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 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:

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 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

How do we characterize the atmosphere?

Winds Temperature Humidity Rainfall Pressure

How do we observe the atmosphere?

Space based platforms

Weather Radars, wind profilers

Traditional observing network Winds, Temperature &

Humidity

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 land-ocean 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

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.)

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

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/

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dFrTT

TAOA

FTA

Atmospheric transportOceanic 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.

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.

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

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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)

Standard Deviation of east-west component of wind (m/s) during northern summer (JJA) averaged over each latitude

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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

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Standard deviation of north-south component of wind (meridional wind, m/s) during northern summer averaged over each latitude circle.

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Northern winter mean zonally averaged northward transport of zonal momentum by transient eddies [u’v’]bar, (m^2/s^2)

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Northern summer mean zonally averaged northward transport of zonal momentum by eddies [u’v’]bar, (m^2/s^2)

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Northern winter mean zonally averaged northward transport of heat by the transient eddies, [v’t’]bar, (m.k/s)

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Northern summer mean zonally averaged northward transport of heat by the transient eddies, [v’t’]bar.

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Required Heat Transport

2/

1

cos2

dFrTT

TAOA

FTA

Atmospheric transportOceanic 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