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Weather & Climate
B. N. Goswami Indian Institute of Science Education and Research, Pune
Reference Books
1. Atmospheric Science : An Introductory Survey, by J.M.Wallace and P.V. Hobbs, Elsevier
Science, 1977
2. Physics of Atmospheres, Third Edition, by John Houghton, Cambridge University Press, 2002
3.Physics of Climate, by J. P. Peixoto and A.H. Oort, AIP Press, Springer, 1992
4. Introduction to Physical Oceanography, by Robert Stewart,
Outline of Lecture: 1.The Climate System
Sun-Earth System; Atmosphere, Ocean, Biosphere, geosphere
Evolution and Composition of the atmosphere and the ocean
Definitions weather and climate, examples
Early knowledge of winds and atmospheric circulation
How do we observe the atmosphere and the Ocean
Observed 3-D structure of Mean temperature, humidity and wind
distribution in the atmosphere
Mean temperature salinity and current distributions in the ocean
Some basic concepts of Dynamics and thermodynamics
Drivers of climate, heat balance. External forcing and Internal feedbacks
Variations of climate. Glacial-interglacial oscillations, millennium scale
oscillations during Holocene, Multidecadal oscillations (AMO, PDO etc),
Interannual variations (e.g. ENSO)
How to model climate, simple examples
Observed vertical and horizontal structure of the atmosphere.
Temperature, winds and humidity fields.
What maintains this distribution?
Solar radiation and earth’s 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.
Observed mean structure of the Atmosphere
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
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.
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.
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.
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.
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.
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.
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.
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.
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.
Long term mean seasonal
average temperature (K)
during NH winter (DJF)
and summer (JJA) at 200
hPa.
Magnitudes decrease with height. But at this level there is a inter hemispheric asymmetry. One pole is warmer than the other that reverses with season.
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.
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.
Climatological zonal mean U Climatological zonal mean T
It may be noted that zonal mean zonal winds and temperature are strongly coupled. Meridional gradient of T is proportional to vertical shear of zonal winds.
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.
How is a three cell meridional structure is maintained?
Precipitation (mm day-1)
Climatological mean precipitation (mm day-1) for January and July.
Zonal Mean Annual Precipitation (mm day-1)
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
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?
Some important features of the observed Mean condition of the atmosphere (contd.)
Ocean Currents, Temperature and Salinity:
Surface currents in the ocean are largely driven by atmospheric winds
Atmospheric winds Ocean surface currents
Deep thermohaline circulation is driven by buoyancy (density)
Average salinity
A typical vertical profile of temperature A temperature-depth section along equator in Pacific Ocean
Geometry of the sun-earth system
What drives the winds and temperature in the Atmosphere?
The motion in the atmosphere (winds) is caused by Heating gradients.
L
H
H
L
Heating Cooling
or No
Heating
Imbalance of heating
Imbalance of temperature
Imbalance of pressure
Wind
Radiation Convection
Latent/Sensible
Conduction
Land/Ocean/Ice
Feedback
Blackbody Radiation
Planck’s Law: The amount of radiation emitted at wavelength λ by
a blackbody at temperature T is given by Planck's Law by,
(1)
Balance of Radiation received and lost by emission leads to Heating of atmosphere/earths surface. Need to review some basic concepts of Blackbody Radiation
Wein Displacement Law: From the Planck’s equation, we can derive that the wavelength at which maximum radiation is emitted by a blackbody is inversely proportional to Temperature, known as the Wein Displacement ;law.
(2)
(3)
The Stephen-Bolzmann Law: The radiance emitted by a black body can be obtained by integrating (1) over all wavelenghts, known as blackbody irradiance is given by,
Blackbody Radiation
T temp. in OK
The Radiation Budget : Incoming Solar (SW) & outgoing LW
(Top) Normalized blackbody radiation for sun (left) and earth (right).
(Bottom) Absorption of solar radiation at 11 km and ground level.
Te- Equilibrium
temperature.
Calculation of Radiative Equilibrium Temperature
Solar constant S0 1365 W m-2
Albedo α = 0.3
Te – Radiative equilibrium temperature
τ – Infrared transmissivity (assuming no
atmosphere, τ = 1.0)
Characteristics of atmospheres of four planets
R – Radius in units of
earth’s radius
A – Albedo
Te – Radiative
equilibrium temp.
Tm – Approx.
measured temp. at the
top of the
atmosphere.
Mr – Molecular
weight of the air.
Role of the Atmosphere
Decreases Long Wave (LW) radiation loss to space
Depends on clouds, Water vapor, and CO2 distributions
Equilibrium Temperature for Venus
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 earth’s 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.
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.
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.
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.
Required Heat Transport
2/
1
cos2
dFrTTTAOA
Atmospheric transport
Oceanic transport
The net heat balance at the TOA also indicates that, for the earth’s climate to be in equilibrium, there must be mechanisms in place that continously transports heat from equatorial regions to the polar regions.
FTA
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.
Governing Equations
Thus, if air rises at the equator and moves to the poles and sink and return to the equator near the surface it would set up ONE huge N-S cell in each hemisphere. What would happen to the surface winds?
If the surface winds were easterlies everywhere as shown, what would happen to the rate of rotation of the earth?
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?
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?
So, What is responsible for the ‘indirect meridional cell?
I mentioned earlier 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 ([v’t’]) and [v’u’].
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.
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.
JJAS
Standard Deviation of east-west component of wind (m/s)
during northern winter (DJF) averaged over each latitude circle
H
E
I
G
H
T
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)
Standard Deviation of east-west component of wind (m/s)
during northern summer (JJA) averaged over each latitude
circle
H
E
I
G
H
T
Similar to the distribution during winter (previous figure). However, there is one major difference in the distribution. What is it?
Standard Deviation of north-south component of wind (meridional
wind , m/s) during northern winter averaged over each latitude circle
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.
H
E
I
G
H
T
Northern winter mean zonally averaged northward transport
of zonal momentum by transient eddies [u’v’]bar, (m^2/s^2)
H
E
I
G
H
T
` `
Northern summer mean zonally averaged northward
transport of zonal momentum by eddies [u’v’]bar, (m^2/s^2)
H
E
I
G
H
T
` `
Northern winter mean zonally averaged northward transport
of heat by the transient eddies, [v’t’]bar, (m.k/s)
H
E
I
G
H
T
V’T’
Northern summer mean zonally averaged northward
transport of heat by the transient eddies, [v’t’]bar.
H
E
I
G
H
T
V’T’
Required Heat Transport
2/
1
cos2
dFrTTTAOA
FTA
Atmospheric transport
Oceanic transport
Relative contributions mean meridional circulation and the eddies in meridional transport of energy.
Transient eddy transport
Stationary eddy transport
MMC transport
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
Waves transport heat and momentum poleward
Decreases dT/dy and stabilizes Baroclinic
Instability