Transcript CHAPTER – 3 - Wayne State

METEOROLOGY
GEL-1370
Chapter Seven
Atmospheric Circulations
Goal for this Chapter
We are going to learn answers to the following questions:
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What are eddies? How are these eddies formed?
How are sea breezes and land breezes formed
How are monsoons are formed?
What are chinook? How they are formed?
What kind of weather sea breeze and chinook bring?
Why & how winds blow around the world the way they do?
How heat is transported from equatorial regions poleward?
What are El Nino? How are they formed
Scales of Atmospheric Motion
• Winds: Workhorse of weather, moves storms and large
fair weather systems around the globe; transports heat,
moisture, dust, insects/bacteria, pollen, etc.
• Circulations are arranged according to their sizes;
hierarchy of motion is called scales of motion --- tiny
gusts to giant storms
• Microscale: Eddies constitute the smallest scale of
motion; few meter in diameter; form by convection or by
the wind blowing an obstruction; short-lived (few
minutes)
• Mesoscale (Meso: middle): Size from few km to ~100
km in diameter; lasts from minutes to a day; include
local winds, thunderstorms, tornadoes, and small
trophical storms
Scales of atmospheric motion; tiny microscale
motions constitute a part of the larger mesoscale
motions and so on
Scale of atmospheric motion with the
phenomena’s average size and life span
Eddies
• Synoptic scale: Weather map scale; extend from 102103 kms; life time: days to weeks
• Planetary (global) scale: Largest wind pattern; wind
pattern extend over the whole earth;
• Macroscale: synoptic + planetary scales
• Eddies: When wind encounters a solid object, eddy
forms on the object’s downwind side; size and shape
of eddy depend on the size of the object and speed of
the wind; wind flowing over a building produces a
larger eddies that can be size of the building
• Mountain Wave Eddy: Strong winds blowing over a
mountain in stable air produce a mountain wave eddy
on the downwind sie, with a reverse flow near the
ground
Eddies – contd.
• Wind Sheer: Rate of change of wind speed or wind
direction over a given surface
• Clear air Turbulence (CAT): Turbulence produced in a
clean air
• Sea breeze: A coastal local wind that blows from the
ocean to the adjoining land; leading edge of the breeze is
called sea freeze front
• Breeze pushes the warmer, unstable humid air to rise and
condense, producing rain showers
• Thermal circulations: Air circulation primarily
resulting from the heating and cooling of air
• No horizontal variation in pressure --- no pressure
gradient --- no wind (Fig.a)
Air flowing past a mountain range creates eddies
eddies many km downwind from the mountain
Thermal circulation produced by heating &
cooling of the atmosphere near the ground
Thermal circulations
• If the atmosphere is cooled in the North & warmed to the south,
isobars bunch close together in the North while in warmed south,
they spread apart (Fig.b); this dipping of the isobars produces
PGF aloft that causes the air to move from higher pressure to
lower pressure
• After the air aloft moves from S to N, air piles up in the northern
area; surface air pressure in the south decreases and north
increases; PGF is established at the earth’s surface from north to
south and surface winds begin to blow from north to south
• When cool surface air flows southward, it warms & becomes less
dense; warm air slowly rises, expands, cools, and flows out the
top at an elevation of ~1 km above the surface; at this level, air
flows horizontally northward toward lower pressure and the
circulation is completed by sinking & flowing out the bottom of
the surface high
Formation of clear air turbulence along a boundary
of increasing wind speed shear
Turbulent eddies forming downwind of a mountain
chain in a wind shear zone produce these billow
clouds
Sea & Land Breezes
Sea Breeze is a type of thermal circulation; uneven
heating of land & water causes these mesoscale coastal
winds; are strongest during the afternoon when the
temperature contrast between land & ocean occurs
Sea Breeze: A coastal local wind that blows from the
ocean onto the land. The leading edge of the breeze is
called Sea breeze front
Land Freeze: A coastal breeze that blows from land to
sea, usually at night, when land cools more quickly
than the water; temperature contrasts are much weaker
are at night hence land breezes are usually weaker than
sea breeze
Development of a sea breeze and a land
breeze
Land Breeze – weaker & occurs during night
time
Sea & Land Breezes – contd.
• Some coastal cities experience the sea breeze by noon &
their highest temperature usually occurs much earlier
than in inland cities
• Sea breeze in Florida help produce state’s abundant
summertime rainfall
• In UP in Michigan, afternoon clouds and showers are
brought to the land by breezes while lakeshore areas
remains sunny, cool and dry
Monsoon – Seasonally changing winds
• Monsoon – derived from Arabic word ‘Mausim’ means
seasons
• Monsoon Wind system: One that changes direction
seasonally, blowing from one direction in summer and
from the opposite direction in winter
• During winter, air over the continent becomes much
colder than the air over the ocean; a large, shallow highpressure area develops over Siberia, producing a
clockwise circulation of air that flows out over the
Indian Ocean and South China Sea; hence winter
monsoon means clear skies, with winds that blow from
land to sea
Annual wind flow patterns associated with winter
Asian Monsoon
Monsoon – contd.
• In summer, air over the continents become much
warmer than air above the water; shallow thermal low
develops over the continental interior; heated air rises;
moisture bearing winds sweeping into the continent
from the ocean; humid air converges with a drier
westerly flow, causing it to rise; lifting air masses cool
and the air reaches the saturation point, resulting in
heavy showers and thunderstorms
• Summer monsoon of southeastern Asia (June –
September) is wet, rainy weather season with winds
blowing from Sea to Land
Changing annual wind flow patterns associated
with summer monsoon
Monsoon – contd.
• Strength of Indian monsoon related to the reversal of
surface air pressure that occurs at regular intervals about
every 2-7 years at opposite ends of the tropical South
Pacific Ocean
• El Nińo: During this event, surface water near the
equator becomes much warmer over the central and
eastern Pacific; over this region near equator, we find
warm rising air, convection, and heavy rain; west of the
warm water (over the region influenced by the summer
monsoon) , sinking air prohibits cloud formation and
convection --- During El Nino period, monsoon is likely
to be deficient
Monsoon – contd.
• Summer monsoon on the southern hills of the Khasi hills
in northeastern India, Cherrapunji, average annual
rainfall is 1080 cm (425 inch)
• Monsoon wind systems can exist if large contrasts in
temperature develop between oceans and continents
• Southwestern US (Arizona and New Mexico),
monsoonlike circulation exists
• Valley Breeze: A local wind system of a mountain valley
that blows uphill during the day
• Mountain Breeze: A local wind system of a mountain
valley that blows downhill at night
• Katabatic Wind: Any wind blowing downslope, usually
cold
Valley Breeze
Mountain breeze
Mountain slopes warm during the day, air rises and
often condenses into cumuliform clouds
Other wind systems
• Chinook Wind: A warm, dry wind on the eastern side
of the Rocky Mountains; source of warmth for a
chinook is compressional heating, as warmer (and drier)
air is brought down from aloft
• Foehn: A warm, dry wind in the Alps
• Santa Ana Winds: A warm, dry wind that blows into
southern California from the east off the elevated desert
plateau; Its warmth is derived from compressional
heating
• Haboob: A dust or sandstorm that forms as cold
downdrafts from a thunderstorm turbulently lift dust
and sand into the air
Other wind systems – contd.
• Haboobs are most common in the African Sudan & in
the desert southwest of the US (e.g. southern Arizona)
• Whirlwinds or dust devils: The spinning vortices so
commonly seen on hot days in dry areas
• Difference between dust devil and Tornadoes:
Circulation of a tornado descends downward from the
base of a thunderstorm; circulation of a dust devil
begins at the surface, normally in sunny weather,
although some form beneath convective-type clouds
City near the warm air-cold air boundary can
experience sharp temperature changes
Conditions that may enhance a chinook
A chinook wall cloud forming over the Colorado
Rockies
Santa Ana conditions in January; downslope winds
blowing into Southern California raised temp into
the upper 80s; elsewhere much lower
Formation of a dust devil; On a hot, dry day, the
atmosphere next to the ground becomes unstable; air rises,
wind blowing past an obstruction twists the rising air
A dust devil forming on a clear, hot summer
day just south of Phoenix, Arizona
Global Winds
• General Circulation: It represents the average air flow
around the world; caused by unequal heating of the
earth’s surface
• What we have learnt:
– Incoming Solar radiation = outgoing earth radiation
– Energy balance is not maintained for every latitude
– Tropics experience a net gain in energy & Polar regions
suffer a net loss
Atmosphere & Ocean transport warm air poleward
and cool air equatorward
General Circulation of the Atmosphere
• General Circulation Models: Single-cell Model & Three
cell Model
• Single-cell Model Assumptions:
– Earth’s surface is uniformly covered with water (differential
heating between the earth & ocean is eliminated)
– Sun is always directed over the equator (winds will not shift
seasonally)
– Earth does not rotate (No Coriolis force and only force is PGF)
A huge thermally driven convection cell in each
atmosphere
Hadley Cell: A thermal circulation proposed to explain
the movement of the trade winds; consists of rising air
near the equator & sinking air near 30° latitude
General circulation of air on a nonrotating earth
uniformly covered with water & with the sun
directly above the equator
Names of different regions and their latitude
Single-cell Model
• Excessive heating of the equatorial area produces a
broad region of surface low pressure, while at the poles
excessive cooling creates a region of surface high
pressure; closed loop with rising air near the equator,
sinking air over the poles, and equatorward flow of air
near the surface, and a return flow aloft. In this manner,
some of the excess energy of the tropics is transported
as sensible and latent heat to the regions of energy
deficit at the poles
• Limitations: Too simplistic, Coriolis force does deflect
the southward-moving surface air in the Northern
Hemisphere to the right, producing easterly surface
winds
Idealized wind and surface pressure distribution
over a uniformly water-covered rotating earth
Three-cell Model
• Features: Tropical regions receive an excess of heat & poles a
deficit
• In each hemisphere, three cells redistribute energy
– Polar Cell: Circulation from the pole to ~60° {cold air aloft sinks and
reaches the surface & flows back toward the polar front)
– Ferrel Cell: Midlatitude cell from ~30° to ~60°
– Hadley Cell: From equator to ~30°
 A surface high-pressure area is located at the poles & a broad
trough of surface low pressure exists at the equator
 Hadley Cell is driven by latent heat released by cumulus
clouds and thunderstorms produced by warm air rising in the
equatorial region
 Doldrums: Region near the equator characterized by low
pressure and light, shifting winds
Three-cell model contd.
• Subtropical Highs: Rising air in the equatorial region
reaches the tropopause, which acts like a barrier, causing
the air to move toward the pole and this air mass gets
deflected by the Coriolis force providing westerly winds
aloft in both hemispheres; this air mass converges due to
radiational cooling at the midlatitudes; convergence
aloft leads to increase in the mass of air above the
surface; convergence of air aloft produces of belts of
high pressure called subtropical highs
• Converging dry air leads to compressional warming;
subsiding air produces clear skies & warm surface temp
--- major deserts of the world
Three-cell model – contd.
• Horse Latitudes: Belt of latitude ~30-35° where the
winds are dry & predominantly light and the weather is
hot and dry
• Trade Winds: Winds that occupy most of the tropics
and blow from the subtropical highs to the equatorial
low (provided an ocean route to the New World)
• InterTrophical Convergence Zone (ITCZ): The
boundary zone separating the northeast trade winds of
the Northern Hemisphere from the southeast trade
winds of the Southern Hemisphere
• Westerlies: Winds that blow in the midlatitudes on the
poleward side of the subtropical high-pressure areas
Names of surface winds & pressure systems over a
uniformly water-covered rotating earth
Generalized wind distribution
• From TX to Canada – commonly winds blow out of the
west, than from the east
• Polar Front: A semipermanent, semicontinuous front
that separates tropical air masses from polar air masses
• Subpolar Low: A belt of low pressure located between
50° and 70 ° (consists of Aleutian low in the North
Pacific & Icelandic low in the North Atlantic in the
Northern Hemisphere)
• Polar Easterlies: A shallow body of easterly winds
located at high latitudes poleward of the subtropic low
• Generalized Picture: At the surface, 2 major high
(~30° & poles) and low pressure areas (~60° & equator)
Wind distribution – contd.
• Summary contd (generalized picture of surface winds):
– Trade winds extend from subtropical high to the equator
– Westerlies from the subtropical high to the polar front
– Polar easterlies from the poles to the polar front
Comparison of three-cell model with observations:
Upper level winds blow from west to east
Middle cell suggests an east wind aloft as air flows
equatorward – does not agree with observations
Model agrees closely with winds & pressure
distribution in the surface
Average surface winds and Pressure
• Four semipermanent pressure systems in the Northern
Hemisphere during January:
– Bermuda high in the Eastern Atlantic (between 30° & 35 °)
– Pacific high in the Pacific (between 25° & 35 °)
– Icelandic Low (in North Atlantic, covers Iceland & Southern
Greenland)
– Aleutian Low (over Aleutian Islands in the N. Pacific)
Other non semipermanent: Siberian high (formed
because of intense cooling of the land)
Sea-level pressure & Surface wind-flow
patterns in January
Sea-level pressure & Surface wind-flow
patterns in July
Formation of Monsoon
• During summer, land warms --- thermal lows are formed (July
map, thermal lows are seen over desert southwest of US, plateau
of Iran & north of India) --- warm, moist air from the ocean is
drawn, producing the wet summer monsoon
• Between January & July, maximum surface heating shifts
seasonally ---major pressure systems, wind belts and ITCZ shift
toward the north in July & toward south in January
• Abundant rainfall where air rises and little where air sinks --areas of high rainfall exist in the tropics where humid air rises &
at 40-55° where midlatitude storms and the polar front force air
upward
• Areas of low rainfall occur near 30° in the vicinity of subtropical
highs and in polar regions where the air is cold & dry
Major pressure systems & idealized air motions (heavy blue
arrows) & precipitation patterns (blue: abundant rainfall)
Pacific high moves northward; sinking air in eastern margin
causes dry weather; in the western margin of Bermuda high,
southerly winds bring humid air leading to abundant rainfall
Average annual precipitation for Los Angeles &
Atlanta
Westerly winds & Jet stream
• Jet Streams: Relatively strong winds concentrated within a narrow
band in the atmosphere
• Several hundred miles long, less than several hundred miles wide,
less than a mile thick; wind speed can exceed 100 knots (100-200
knots); usually found at the tropopause at 10-14 km
• In the Northern hemisphere, situated along the boundary layer where
cold, polar air lies to the north & milder, subtropical air lies to the
south; sharp contrast in temp produces rapid horizontal pressure
changes --- steep pressure gradient ---- PGF causes the jet stream
• N-S temp contrast along the front is strongest in winter and weakest
in summer --- seasonal variations -- Winds blow stronger in winter
and jet moves farther south; in summer, jet stream is weaker and is
usually found farther north (such as southern Canada)
Jet stream – swiftly flowing current of air; colder
air lies to the north & warmer air to the south
Jet streams – contd.
• There are two jet streams, located in the tropopause
gaps, where mixing of tropospheric & stratospheric air
takes place
• Subtropical jet stream: 13 km above the subtropical high
• Polar front jet stream: 10 km above & near the polar
front
• Jet streams play a major role in the global transfer of
heat; they tend to meander; pollutants are transported to
farther distances by jet streams
Position of polar stream & subtropical jet stream at 300-mb
during March 10, 1998; solid gray lines: Isotachs Heavy lines:
position of jet stream; Heavy blue lines: direction of cold air
southward; heavy red: direction of warm air
Global wind patterns & the oceans
• Wind causes the surface water to drift --- moving water
piles up, creating pressure differences within water itself
• In North Atlantic, Gulf Stream, a warm water current,
flows northward along the east coast of US, carries
warm, tropical water into the higher latitudes; Gulf
stream provides moisture and heat for developing mid
latitude cyclones
• As Gulf Stream moves toward Europe, it merges with
North Atlantic Drift current system; other part flows
southward as the Canary Current equatorward;
• Atmospheric and ocean circulation are closely linked;
wind and ocean transport heat to higher latitude; leads to
energy balance
Major ocean currents: Blue: cold currents; Red: warm
currents; 1: Gulf Stream; 2: North Atlantic Drift; 6: Canary
current; 16: California current
Winds & Upwelling
• When wind blows over the ocean, surface water is set in
motion; it bends slightly right due to Coriolis effect;
water drifts away from coast in California current
system; cold, nutrient-rich from below rises – upwelling
• Benefits of upwelling: food for fish
• Link between Ocean – Atmosphere - pocketbook:
– Once 2-7 years, surface atmospheric pressure pattern
break down (WHY??), as air pressure over western Pacific
and falls over the eastern Pacific ---weakens trades and
during strong pressure reversals, east winds are replaced
by west winds
– A warm current of nutrient-poor tropical water moves
southward, replacing the cold, nutrient-rich surface water
– El Nino (spanish for boy child) referring to Christ child
Average position of the polar front jet stream &
subtropical jet stream in winter; both jet streams are
flowing into the page
El Nino & Southern Oscillation
• During El Nino event, large numbers of fish & marine
plants may die; dead fish & birds litter the beaches of
Peru
• El Nino of 1972-1973 reduced Peruvian anchovy catch
from 10.3 million metric tons in 1971 to 4.6 million
metric tons in 1972 --- fishmeal production dropped in
1972 --- Animal feed prices went up --- poultry prices
went up by 40%
• Southern Oscillation: See-Saw pattern of reversing
surface air pressure at opposite ends of the Pacific
Ocean; pressure reversals and ocean warming are more
or less simultaneous --- El Nino/Southern Oscillation or
ENSO
Ordinary condition - Higher pressure over the
southeastern Pacific & lower pressure near Indonesia
El Nino condition: Atm pressure decreases over the eastern
Pacific and rises over the W. Pacific; trade winds weaken or
reverse direction; thermocline changes
SST during non El Nino conditions – upwelling along the
equator and Peru coast keeps the water cool (blue color) in the
tropical eastern Pacific
SSTs: upwelling is greatly diminished, and warm water (red
color) from the Western Pacific has replaced the cool water
Regions of climatic abnormalities due to ENSO; months in
black: during the same year; red: following year
El Nino & its impacts
• In the eastern equatorial Pacific, as high as 6°C than the
normal has been observed
• Warm water along the coastal areas of Ecuador & Peru
chokes off the upwelling that supplies cold, nutrient-rich
water to South America’s coastal region
• Warm tropical water fuels the atmosphere with
additional warmth and moisture --- additional
storminess & rainfall
• Certain regions of the world experience too much
rainfall & other regions have very little
• Over the warm tropical central Pacific, the frequency of
typhoons increases
Effects of El Nino
• Tropical Atlantic, between Africa and Central
America: fewer hurricanes
• Summer monsoon conditions tend to get weaker
• Drought is felt in Indonesia, southern Africa,
Australia
• Heavy rains & flooding in Ecuador & Peru
• Storms in to California
• Heavy rain into the Gulf Coast states
• La Nina: Cold surface water moving to Central and
eastern Pacific & warm water confined to western
tropical Pacific
What Causes El Nino
• Within the changing of seasons, especially
the transition periods of spring & fall
• Winter monsoon plays a major role in
triggering a major El Nino event
• ENSO and monsoon system are linked
• Linked to out pocket books!!
chapter –7- Summary
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Micro and macro-scale motion
Wind sheer; sea breeze and land breeze
Monsoon depression; development, causes
Valley breeze, katabatic wind; chinook wind, Santa Ana winds
Dust devil
ITCZ, location of Detroit, Chicago, Barrow, Honolulu – what
types of wind system
Semi-permanent high and low pressure areas
Converging/diverging along polar front
Three- and one-cell general circulation model-driest areas
Westerlies and easterlies
Hadley, Ferrel cells
Subpolar lows, doldrums, horse latitudes
Where we do see deserts
Summary – contd.
• Polar front jet stream, jet stream blow direction
• Upwelling
• North Atlantic Drift, Gulf Stream current, California
current, Labrador
• Major currents that flow parallel to the coast of North
America
• El Nino; where does the warming occur; Southern
Oscillation