Download Lecture 12 Local Wind Systems - School of Ocean and Earth

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 generation
Kelvin-Helmholtz instability Æ bumpy rides near the jet stream
Billow clouds
2
Thermal circulation
Heating causes air column
to expand vertically;
Æ High pressure in the
upper column;
Æ Upper air flows from the
heated to cooled region;
Æ Low pressure forms near
surface in the heated
region;
Æ Surface air flows from the
cooled to heated region;
Æ 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 Æ 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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Advancing sea breeze front
1999/07/12
Cumulonimbus due to
converging sea breezes on
Florida Key
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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Mountain/Valley Winds
• 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
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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Katabatic winds
• 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.
Elevation ~ 4 km
Katabatic
winds over
Antarctica
Time-average winter surface wind flow pattern in East
Antarctica. Cape Denison is the windiest place on
Earth.
6
Downslope wind on the Island of Hawaii
Cold air
rushing down
Easterly trade winds
Downslope
wind
Hilo
4:00 HST along 19.6oN on the windward side
Feng and Chen (2001, Monthly Weather Review)
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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• 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
Chinook (Foehn)
Downslope Winds
• Latent heat release
from condensation
during upwind ascent
also contributes
– If condensed water is
removed as precipitation
on upwind slope
Other Chinook Features
Chinook wall cloud seen toward the west (Rockies)
Temperature changed by 49oF in two minutes at Spearfish,
South Dakota in 1/22/1943.
Inversion
8
Surface weather map in January
Santa Ana Winds (a kind
of gap winds) are warm
and dry from adiabatic descending,
setting stage for brush fires.
Central American Gap Winds
(often exceed 20 m/s in winter)
Wind stress & sea surface temperature (oC)
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High pressure over winter continent
Low pressure over summer continent
(The Tibetan Plateau effectively blocks
cold Siberia air from entering the Indian
Ocean Æ Northern India has a much
milder winter than southern China.)
(The monsoon is of continental scale,
instead of peninsular scale as depicted in
this schematic.
Wettest spot on Earth: Cherrapunji, India at 10,800 mm/year
(Mount Waialeale, Kaui, comes as a close second)
Great Monsoon of Asia
July
• 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.
January
• 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.
Land topography (km) & wind (m/s) at 925 hPa
10
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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Trails of
Dust Devils
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.
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