G109 11. Wind Systems

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11. WIND SYSTEMS A&B: Ch 8 (p 214-238) Concepts: I. Scale

II. Differential heating III. Wind direction 1. Scales: Three major divisions

Space Time Micro meters seconds - minutes Meso kilometers seconds - hours Macro Synoptic

Planetary 100 - 1000 km

1000+km - Globaldays

days - weeks 2. Differential Heating Spatially - get differences in surface heating

e.g. Micro: grass - concrete - Lab 5

Meso: land - lake Macro: equator - poles • T difference

3. Wind direction Based on where the wind is coming from

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e.g. sea breeze - air coming from the sea valley breeze - air coming from the valley Micro-scale

Example: Turbulent eddies - small whirls of air - dust devils - gusts

Meso-scale a. Land-sea breeze (land - lake breeze) Dail y T difference between land and sea Daytime:

• land heated more intensively compared to water (remember temperature section of course)

http://www.indiana.edu/~geog109/topics/temp/temp.htm

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• • • Pressure gradient force: H ⇒ L

Nighttime: • Reverse • Land cools more rapidly than water • Warmer over the water • Land breeze

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Sea breeze - can have a significant modifying effect on

the T in coastal areas e.g. Lake breeze - Chicago Size of breeze varies. b. Mountain - Valley Breeze Day:

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

• Valley breeze • Most common in

summer

Sunset/Night • Rapid cooling of

slopes • Mountain breeze • Most common in

winter •

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⇒ seasonal preference for winds c. Foehn, Chinook, (Santa Ana) winds Chinook – Rockies (Montana and Wyoming) Foehn - Alps, N.Z. Santa Ana – California –special case (high pressure)

• Migration of surface and upper level weather systems

• Low pressure system on the lee side of a mountain barrier

⇒ Pulls the air across • Adiabatically warms

as it comes down mountain

• Usually occur winter /spring

When T cold, melts snow • T can rise by 20 C

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Special cases of ⇒ d. Katabatic Winds --- from local cooling over high- elevation plateau

• downslope wind • winter time • Bora - Adriatic sea Mistral - France

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Bora--Adriatic Sea: Cold, north to northeast katabatic wind flowing through mountain passages into the Adriatic Sea east of Italy. Bora--Adriatic Sea may be associated with stormy weather, with winds reaching 100 kts or more. Bora-Adriatic Sea develops in the winter when cold polar air builds over the Balkans and flows katabatically through the valleys of the Dinaric Alps and into the Adriatic Sea.

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Mistral -- France

Mistral: Cold, northerly katabatic wind flowing into the Gulf of Lion from the southern coast of France. Mistrals are most common during winter and spring. In the winter, wind speeds can reach over 100 knots off the southern coast of France. Gale-force Mistrals often develop when cyclogenesis occurs over the Gulf of Genoa with the passage of the 500 mb trough (see forces and pressure notes) through eastern France.

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Synoptic and Planetary Scale Wind systems Larger scale circulations influence the smaller scale winds i.e. it is important what the synoptic situation is Global Circulation

• Differential heating between equator and poles ⇒ pressure differences

Three cell circulation model

• Realistic model – base on earth rotation and energy gradient between the equator and the poles

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• Hadley Cell : Equator and Subtropics --thermally driven circulation—intense heating upward expansion

o ITCZ – Very strong zone of low pressure at the equator --Convergence--( small deflection -- Tradewinds)

• Ferrel Cell : Midlatitudes (NH-deflection to right—SH deflection to left--westerlies) • Polar Cell : Poles –thermally driven circulation—Very cold condition--high surface pressure--sinking air—strong deflection

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Close approximation to real world patterns Reality

ITCZ Trade winds Hadley Circulation Cell

Break down of Ferrel and Polar cells Surface winds

Westerlies esp. SH • Real world --- alternating semi-permanent high and low pressure cells

January

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July

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Jet stream • An area of increased wind speeds

o Narrow band: 100 - 500 km wide o 9 – 12 km above sea level o Speeds: 200 - 500 km h-1

40F 45F 75F 80F • • • • • • • Most powerful jetstream: polar jet stream

o Moves north and south with the seasons

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World Jet stream positions 11/14/05

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US Jet Stream Position 11/21/05 http://www.weatherimages.org/data/imag192.html

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US Jet Stream Position 11/05/2007

http://www.nws.noaa.gov/outlook_tab.php