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Lecture 12 Local Wind Systems

Scales of Atmospheric Motion

Small turbulent eddies (swirls)

A synoptic eddy

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Coriolis Effect

The larger the scale, the longer the life time.

Wind shear and turbulent eddy generationKelvin-Helmholtz instability bumpy rides near the jet stream

Billow clouds

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Thermal circulation

Heating causes air column to expand vertically;High pressure in the upper column;Upper air flows from the heated to cooled region;Low pressure forms near surface in the heated region;Surface air flows from the cooled to heated region;Air rises in the heated region (cloudy) while sinking in the cooled region (clear).

Sea Breeze• Onshore wind near surface• Sea breeze front advances

inland maximum temperature occurs earlier near the coast than inland.

• Rising motion (cloud) on the sea breeze front while subsidence and clear sky prevail over the ocean.

Land breeze takes place at night, from land to sea.

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Cumulonimbus due to converging sea breezes on Florida Key

1999/07/12 Advancing sea breeze front

Sea Breeze• Converging Gulf of Mexico and Atlantic sea breezes produce

uplift and thunderstorm development in Florida• Disruption of sea breezes reduces rainfall and can lead to a

bad fire season

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• Sunlight heats mountain slopes during the day and they cool by radiation at night

• Air in contact with surface is heated/cooled in response• A difference in air density is produced between air next to the mountainside

and air at the same altitude away from the mountain• Density difference produces upslope (day) or downslope (night) flow• Daily upslope/downslope wind cycle is strongest in clear summer weather

when prevailing winds are light

Mountain/Valley WindsMountain/Valley Winds

Consequences of Mountain/Valley winds• Upslope flow during the day leads to formation of clouds

and precipitation along mountain ranges– When is the best time for hiking and climbing?

• Upslope flow along the Front Range transports pollutants from the urban corridor into the high country

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• Local to mountainous regions.• Occur under calm, clear

conditions. • Cold, dense air flows down

valleys onto the land below. • Drainage winds can be

extremely cold and strong.

Katabatic winds

Time-average winter surface wind flow pattern in East Antarctica. Cape Denison is the windiest place on Earth.

Katabatic winds over Antarctica

Elevation ~ 4 km

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4:00 HST along 19.6oN on the windward sideFeng and Chen (2001, Monthly Weather Review)

Cold airrushing down

Downslopewind

Downslope wind on the Island of Hawaii

Easterly trade winds

Hilo

Hilo 10/4/06

14:00, surface• onshore flow• dry adiabatic lapse rate• Td=19 C

2:00 am, surface• Offshore katabatic flow• Surface temperature inversion• Dry humidity (Td=17.3)

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Chinook (Foehn) Downslope Winds

• Main source of heating is compression during downslope flow– Key is loss of moisture

on upwind slope so downslope heating occurs at higher dry adiabatic rate

• Latent heat release from condensation during upwind ascent also contributes– If condensed water is

removed as precipitation on upwind slope

Other Chinook FeaturesChinook wall cloud seen toward the west (Rockies)

Inversion

Temperature changed by 49oF in two minutes at Spearfish, South Dakota in 1/22/1943.

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Santa Ana Winds (a kind of gap winds) are warm and dry from adiabatic descending, setting stage for brush fires.

Surface weather map in January

Central American Gap Winds(often exceed 20 m/s in winter)

Wind stress & sea surface temperature (oC)

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Wettest spot on Earth: Cherrapunji, India at 10,800 mm/year (Mount Waialeale, Kaui, comes as a close second)

High pressure over winter continent(The Tibetan Plateau effectively blocks cold Siberia air from entering the Indian Ocean Northern India has a much milder winter than southern China.)

Low pressure over summer continent(The monsoon is of continental scale, instead of peninsular scale as depicted in this schematic.

Land topography (km) & wind (m/s) at 925 hPa

July

January

• Monsoon is not confined to India but spans from west Africa, through the Bay of Bengal, all the way to the western North Pacific.

• The summer monsoon of Asia is dominated by the southwesterly winds in the lower atmosphere.

• The Tibetan Plateau: an elevated heat source in summer and a wind block in winter.

• Tectonic mountain building since 50 million years ago when the Indian Plate collided with the Eurasian Plate.

Great Monsoon of Asia

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North American Summer Monsoon brings convective rain to the US southwest in July 2001, fueled by the moisture from the Pacific and Gulf of Mexico. Cold clout tops are colored red and yellow in this satellite infrared image.

Dust Devils• Surface heating produces convection• Wind blowing past object twists rising air• Air rushes into rising column lifting dirt and debris

(like a skater stretching hands upward)• Low pressure inside

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Scientists have for years enjoyed images of dark trails on the surface of Mars left by dust devils, which are atmospheric phenomena similar to tornadoes. But few are as dramatic as this NASA photo, taken in September and released earlier this month.

The Mars Global Surveyor (MGS) image shows several dark streaks created during the recent southern spring by dust devils, as they passed over and around an old, nearly filled, meteor impact crater.

The circular feature is the former crater. Dark dots and specks on its rims are boulders, said astronomers at Malin Space Science Systems, which operates the spacecraft's camera.

Dust devils create streaks by removing or disrupting thin coatings of fine, bright, dust on the surface, the astronomers said. The features will disappear before the next spring arrives in 2005. The crater is located near 57.4°S, 234.0°W. The image covers an area 1.9 miles (3 kilometers) wide and is illuminated from the upper left.

Trails of Dust Devils