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Meteorology ENV 2A23 Radiation Lectures. How is energy transferred?

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Presentation on theme: "Meteorology ENV 2A23 Radiation Lectures. How is energy transferred?"— Presentation transcript:

1 Meteorology ENV 2A23 Radiation Lectures

2 How is energy transferred?

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5 Conduction Convection Radiation

6 Conduction Convection Radiation

7 Conduction Convection Radiation

8 How is energy transferred? Conduction – energy transfer from molecule to molecule Convection – spatial mixing of air parcels i.e. masses of air Radiation – primary source of energy for the Earth Radiation imbalances drive the circulation of the atmosphere and ocean

9 Electomagnetic radiation in the range 0.1 to 10 micrometres ( m), i.e x10 -6 m

10 Electomagnetic radiation travels in packets (quanta), whose energy is given by E = hc/8, where 8 is wavelength, h is Plancks constant (6.625x J s -1 ) c is speed of light (3x10 8 m s -1 )

11 The Sun Most solar radiation is emitted from the photosphere (T~6000 K) Sun powered by nuclear fusion, H to He Plasma ejected as solar wind

12 The Sun The suns radiative output is centred on visible wavelengths

13 The Sun The suns output is not constant Sunspot cycle ~11 years Periods of high/low activity

14 Sun-Earth Geometry Axial tilt = 23.5 o Eccentricty = 0.02 Aphelion = 1.50x10 8 km, 3 July Perihelion = 1.45x10 8 km, 3 January –SH receives more solar radiation in summer than NH –Is it warmer?

15 Sun-Earth Geometry Equinoxes = equal days and nights

16 Sun-Earth Geometry Solstice = sun stands still, longest/shortest days

17 Changes in orbital parameters result in changes in incoming solar radiation and distribution (Milankovitch 1930) Orbital featureRangePeriod (years) Radiation changes Tilt21.8 o to 24.4 o 40,000Seasonal radiation balance only Eccentricity0 to ,000Seasonal balance and total radiation by ±15% Precession of equinoxes orbit21,000Seasonal affects

18 The Suns energy output The solar constant is the radiation flux density at the top of the atmosphere, for the mean sun-earth distance i.e. the amount of radiation falling on the top of the atmosphere (per unit area) S 0 = 1360 W m -2

19 The Suns energy output The sun is an almost perfect emitter of radiation, i.e. emits maximum possible radiation for its temperature It is a blackbody emitter and so governed by Stephan-Boltzmann Law: F = FT 4, where, F is flux density W m -2, T is temperature, F = 5.67x10 -8 W m -2 K -4

20 Radiation flux density at the Earth F = FT 4 per unit area So over sphere 4Br s 2 FT 4 Hence at distance of earth (r d ): 4Br s 2 FT 4 / 4Br d 2 i.e. S 0 = r s 2 /r d 2 FT 4, an inverse square law sun rsrs rdrd earth

21 Emission temperature of a planet The emission temperature of a planet is the blackbody temperature with which it needs to emit radiation in order to achieve energy balance. To calculate this for the Earth, equate blackbody emission with amount of solar energy absorbed. - see radiation practical

22 Emission temperature of a planet Energy incident on planet = solar flux density x shadow area But not all radiation is absorbed, some is reflected: albedo (α) = reflected/incident radiation Absorbed solar radiation = S 0 (1- α)π r e 2 (W) Absorbed solar radiation per unit area = S 0 (1- α)/4 (W m -2 ) This must be balanced by terrestrial emission. If we approximated F e as a blackbody: F Earth = σT e 4, where T e is the blackbody emission temperature. => T e 4 = S 0 (1- α)/σ4 For Earth, T e = 255 K. Note this is well below the average surface air temperature of the Earth = 288 K.

23 Distribution of Insolation Seasonal & latitudinal variations in temperature are driven primarily by variations in insolation The amount of solar radiation incident on the top of the atmosphere depends on:

24 Distribution of Insolation Seasonal & latitudinal variations in temperature are driven primarily by variations in insolation The amount of solar radiation incident on the top of the atmosphere depends on: –Latitude –Season –Time of day

25 Distribution of Insolation The solar zenith angle (2 s ) is the angle between the local normal to the Earths surface & the line between the Earths surface & the sun The (daily) solar flux per unit area can be calculated as: where S 0 is the solar constant, and d is the sun-earth distance earth 2s2s

26 Distribution of Insolation The season ~ declination angle *, –i.e. latitude on Earths surface directly under the sun at noon - * varies between 23.5 & o The time of day ~ hour angle h, –Longitude of subsolar point relative to its position at noon Then cos θ s = sinφ sinδ + cosφ cosδ cosh, for latitude φ

27 Distribution of Insolation

28 Equator receives more solar radiation than the poles (at the top of the atmosphere)

29 As well as the distribution of insolation, the amount of energy absorbed and emitted depends on atmospheric and surface conditions. Energy balance at the top of the atmosphere

30 albedo (α) = reflected/incident radiation

31 Energy balance at the top of the atmosphere Outgoing longwave radiation

32 Energy balance at the top of the atmosphere The net radiation can be calculated from R = SW d – SW u + LW d – LW u, Where SW = shortwave (solar) radiation, LW = longwave (terrestrial radiation) => R = SW d (1-α p ) –LW u at the top of the atmosphere, where α p is the planetary albedo. Net radiation

33 Energy balance at the top of the atmosphere => R = SW d (1-α p ) –LW u at the top of the atmosphere, where α p is the planetary albedo.

34 Energy balance at the top of the atmosphere There must be a poleward transport of energy to balance out the net gain at the equator and the net loss at the poles.

35 Radiation Flux and Radiation Intensity The radiation flux density (or irradiance), F (units W m -2 ) is the radiant energy crossing a unit area in unit time. It does not discriminate between different directions. The radiation intensity (or radiance), I, (units W m -2 steradians -1 ) includes information on directionality. Special Case : Radiation intensity I is isotropic, Then F = BI For example: emission from a blackbody, emission from the atmosphere Animation…

36 What about the wavelength of the radiation? In other words, radiation intensity depends on frequency (or equivalently wavelength) of emission. i.e. B B v (T) dv = FT 4 Plancks Law

37 What about the wavelength of the radiation? Weins Law Suns emission peaks ~ 4.8 microm Earths emission peaks ~ 10 microm Brightness temperatures of the sun and Earth are ~6000 K and 255 K

38 What about the wavelength of the radiation? When an object is not a blackbody, then its radiation flux density can be written F = eσT 4, where e is the emissivity. Usually e λ = e(λ) is a function of wavelength. If we define absorptivity a λ as the fraction of incident radiation that is absorbed. It can be shown that e λ = a λ, this is Kirchoffs Law. i.e. an object emits radiation at each wavelength as efficiently as it absorbs it.

39 Radiation in the atmosphere Earlier we found the blackbody emission temperature T e = 255 K, much colder than the observed T surface = 288 K. Why ?

40 Radiation in the atmosphere Difference is due to selective scattering, absorption and emission of radiation by the atmosphere. These depend upon the structure of the molecules present. sketch

41 Radiation in the atmosphere Difference is due to selective scattering, absorption and emission of radiation by the atmosphere. These depend upon the structure of the molecules present.

42 Scattering Scattering decreases the intensity of the solar beam. It depends upon λ (wavelength) and d (particle size). Three cases:

43 (1) Rayleigh Scattering occurs when d << λ For example from O 2 or N 2, the major tropospheric gases, where d = m and λ = 0.5x10 -6 m. Scatters equal amounts of radiation forward and backward The amount of scattering strongly dependent on λ: the volume extinction coefficient is a function of 1/ λ 4 Rayleigh scattering explains why the sky is blue and sunsets are red. - blue (short λ) scattered more than red (long λ) light

44 (2) Diffuse scattering occurs when d >> λ Diffuse scattering occurs when d >> λ, for example from dust or cloud droplets Typically ~10 m Diffuse scattering is independent of λ. –Clouds appear white and polluted skies are pale Full consideration requires Mie theory.

45 (3) Complex Scattering occurs when d = λ Diffraction

46 Absorption All gases absorb and re-radiate energy at specific wavelengths depending on their molecular structure. –Electronic excitation – visible uv –Vibrational excitation – IR –Rotational excitation – thermal IR Molecules need a permanent electric dipole, e.g. H 2 O H H O + -

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48 Aborption occurs at specific wavelengths (lines) according to the excitational properties of the gas (or gases) involved. However these lines are broadened by various mechanisms into absorption bands.

49 Absorption line broadening 1.Natural broadening – associated with the finite time of photon emission and the uncertainty principle 2.Pressure broadening (or collision broadening) – collisions between molecules supply or remove small amounts of energy during radiative transitions. - Primary mechanism in the troposphere (why?) 3.Doppler broadening – results from the movement of molecules relative to photons. - dominant at higher altitudes

50 Groups of lines within a frequency interval are termed absorption bands In the thermal infra- red there are important absorption bands due to H 2 O, CO 2, O 3, CH 4, N 2 O, etc

51 Bottom panel shows atmosphere is generally opaque to IR radiation There are important windows at 8-9 m and m. It is through these windows that most passive satellite sensors observe radiation emissions

52 For example, this geostationary Meteosat image shows radiation emitted in the IR at m.

53 Clouds and radiation Clouds consist of liquid water droplets or ice particles suspended in the atmosphere The droplets or ice particles interact with both solar and terrestrial (IR) radiation, depending on their size and shape.

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58 i.e. the cloud albedo is a function of total liquid water content and solar zenith angle.

59 Thick clouds (e.g. 1 km), e.g. cumulus, = 0.9 Thin clouds (e.g. 100 m), e.g. stratus, = 0.7 Very important for planetary albedo

60 Global (1 dimensional) Energy Balance Observations from the ground & space of emitted radiation, combined with climatological surface energy flux observations have allowed an average (1D) picture of energy transfer through the Earths atmosphere to be estimated.

61 SH = sensible heat fluxes, LE = latent heat fluxes

62 Solar: 100 units incoming, 70 absorbed, 30 reflected or scattered Terrestrial 110 emitted from surface! The strong downward LW emission (89) is responsible for modulating the diurnal cycle

63 Further reading: Chapters 2 and 3 Ahrens


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