Transcript Slide 1

Atmospheric circulation
Moving air and heat around
Upper level flow
• Lower level flow in “cells”; upper
level flow to poles
• Warm air in tropics and cooler air
at poles
• Depth of troposphere changes
with latitude
• Steepest pressure gradient at midlatitudes
• At given altitude, pressure higher
at equator
• High altitude flow is from equator
to poles down pressure gradient
• Wind speeds greatest at midlatitudes (jet streams)
Circulation in Hadley cells
• Buoyancy
– Vertical air movements related to moisture and
heat (DENSITY)
• Pressure differences
– Hadley cells
– Horizontal air movements
– Air move from high pressure to low pressure
• Coriolis effect
– Apparent deflection to right or left
– Vertical component – centrifugal
– Pushes an object away from center of rotation
• ITCZ – convergence and uplift of hot, moisture
laden air
• Subsidence at 30 degrees – cool, dry air
• Divergence due to high pressure at Earth’s
surface
Meridonal circulation
(N-S flow)
Pressure
Coriolis effect and atmospheric
circulation
• Coriolis effect influences wind direction
• Air only makes it about 1/3 of the way to the poles
before it becomes dense enough to sink
• End up with 3 sets of cells – by 30 deg, flow has been
deflected 90 deg
• Descending air turns back toward equator when it
reaches the surface because it is again deflected to the
right
• Heats up when it gets back to equator and rises again.
• On a CCW (eastward)
rotating earth
• Change in rotation rate
with latitude (angular
momentum
• Winds due east or due
west affected by
centrifugal force –
horizontal component is to
the right in N hemisphere
• Coriolis effect increases as
speed increases
• Coriolis effect increases
with latitude counterintuitive
• No Coriolis at the equator
Net result – sort of
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Very low temperature at poles
Increased air density near the surface
Movement of cold air outward (divergence)
Equatorward-moving cold air
Steep temperature gradient at polar front
Little mixing – wavelike structure around the hemisphere
Exact latitude varies
Fig. 4-7
Hadley Cell
Fig. 4-6
Hadley Cell
More realistic near poles
Fig. 4-11
Features of the model
• At boundaries, air is moving vertically
– Surface winds are weak and erratic
• Equatorial region
– Lots of rain as humid air rises and loses moisture (rain
forests)
– Doldrums
– Intertropical convergence zone (ITCZ) – winds converge
• 30oN and S region
– Sinking air is arid and evaporation >> precipitation
(deserts and high salinity)
– Horse latitudes
Features of the model
• Air moves horizontally within the cells from areas of high
pressure to areas of low pressure
• Tropical areas – Hadley cells
– Surface winds are strong and dependable
– Trade winds or easterlies centered at ~15oN (northeast trade
winds) and ~ 15oS (southeast trade winds)
– Surface wind moves from horse latitudes to doldrums so come
out of northeast in N hemisphere
• Mid-latitude areas – Ferrel cells
– Westerlies centered at ~ 45oN and ~45oS
– Surface wind moves from horse latitudes to polar cells so
comes out of southwest in the N hemisphere
• Major surface wind and pressure systems of the
world and their weather
• These wind patterns move 2/3 of heat from tropics
to poles.
Circulation of the Atmosphere
• Over long term – 6-cell model is pretty good
for describing average flow
• Most of the variation from the 6-cell model is
due to
– Geographical distribution of landmasses
– Different response of land and ocean to solar
heating
– Chaotic flow
The 6-celled model
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Not exactly correct either
North - South variation
East-West variation
Seasonality
Land versus water distribution
– Equator to pole flow of air different depending on
amount of land at a particular longitude
– ITCZ narrower and more consistent over land than
ocean
– Seasonal differences greater in N hemisphere
(remember, more land)
Distributions of land masses
-Differential heating and cooling
-Land heats up and cools more rapid
Winds over the Pacific
on two days in Sept
1996
Stronger winds in redorange
Notes:
Deviates from 6-cell
model
Strong westerlies hitting
Canada
Strong tradewinds
(easterlies) over Hawaii
Extratropical cyclone
east of New Zealand
West-East variations
• Air over chilled continents becomes cold and dense
in the winter
• Air sinks creating high pressure over continents
• Air over relatively warmer waters rises (possibly with
water vapor) creating low pressure zones over water
• Air flows from high pressure to low pressure
modifying air flow within cells
• Reverse situation in summer
• Effects pronounced in N hemisphere (mid-latitudes)
where there is about the same amount of land &
water
Monsoons
• Pattern of wind circulation that changes with
the season
• Generally wet summers and dry winters
• Linked to different heat capacities of land and
water and to N-S movement of the ITCZ
Seasonality important
Shifts in polar front and the ITCZ – meteorological equator
Wet season
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In the spring, land heats (faster than water)
Warm air over land rises creating low pressure
Cool air flows from ocean to land
This humid air heats and rises (rains form)
Dry Season
• Land cools (faster than ocean)
• Air cools and sinks over land creating high
pressure
• Dry surface wind moves seaward
• Warms and rises over water (with or without
evaporation and rain over water)
Monsoons
• Most intense over Asia where you have a huge
land mass in the N and a huge ocean to the S
• Monsoon over India causes wet season
(summer) from April – October (up to 10
meters – 425 inches of rain per year)
• Smaller monsoon in N America (Gulf of
Mexico and SE)
Dry season
Wet season
ITCZ
ITCZ
Sea and Land breezes
• Daily changes in wind direction due to unequal
heating and cooling of land versus water
• Warm air during day on land rises and cool air from
sea moves onshore (with or without water vapor)
• Warmer air over water rises and cool air on land
during the night sinks and moves offshore
Daytime Onshore Breeze
Nighttime Offshore Breeze
Weather and our model
• Cells are not really continuous features
• Clusters of convective cells
• Vary seasonally and locally and from day to
day
• Exact location of subtropical highs and polar
front can create cyclonic flow
Weather
• A result of smaller atmospheric motions and
eddies
• Usually caused by differences in atmospheric
pressure, temperature and humidity (remember
all of these affect density)
• Weather forecasts try to predict smaller scale air
movements
Cyclonic flow
• Localized circular flow to the right (CCW – N
hemisphere). Air moving into a low pressure
center
• Anticyclonic (CW – N hemisphere) flow – air
flowing out of a high pressure center
• Low pressure systems forming outside the
tropics are extratropical cyclones
– Extensive uplift of warm air
– Features move along polar front
Air masses
• Comparable to a water mass
• Large body of air with uniform temperature
and humidity (so density) throughout
• Air over land or water will take on
characteristics of surface below
– Cold, dry land yields cold, dry air (high pressure)
– Warm ocean surface yields warm, wet air (low
pressure)
Air masses
• Air masses form over land and water acquiring
characteristics of their sources
– Dry, cold air forms over Canada and Siberia…
– Wet, moist air forms over equatorial waters…
• When air masses move, they change
characteristics
– Temperature changes
– Humidity or water content changes (lose water)
Air masses
• Air masses can move within or between cells
• Density differences prevent air masses from
mixing (like water) – dense air slides beneath
• Turbulence at boundaries between air masses
• Fronts are boundaries between air masses of
different densities
– Fronts marked by changes in temperature and humidity
Fronts
• A cold front is the leading edge of a cold-air
mass advancing on a warm air mass
– Displaces warm air
– Cold air pushes under warm air (more toe shaped)
– Get precipitation (rain or snow) just behind the front
• A warm front is the advancing edge of a warm
air mass
– Displaces cold air
– Rises over cold air in a wedge shape
– Drops water in front of its leading edge
Polar front
• About 50o N and S
• Persistent boundary between converging warm
and cold air masses
• Get highly variable weather at these latitudes
• Made up of a succession of waves that appear
on weather maps as warm or cold fronts
– Succession of warm, moist, subtropical air
and cold dry polar air
– Weather typical of N America and Europe
• Narrow bands of strong winds called jet streams
at altitudes of about 10 km
Ocean influence on weather
• At mid-latitudes, warm and cold water masses
steer weather patterns on land
• Size and energy of water masses permits this
• Large cold water masses in the N Pacific shift
prevailing westerlies blowing across E North
America
• Cold, dry air from Canada displaces warm, moist
air from the Gulf of Mexico and the tropical N
Atlantic
• So get cooler winters in the SE USA
• Shifts in positions of water masses can cause
changes in patterns
• Warm equatorial surface waters in the Atlantic
cause prolonged drought in Africa?
Storms
• Regional atmospheric disturbances characterized by
strong winds and, often, precipitation
• Cyclones are intense storms around low pressure
centers
• Tropical disturbances (in Hadley cells/tropics) –
cyclones (hurricanes)
• Extratropical disturbances (in Ferrel cells/midlatitudes) – also cyclones, usually in winter
Cyclones
• Low pressure air
• Rotates as winds converge and ascend (may
bring water with them so get precipitation)
• Form between or within air masses
Extratropical cyclones
• Form at the boundary of polar and Ferrel cells (polar
front) – mid-latitudes
• Occur mainly in winter when temperature and
density differences across the front are most
pronounced
• Cold air poleward of front is moving from the pole
and east (more dense)
• Warm air equatorward of the front is moving from
the equator and west (less dense)
• Cold air tries to slide below the warm air at the low
pressure interface of a stationary front
Extratropical cyclone
• May get alternating high and low pressure systems
that bend the front
• May get a twist in front due to opposite wind
directions
• Twisting air mass becomes cyclonic and circulates
CCW in the N hemisphere (opposite Coriolis)
• CCW flow is Coriolis driven because of the dominant
flow of air masses at the edges
• Part of the front is cut off
• Wind speed increases as storm condenses
• Air rushing toward center rises making a low
pressure zone (air rises and loses moisture)
Cold air tries to dive below or push under warm air
Higher pressure N of cold front so bending
is towards lower pressure
Cold air pushes warm air/front
Low pressure intrusion into cold front
Warm air rises (with or without water) at both
fronts & yields precipitation at the fronts
Cold front pushes warm front
Eventually part of front is cut off and moves
east
Extratropical cyclones
• Cyclone gets embedded in the westerly winds
so moves eastward
• Typically 1000-2500 km in diameter
• Last 2-5 days
Extratropical cyclones
• Precipitation begins as circular flow develops
• Precipitation caused by the lifting and cooling of the
mid-latitude air (warmer air from the Ferrel cell)
involved in the twist
• Cold air advances behind it and does the lifting
creating a cold front
• Warm front occurs as the warm air is lifted on top of
the retreating cold edge
• Often these are called frontal storms and are the
principle cause of weather in mid-latitudes
Nor’easters
• Most powerful wind approaches from the east
(polar cells)
• Occur along the east coast of the US in winter
Tropical cyclones
• Masses of warm, humid, rotating air
• Occur in all tropical oceans except the equatorial
South Atlantic
• Large tropical cyclones (winds at least 119 km/hr)
are:
– hurricanes in the North Atlantic & eastern Pacific (about
100/year)
– Typhoons in the western Pacific
– Tropical cyclones in the Indian Ocean
– Willi-willis in the waters near Australia
• Smaller tropical cyclones are tropical storms or
depressions
Tropical cyclones
• Masses of warm, humid, rotating air
• Occur in all tropical oceans except the equatorial
South Atlantic
• Large tropical cyclones (winds at least 119 km/hr)
are:
– hurricanes in the North Atlantic & eastern Pacific (about
100/year)
– Typhoons in the western Pacific
– Tropical cyclones in the Indian Ocean
– Willi-willis in the waters near Australia
• Smaller tropical cyclones are tropical storms or
depressions
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Appear as circular spirals
May be 1000 km in diameter and 15 km high
Calm center is the eye & can be 13-16 km
Occur June – November in N hemisphere
Internal structure of a mature hurricane.
Tropical cyclones
• Usually generated within one air mass
• Usually generated between 10o and 25o latitude
(Coriolis effect closer to equator is too weak to
initiate rotary motion)
• Typically last ~9 days
• Origins not well understood
– Convergence of warm, wet winds that rise
• Usually develop from a tropical depression
• Power if from the condensing water vapor and rising
air currents at the eye
Tropical cyclones
• Tropical depressions form in easterly waves
– areas of lower pressure within the easterly tradewinds
– thought to originate over a large, warm land mass.
• Air containing the disturbance is heated over tropical
water
• Circular winds begin to blow in the vicinity of the
wave
• Some warm, humid air is forced upward
• Condensation begins
Where hurricanes form (areas of high humidity and warm air over warm water)
Hurricanes
• Develop in 2-3 days from tropical cyclones under
ideal conditions
• Centers move westward and poleward (within
easterlies) in N hemisphere at 5 to 40 km/hr
• Poleward motion due to general atmospheric
circulation
• Hurricanes lose strength over land (friction and loss
of water vapor supply) or relatively cold surface
water (decreases rising wind speed in eye)
Tropical cyclone tracks – breeding grounds shown in orange