The General Circulation of the Atmosphere

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The General Circulation of the Atmosphere B.
N. Goswami Centre for Atmospheric and Oceanic
Sciences Indian Institute of Science Bangalore
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Introduction to Observed General Circulation of
the Atmosphere or the Climate (the mean
condition) Example of Weather producing systems
(fluctuating component)
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Weather and Climate Weather is what you
see Climate is what you expect In other words
Weather is the Instantaneous State of the the
Ocean-atmosphere system or the day-to-day
fluctuations Climate is the Mean state of the
Ocean-Atmosphere System on which the day-to-day
fluctuations or the Weather rides. Normally
mean refers to time mean (e.g. seasonal
mean) Examples
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An example of weather and climate
Daily time series of precipitation (PPT),
eastward component of wind at 850 hPa level
(U850) and temperature near the surface at 925
hPa at a tropical station around Bombay. The red
line is the annual cycle or expected values.
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Another example of weather and climate
Daily time series of precipitation (PPT),
eastward component of wind at 850 hPa level
(U850) and temperature near the surface at 925
hPa at a high latitude station (70E,55N). The
red line is the annual cycle or expected values.
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Fig.5 Polar stereographic projection of
geopotential height at 500 hPa in the NH on a
typical day. The large scale waves with
wavelength 3000-4000 km are seen.
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Fig.6 Polar stereographic projection of
geopotential height at 500 hPa in the SH on a
typical day. The large scale waves with
wavelength 3000-4000 km are seen.
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An example of long waves in the middle latitude
westerlies
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One heavy rainfall producer in the tropical
region is the Tropical Cyclone. We observe, using
a Geosynchronous satellite similar to NOAAs GOES
series, a Cyclone originating in the Indian Ocean
in May of 1999. These storms can end droughts or
cause devastating floods on the Indian
Subcontinent.
Hurricanes are hazardous for residents along the
East Coast and Gulf of Mexico. Hurricane Floyd
was a devastating flood-producer along the
eastern U.S. coast in 1999. This view of Floyd is
from one of NOAAs GOES satellites, which was
developed and launched by NASA.
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Another ex. of Weather in the tropics A Low
Pressure System on the ITCZ gives copious rain in
Rajasthan-Gujarat, 5-8-04
IR picture from METEOSAT at 18UTC 05-08-2004
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Observed mean structure of the Atmosphere

• Observed vertical and horizontal structure of the
  atmosphere.
• Temperature, winds and humidity fields.
• What maintains this distribution?
• Solar radiation and earths radiation and
  radiation balance.
• Simple estimate of global mean surface
  temperature.
• Greenhouse effect and examples of surface
  temperature of some other planets and their
  radiative equilibrium.

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How do we characterize the atmosphere? Winds
Temperature Humidity Rainfall
Pressure
How do we observe the atmosphere?

• Traditional observing network Winds,
  Temperature Humidity

• Space based platforms

• Weather Radars, wind profilers

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From a network of roughly 900 upper-air stations,
radiosondes, attached to free-rising balloons,
make measurements of pressure, wind velocity,
temperature and humidity from just above ground
to heights of up to 30km. Over two thirds of the
stations make observations at 0000UTC and
1200UTC. Between 100 and 200 stations make
observations once per day, while about 100 have
"temporarily" suspended operations. In ocean
areas, radiosonde observations are taken by 15
ships, which mainly ply the North Atlantic,
fitted with automated shipboard upper-air
sounding facilities.
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Why Mean ? What Mean ?

• The atmosphere variables fluctuates in a wide
  range of time scales
• In this lecture, we do not address the variation
  but concentrate on time mean state of the
  atmosphere
• However, there are clear differences between
  summer and winter. Therefore time mean will refer
  to seasonal mean. We shall show summer and winter
  separately

The atmosphere has a 3-dimensional structure

• There are east-west variations, north-south
  variations and variations in the vertical

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Another example of weather and climate
Daily time series of precipitation (PPT),
eastward component of wind at 850 hPa level
(U850) and temperature near the surface at 925
hPa at a high latitude station (70E,55N). The
red line is the annual cycle or expected values.
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Long term mean seasonal average vector winds
during NH winter (DJF) and summer (JJA) at the
surface. This is based on 40 years of NCEP/NCAR
reanalysis. Colors indicate wind magnitude.
Easterlies in the tropics and westerlies in the
middle latitudes may be noted. Reversal of winds
between the two seasons over the monsoon regions
is seen.
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Long term mean seasonal average vector winds
during NH winter (DJF) and summer (JJA) at 850
hPa. This is based on 40 years of NCEP/NCAR
reanalysis. Colors indicate wind magnitude.
Easterlies in the tropics and westerlies in the
middle latitudes may be noted. Reversal of winds
between the two seasons over the monsoon regions
is seen.
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Long term mean seasonal average vector winds
during NH winter (DJF) and summer (JJA) at 500
hPa. This is based on 40 years of NCEP/NCAR
reanalysis. Colors indicate wind magnitude.
Easterlies in the tropics and westerlies in the
middle latitudes may be noted. Winds at this
level over the monsoon regions are weak during
both seasons.
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Long term mean seasonal average vector winds
during NH winter (DJF) and summer (JJA) at 200
hPa. Colors indicate wind magnitude. Easterlies
in the tropics and jet-like strong westerlies are
seen in the sub-tropics. Westerly jet in the
winter hemisphere is stronger than that in the
summer hemisphere.
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Long term mean seasonal average vector winds
during NH winter (DJF) and summer (JJA) at 100
hPa. Colors indicate wind magnitude. Easterlies
in the tropics and jet-like strong westerlies are
seen in the sub-tropics. An easterly jet over the
equatorial monsoon region during summer. Also a
massive anticyclonic circulation sits over the
Tibet during summer.
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Long term mean seasonal average vector winds
during NH winter (DJF) and summer (JJA) at 50
hPa (lower stratosphere). Colors indicate wind
magnitude. The striking feature is that westerly
jet is asymmetric about the equator at this
level. Summer hemisphere does not have westerly
jet and the jet is located closer to the winter
hemispheric polar region.
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Eastward component of the winds (zonal winds, u)
averaged along a latitude circle (zonal average)
as a function of latitude and height (represented
in pressure from 1000 hPa to 10 hPa. In the
troposphere (below 100 hPa), subtropical westerly
jets in both hemispheres may be seen. Westerly
jet in the summer hemisphere and easterly jet in
the winter hemisphere are seen the stratosphere.
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Long term mean seasonal average temperature (K)
during NH winter (DJF) and summer (JJA) at the
surface. This is based on 40 years of NCEP/NCAR
reanalysis. In the tropics (between 30S and
30N), latitudinal variations of temp. is very
weak. It is rapid in the middle latitude. The
equator-to-pole temp. difference is around 60K
(40K)in winter (summer) hemisphere.
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Long term mean seasonal average temperature (K)
during NH winter (DJF) and summer (JJA) at 850
hPa.
Similar to that at surface but the magnitude has
decreased. The wave like structure of Temp.
contours in NH winter (DJF) is due to land-ocean
contrasts.
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Long term mean seasonal average temperature (K)
during NH winter (DJF) and summer (JJA) at 500
hPa.
Similar to that at 850 hPa but the magnitude has
further decreased. The wave like structure of
Temp. contours in NH winter (DJF) is due to
land-ocean contrasts.
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Long term mean seasonal average temperature (K)
during NH winter (DJF) and summer (JJA) at 200
hPa.
Similar to that at 500 hPa but the magnitude has
further decreased.
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Long term mean seasonal average temperature (K)
during NH winter (DJF) and summer (JJA) at 100
hPa. It may be noted that at this level, the
equator is colder than the polar region reversing
the equator to pole temperature gradient at this
level compared to that at the surface.
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Temperature (K) averaged along a latitude circle
(zonal average) as a function of latitude and
height (represented in pressure from 1000 hPa to
10 hPa. The temperature decreases to a height
(tropopause) and increases thereafter. Height of
the tropopause in the tropics is about 100 hPa
while it is 300 hPa in polar regions. The
symmetry of the temperature profile around the
equator in the troposphere and its asymmetry in
the stratosphere may be noted.
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Specific humidity (g/kg) averaged along a
latitude circle (zonal average) as a function of
latitude and height (represented in pressure from
1000 hPa to 300 hPa.
Pressure vertical velocity (hPa/s) averaged along
a latitude circle (zonal average) as a function
of latitude and height (represented in pressure
from 1000 hPa to 100 hPa. Negative values
represent upward motion.
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How is a three cell meridional structure is
maintained?
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Precipitation (mm day-1)
Climatological mean precipitation (mm day-1) for
January and July.
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Zonal Mean Annual Precipitation (mm day-1)
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Some important features of the observed Mean
condition of the atmosphere

• Surface easterlies in the tropics surface
  westerlies in the middle latitudes
• Westerly jet stream in the upper atmosphere
  subtropics. Winter hemisphere jet tends to be
  stronger than the summer hemisphere one.
• Easterly jet in the upper atmosphere over the
  equatorial region during summer monsoon region
• Three cell meridional structure

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Some important features of the observed Mean
condition of the atmosphere (contd.)

• Equator to pole temperature difference is about
  600K in the winter hemisphere and about 350K in
  the summer hemisphere
• The temperature gradient in the meridional
  direction is weak in the tropics and strong in
  the middle latitude.
• Height of the tropopause is much lower in the
  polar region as compared to the equatorial region

What drives this temperature and wind
distribution in the Atmosphere?
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Geometry of the sun-earth system
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The Radiation Budget Incoming Solar (SW)
outgoing LW
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(Top) Normalized blackbody radiation for sun
(left) and earth (right). (Bottom) Absorption of
solar radiation at 11 km and ground level.
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Calculation of Radiative Equilibrium Temperature
Te Radiative equilibrium temperature t
Infrared transmissivity (assuming no atmosphere,
t 1.0)
Solar constant S0 ? 1365 W m-2 Albedo a 0.3
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Characteristics of atmospheres of four planets
R Radius in units of earths radius A
Albedo Te Radiative equilibrium temp. Tm
Approx. measured temp. at the top of the
atmosphere. Mr Molecular weight of the air.
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Role of the Atmosphere

• Decreases Long Wave (LW) radiation loss to space
• Depends on clouds, Water vapor, and CO2
  distributions

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Equilibrium Temperature for Venus
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However, if the earth had one uniform
temperature, there would be no pressure gradient
and no motion (winds)! So, the energy balance
model, just described is only a zero-order model
of the earths climate! In reality, due to the
sphericity of the earth and its inclination of
its axis in the ecliptic plane, radiation
received varies with latitude. Next, the
latitudinal variation of radiation balance is
described.
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Zonal mean incoming solar radiation (W m 2 ) at
the top of the atmosphere, annual mean (thick
solid), JJA (dashed line) and DJF (thin solid) as
a function of latitude.
Zonal mean reflected solar radiation (W m 2 ) at
the top of the atmosphere, annual mean (thick
solid), JJA (dashed line) and DJF (thin solid) as
a function of latitude.
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Zonal mean Albedo () at the top of the
atmosphere, annual mean (thick solid), JJA
(dashed line) and DJF (thin solid) as a function
of latitude.
Zonal mean absorbed radiation (W m 2 ), annual
mean (thick solid), JJA (dashed line) and DJF
(thin solid) as a function of latitude.
Zonal mean emitted radiation (W m 2 ), annual
mean (thick solid), JJA (dashed line) and DJF
(thin solid) as a function of latitude.
Zonal mean net radiation (W m 2 ) at the top of
the atmosphere, annual mean (thick solid), JJA
(dashed line) and DJF (thin solid) as a function
of latitude.
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• Positive net heat flux at the top of the
  atmosphere and negative net heat flux over the
  polar region indicates that,
• Air should rise over the tropics and sink over
  the polar region.
• One large meridional cell?
• Early attempts to explain the general circulation
  assumed a single meridional circulation.
• But this cannot explain westerlies in the middle
  latitude. In this case we should have easterlies
  at the surface over the whole globe.

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The net heat balance at the TOA also indicates
that, for the earths climate to be in
equilibrium, there must be mechanisms in place
that continously transports heat from equatorial
regions to the polar regions.
FTA
Required Heat Transport
Atmospheric transport
Oceanic transport
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How are the Atmospheric motions
generated? Positive net heating in Tropics
negative net heating in polar regions ?Warmer
tropics Colder polar regions ?Lower Pressure in
the tropics and higher pressure in the polar
regions ?Air moves under the action of the
pressure gradient force and motion is
generated. ?As the earth is rotating, Coriolis
force modifies this motion and observed
circulation is generated.
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Governing Equations
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How do we explain surface easterlies in tropics
and westerlies in middle latitudes? A three cell
meridional circulation is required.
How is a three cell meridional structure is
maintained?
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• Thus, estimation of the mean meridional
  circulation (e.g.zonal mean vertical velocity)
  indicates the existence of three meridional cells
  in each hemisphere.
• Three meridional cells in each hemisphere are
  also required to explain the surface easterlies
  in the equatorial region and surface westerlies
  in middle latitude.
• The middle cell where ascending motion takes
  place around 60 deg where the surface is
  relatively warmer and descending motion takes
  place around 30 deg where the surface is
  relatively warmer is a thermally indirect cell,
  also called Ferrel cell.
• What is responsible for the indirect Ferrel
  cell? What makes air to rise over a surface which
  is colder than over its descending region?

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So, What is responsible for the indirect
meridional cell? I mentioned that large
amplitude Rossby waves are important part of
middle latitude circulation. Could these waves
play a role is causing the indirect meridional
cell? What are the amplitudes of these waves?
Plot standard deviation. Can they transport heat
and momentum? We shall calculate transport of
heat (vt) and vu.
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An example of amplitude daily fluctuations of
wind at 200 hPa level at a point in middle
latitude (shown by the dot) U and V winds during
summer (red) and winter (blue) are highlighted.
It may be noted that 20-40 m/s wind variation
from one day to another takes place.
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JJAS
Standard deviation of daily fluctuations of U
wind at 200 hPa level during summer season over
all grid points Note that S.D. is generally
uniform along a latitude circle. Also note that
the S.D is small in tropics and large in middle
latitudes.
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Note large day-to-day fluctuations (15 m/s) of
zonal winds in middle lat. Upper atmos. In the
exit region of the subtropical westerly jets. It
is small in the tropics (3-5 m/s)
H E I G H T
Standard Deviation of east-west component of wind
(m/s) during northern winter (DJF) averaged over
each latitude circle
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Similar to the distribution during winter
(previous figure). However, there is one major
difference in the distribution. What is it?
H E I G H T
Standard Deviation of east-west component of wind
(m/s) during northern summer (JJA) averaged over
each latitude circle
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H E I G H T
Standard Deviation of north-south component of
wind (meridional wind , m/s) during northern
winter averaged over each latitude circle
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H E I G H T
Standard deviation of north-south component of
wind (meridional wind, m/s) during northern
summer averaged over each latitude circle.
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H E I G H T
Northern winter mean zonally averaged northward
transport of zonal momentum by transient eddies
uvbar, (m2/s2)
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H E I G H T
Northern summer mean zonally averaged northward
transport of zonal momentum by eddies uvbar,
(m2/s2)
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H E I G H T
Northern winter mean zonally averaged northward
transport of heat by the transient eddies,
vtbar, (m.k/s)
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H E I G H T
Northern summer mean zonally averaged northward
transport of heat by the transient eddies,
vtbar.
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FTA
Required Heat Transport
Atmospheric transport
Oceanic transport
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Relative contributions mean meridional
circulation and the eddies in meridional
transport of energy.
Transient eddy transport
Stationary eddy transport
MMC transport
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Maintenance of General Circulation of the
Atmosphere
Solar Input Net Q ve in tropics, -ve in polar
regions
Equator to Pole Temp. Gradient dT/dy
Thermal Wind dU/dz
Baroclinic Instability
Decreases dT/dy and stabilizes Baroclinic
Instability
Waves transport heat and momentum poleward