8How is energy transferred? Conduction – energy transfer from molecule to moleculeConvection – spatial mixing of “air parcels” i.e. masses of airRadiation – primary source of energy for the EarthRadiation imbalances drive the circulation of the atmosphere and ocean
9Electomagnetic radiation in the range 0.1 to 10 micrometres (mm), i.e. 0.1-10 x10-6 m
10Electomagnetic radiation travels in packets (quanta), whose energy is given by E = hc/8,where 8 is wavelength,h is Planck’s constant (6.625x10-34 J s-1)c is speed of light (3x108 m s-1)
11The SunMost solar radiation is emitted from the photosphere (T~6000 K)Sun powered by nuclear fusion, H to HePlasma ejected as “solar wind”
12The SunThe sun’s radiative output is centred on visible wavelengths
13The Sun The sun’s output is not constant Sunspot cycle ~11 years Periods of high/low activity
14Sun-Earth Geometry Axial tilt = 23.5o Eccentricty = 0.02 Aphelion = 1.50x108 km, 3 JulyPerihelion = 1.45x108 km, 3 JanuarySH receives more solar radiation in summer than NHIs it warmer?
15Sun-Earth GeometryEquinoxes = “equal” days and nights
16Sun-Earth GeometrySolstice = “sun stands still”, longest/shortest days
17Changes in orbital parameters result in changes in incoming solar radiation and distribution (Milankovitch 1930)Orbital featureRangePeriod(years)Radiation changesTilt21.8o to 24.4o40,000Seasonal radiation balance onlyEccentricity0 to 0.0696,000Seasonal balance and total radiation by ±15%Precession of equinoxesorbit21,000Seasonal affects
18The Sun’s energy output The solar constant is the radiation flux density at the top of the atmosphere, for the mean sun-earth distancei.e. the amount of radiation falling on the top of the atmosphere (per unit area)S0 = 1360 W m-2
19The Sun’s energy output The sun is an almost perfect emitter of radiation, i.e. emits maximum possible radiation for its temperatureIt is a blackbody emitter and so governed by Stephan-Boltzmann Law:F = FT4, where, F is flux density W m-2,T is temperature,F = 5.67x10-8 W m-2 K-4
20Radiation flux density at the Earth sunrsrdearthF = FT4 per unit areaSo over sphere 4Brs2FT4Hence at distance of earth (rd): 4Brs2FT4/ 4Brd2i.e. S0 = rs2/rd2 FT4, an inverse square law
21Emission 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
22Emission temperature of a planet Energy incident on planet = solar flux density x shadow areaBut not all radiation is absorbed, some is reflected:albedo (α) = reflected/incident radiationAbsorbed solar radiation = S0(1- α)π re2 (W)Absorbed solar radiation per unit area = S0(1- α)/4 (W m-2)This must be balanced by terrestrial emission.If we approximated Fe as a blackbody:FEarth = σTe4 , where Te is the blackbody emission temperature.=> Te4 = S0(1- α)/σ4For Earth, Te = 255 K.Note this is well below the average surface air temperature of the Earth = 288 K.
23Distribution of Insolation Seasonal & latitudinal variations in temperature are driven primarily by variations in insolationThe amount of solar radiation incident on the top of the atmosphere depends on:
24Distribution of Insolation Seasonal & latitudinal variations in temperature are driven primarily by variations in insolationThe amount of solar radiation incident on the top of the atmosphere depends on:LatitudeSeasonTime of day
25Distribution of Insolation The solar zenith angle (2s) is the angle between the local normal to the Earth’s surface & the line between the Earth’s surface & the sunThe (daily) solar flux per unit area can be calculated as:where S0 is the solar constant, and d is the sun-earth distance2searth
26Distribution of Insolation The season ~ declination angle *,i.e. latitude on Earth’s surface directly under the sun at noon- * varies between 23.5 & -23.5oThe time of day ~ hour angle h,Longitude of subsolar point relative to its position at noonThen cos θs = sinφ sinδ + cosφ cosδ cosh, for latitude φ
28Distribution of Insolation Equator receives more solar radiation than the poles (at the top of the atmosphere)
29Energy balance at the top of the atmosphere As well as the distribution of insolation, the amount of energy absorbed and emitted depends on atmospheric and surface conditions.
30Energy balance at the top of the atmosphere albedo (α) = reflected/incident radiation
31Energy balance at the top of the atmosphere Outgoing longwave radiation
32Energy balance at the top of the atmosphere Net radiationThe net radiation can be calculated fromR = SWd – SWu + LWd – LWu ,WhereSW = shortwave (solar) radiation,LW = longwave (terrestrial radiation)=> R = SWd(1-αp) –LWuat the top of the atmosphere,where αp is the planetary albedo.
33Energy balance at the top of the atmosphere => R = SWd(1-αp) –LWuat the top of the atmosphere,where αp is the planetary albedo.
34Energy 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.
35Radiation 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 = BIFor example: emission from a blackbody, emission from the atmosphereAnimation…
36What about the wavelength of the radiation? Planck’s Lawi.e. B Bv(T) dv = FT4In other words, radiation intensity depends on frequency (or equivalently wavelength) of emission.
37What about the wavelength of the radiation? Wein’s LawSun’s emission peaks ~ 4.8 micromEarth’s emission peaks ~ 10 micromBrightness temperatures of the sun and Earth are ~6000 K and 255 K
38What about the wavelength of the radiation? When an object is not a blackbody, then its radiation flux density can be writtenF = eσT4, where e is the emissivity.Usually eλ = e(λ) is a function of wavelength.If we define absorptivity aλ as the fraction of incident radiation that is absorbed. It can be shown thateλ = aλ , this is Kirchoff’s Law.i.e. an object emits radiation at each wavelength as efficiently as it absorbs it.
39Radiation in the atmosphere Earlier we found the blackbody emission temperature Te = 255 K, much colder than the observed Tsurface = 288 K.Why ?
40Radiation 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
41Radiation 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.
42Scattering 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 O2 or N2, the major tropospheric gases, where d = m and λ = 0.5x10-6 m.Scatters equal amounts of radiation forward and backwardThe amount of scattering strongly dependent on λ:the volume extinction coefficient is a function of 1/ λ4Rayleigh 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 dropletsTypically ~10 mmDiffuse scattering is independent of λ.Clouds appear white and polluted skies are paleFull consideration requires Mie theory.
45(3) Complex Scattering occurs when d = λ Diffraction
46AbsorptionAll gases absorb and re-radiate energy at specific wavelengths depending on their molecular structure.Electronic excitation – visible uvVibrational excitation – IRRotational excitation – thermal IRMolecules need a permanent electric dipole, e.g. H2OO-HH+
48Aborption 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.
49Absorption line broadening Natural broadening – associated with the finite time of photon emission and the uncertainty principlePressure broadening (or collision broadening) – collisions between molecules supply or remove small amounts of energy during radiative transitions.- Primary mechanism in the troposphere (why?)Doppler broadening – results from the movement of molecules relative to photons.- dominant at higher altitudes
50Groups of lines within a frequency interval are termed absorption bands In the thermal infra-red there are important absorption bands due to H2O, CO2, O3, CH4, N2O, etc
51Bottom panel shows atmosphere is generally opaque to IR radiation There are important “windows” at 8-9 mm and mm.It is through these “windows” that most passive satellite sensors observe radiation emissions
52For example, this geostationary Meteosat image shows radiation emitted in the IR at 10.5-12.5 mm.
53Clouds and radiationClouds consist of liquid water droplets or ice particles suspended in the atmosphereThe droplets or ice particles interact with both solar and terrestrial (IR) radiation, depending on their size and shape.
58i.e. the cloud albedo is a function of total liquid water content and solar zenith angle.
59Thick clouds (e.g. 1 km), e.g. cumulus, a = 0.9 Thin clouds (e.g. 100 m), e.g. stratus, a = 0.7Very important for planetary albedo
60Global (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 Earth’s atmosphere to be estimated.
61SH = sensible heat fluxes, LE = latent heat fluxes
62Solar: 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