Transcript Course material from Prof Goswami

Weather, Climate & General Circulation
B. N. Goswami
Indian Institute of Tropical Meteorology
Introduction to
Observed General Circulation of the
Atmosphere or the Climate (the
mean condition)
Example of Weather producing
systems (fluctuating component)
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 OceanAtmosphere System on which the day-to-day
fluctuations or the Weather rides. Normally
‘mean’ refers to time mean (e.g. seasonal mean)
Examples:
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.
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.
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.
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.
An example of long waves in the middle latitude westerlies
One heavy rainfall producer in the
tropical region is the Tropical Cyclone.
We observe, using a Geosynchronous
satellite similar to NOAA’s 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.
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
Meteosat cloud picture of June 09, 2008. Weather Vortices
on the ITCZ
Daily surface water temp as measured by satellite
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 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.
How do we characterize the atmosphere?
Winds
Pressure
Temperature
Humidity
How do we observe the atmosphere?
 Traditional observing network
Winds, Temperature & Humidity
 Space based platforms
 Weather Radars, wind profilers
Rainfall
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.
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 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.
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 landocean contrasts.
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.
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.
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.
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
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?
Geometry of the sun-earth system
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.
Calculation of Radiative Equilibrium Temperature
Solar constant S0  1365 W m-2
Te – Radiative equilibrium temperature
Albedo α = 0.3
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.
FTA
Required Heat 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.
 /2
T  T    2r cosF d
A
O
1
Atmospheric transport
Oceanic transport
TA
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
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 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.
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.
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 winter (DJF) 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 east-west component of wind (m/s)
during northern summer (JJA) 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 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
Northern summer mean zonally averaged northward
transport of heat by the transient eddies, [v’t’]bar.
FTA
Required Heat Transport
 /2
T  T    2r cosF d
A
O
1
Atmospheric transport
Oceanic transport
TA
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
Decreases dT/dy and
stabilizes Baroclinic
Instability
Waves transport heat and
momentum poleward