale) winds influence the smaller scale (mesoscale and microscale) winds

• Global CirculationDifferential heating between equator and poles

→Global scale pressure differences

→Persistent large-scale motion



• Single Cell Model –Differential heatingAssumptions:

• Earth is uniformly covered with water

• Sun is directly over equator

→ Single-cell pattern of flow – Hadley Cell

• Warm air rises at

• Cold air sinks at



• Single Cell Model – Hadley CellEarth’s rotation →Coriolis force: winds deflected to right in Northern hemisphere, to left in Southern hemisphereWinds: winds from poles to equator

• Single-cell pattern is not what we observe

Breaks down due to:••



• Three-Cell Model – more realistic model


• Hadley Cell:

• Inter-Tropical Convergence Zone (ITCZ) (0o)

Very strong low pressure zone – rising air

Light winds: doldrums

• Sub-tropical High (30o N/S)

Sinking air

Light winds: horse latitudes

• Trade winds (0-30oN/S)

Macroscale Winds: Global CirculationThree-Cell Model – more realistic model



Three-Cell Model – more realistic model

• Ferrel cell –

Some of sinking air at subtropical high diverges poleward

• (mid-latitudes)


• Polar cell: high latitudesThermally driven circulation

• Polar High (90o)Very cold conditionsSinking, diverging air

• Sub-polar Low (60o N/S)Rising air

• Polar Flow from Very strong deflection by Coriolis force

Macroscale Winds: Global CirculationThree-Cell Model – more realistic model



• Zonal Precipitation Patterns• Equa• Equatorial Low

Rising air →

• Sub-tropical HighSinking air →

Migrates N / S with seasons

• Sub-polar LowRising air →

• Polar HighSinking air →


Macroscale Winds: Global & Synoptic

• Three-cell model not quite true: doesn’t include land/water differences

• Three-cell model breaks down in upper-level winds – do not have the distinct structure of Ferrel cell and polar cell, although surface winds are correct there

• But it was a very useful starting point for considering global circulation

• In the real atmosphere, we instead find a number of semi-permanent High and Low pressure cells



• Semi-permanent Pressure CellsJanuary



• Semi-permanent Pressure CellsJuly


Macroscale Winds: Asian Monsoon

• Seasonal wind due to seasonal changes in mean pressure

• Winter: Sinking air from jet stream →

• Summer:Strong heating over continent → .

Draw moisture from warm Indian Ocean toward India and Asia

Himalayan Mountains cause strong orographicuplift


Macroscale Winds: Jet stream

• An area of increased wind speeds Narrow band: 100 - 500 km wideSpeeds: 200 - 500 km h-1

Height: 9 - 12 km ( )• Typically found above the largest horizontal T

gradient – e.g., at polar front• Move north and south with

the seasons • Stronger in the when

the T gradients are largest• Most powerful jet-stream:

• Weaker jet-stream:


Macroscale Winds: Rossby Waves

• Recall: Upper air (zones of low pressure extending equator-ward) and .

(zones of high pressure extending poleward)

→ Wavelike flow around earth at mid-latitudes

• Rossby• Rossby waves: “long waves”in flow

Usually 3-7 Rossby waves encircling earth

Migrate west to east

Change in wavelength and amplitude


Macroscale Winds: Rossby Waves

• Large amplitude Rossby waves ( .

flow) transport:Warm air from subtropics to high latitudesCold polar air to low latitudes

• Small amplitude Rossby waves ( flow)Flow is more westerly, less equator-pole exchange of heat

• Changes in the flow along the wave lead to:Divergence aloft

• Draws air • Leads to

Convergence aloft• Forces air • Inhibits


El Niño Southern Oscillation

• El Niño – weak warm current occurring along the west coast of South America (particularly Peru)

Appears every 3-7 years around Christmas time

Lasts about 1 year

Warm current is not good for fishing industry

1997-98 was warmest event ever recorded

• Occurs due to a reversal in “Walker Circulation” – the interaction between atmospheric circulation and ocean circulation in the equatorial Pacific



• During a normal (non-El Niño) year:

Easterly trade winds drag warm surface water from East to West across Pacific

Upwelling of cold water along the west coast of South America

Low pressure area:

High pressure:



• A normal (non-El Niño) year



• During an El Niño year:Weakening or reversal of trade winds drag warm surface water from W to E across Pacific

No upwelling of cold ocean water

Sea Surface Temps (SST’s) in Eastern Pacific become warmer than normal

Low pressure area shifts to Eastern Pacific → .

along west coast of South America, Central America and even California

High pressure shifts from to western Pacific

The reversal in surface pressure is called the



• During El Nino year:



• When El Niño dissipates:Normal (non-El Niño) conditionsOR La Niña conditions

• During a La Niña year:Very strong easterly trade-winds in the PacificVery strong upwelling of cold water along the west coast of South AmericaSST’s become colder than normalIn Western Pacific: warm water promotes uplift, which intensifies surface low, and intensifies easterly trade windsAlong west coast of America’s: very High pressure →

wind systems - indiana university

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