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