Wind: Small Scale and LocalWind: Small Scale and Local SystemsyChapter 9Chapter 9

Part 2

Mountain/Valley circulationsMountain/Valley circulations

• Mountain-valley circulations similar in concept to Mountain valley circulations similar in concept to seabreeze/landbreeze

• Heating of mountain slopes makes high pressure aloft g p g pcompared to same elevation above valley pressure gradient force aloft from mountain to valley- air moves

f t i i t l b ll l i taway from mountain into column above valley- less air at surface over mountain, more over valley causes high pressure over valley low at surface over slope pressurepressure over valley, low at surface over slope, pressure gradient force at surface moves air upslope

• At night, cool air over mountain and circulation reverses,At night, cool air over mountain and circulation reverses, cold air drainage down mountain

• Often get thunderstorms over mountains from rising airg g

Valley and Mountain Slope WindsValley and Mountain Slope Winds

Examples of Slope WindsExamples of Slope Winds• California central valley affected by sea breeze andCalifornia central valley affected by sea breeze and

valley circulations- cool Pacific coastal water- strong heating in central valley produces strong sea breeze, much channeled through Sacramento River delta-much channeled through Sacramento River deltameanwhile, mountain valley wind systems carry flow up Sacramento and San Joaquin Valley and slopes of Sierra Nevada Mountain rangeof Sierra Nevada Mountain range

• In Los Angeles Basin, sea breeze channeled through passes into Mohave DesertFl T h h i P t th d f S• Flow over Tehachapi Pass at south end of San Joaquin Valley converges with flow through passes east of LA Basin flow continues to the northwest over Mojave Desert due to heating over Colorado Plateau

• In Las Vegas, in absence of strong winds, up valley and upslope (e g Spring Mountains day- downand upslope (e.g. Spring Mountains day down valley, down slope at night

Thermallly induced mesoscale winds• In valleys, if pressure gradient force is located

along valley winds blow from high to low

y

along valley, winds blow from high to low pressure- along (up or down) valley, rather than across valleys- wind speed enhanced

• If geostrophic flow aloft is oriented along valley, pressure gradient at surface is along valley, and atmosphere has near neutral stability can getatmosphere has near neutral stability can get very strong winds

• Santa Ana winds- typically autumn with coolSanta Ana winds typically autumn with cool, high pressure over Great Basin, lower pressure southern California- get strong flow through

d d d t d tpasses and canyons- warm, dry due to desert origin and compressional heating- high fire dangerdanger

Typical weatherTypical weather map for Santa Ana windsAna winds

Thermallly induced mesoscale windsThermallly induced mesoscale winds

Columbia River Gorge winter cold air high pressure• Columbia River Gorge- winter cold air, high pressure east of gorge- lower pressure west of gorge strong easterly winds develop, especially near gorge exiteasterly winds develop, especially near gorge exit

• CRG- summer, desert area east of gorge heats, modified sea breeze west of gorge- consistent winds up gorge-g g p g gfamous for wind surfing

• Chinook winds- strong winds down east slope of Rocky Mountains- warm, dry

• Haboob- strong winds from thunderstorm downdraft (from evaporative cooling) lifting dust into the air

Asian MonsoonAsian Monsoon

• Seasonally changing wind system