AOSC 621Physics and Chemistry ofthe Atmosphere II:Radiative TransferLESSON 1
Purpose• The purpose of this course is twofold:• (1) To examine how solar radiation istransferred through the atmosphere,absorbed by the Earth, and re-emitted bythe Earth and atmosphere.• (2) To examine how this radiation drivesthe dynamics and chemistry of theatmosphere.
Earth’s (long-term) energy balancesolar incident=solar reflected + Earth emitted
Composition of the Earth’s TroposphereH 2O 2N 2CHPM4CON 2 OO 3←SO 2, NO 2 ,CFC’s, etcArCO 2Inert gases
Height profiles for minor species• Species:Chlorofluorocarbons, CCl 3 F and CCL 2 F 2Nitrous oxide, N 2 OMethane, CH 4Nitric acid, HNO 3Carbon monoxide, COCarbon dioxide, CO 2.• Note that the CO 2 line is vertical. This signifies that CO 2is well mixed in the atmosphere and has no significantchemical loss or production• The lines for the chlorofluorocarbons and nitrous oxideare vertical in the troposphere (well mixed) but sufferchemical loss in the stratosphere.
Height profiles for minor species
HYDROSTATIC EQUATIONWhere H is called the atmospheric scale height
Pressure and Atmospheric Mass•The hydrostatic relationship tellsus that the pressure decreasesexponentially with height.•A hydrostatically balancedatmosphere does not mean thatthere is no vertical motion. Therestill can be vertical motion(topographically p forced verticalmotion). , but no acceleration ordeceleration.•But a non-hydrostatic balanceatmosphere column would havesevere up/down draft (gravitywaves) => typically y inthunderstorm (very short timeover a small area)
Trenberth et al., 2009, BAMS
Radiation Primary Energy transfer mechanisms in the atmosphere and oceans Phase Change (ice water water vapor) Vertical convection (including turbulent heat and momentumfluxes at the (land and ocean) surface. Meridional mass circulation driven by equator-pole heatingcontrast. West-east mass circulation driven by west-east thermal andmechanic contrasts (land-ocean, sea surface temperaturevariation, and orography).
Radiation - quantity and quality To describe electromagnetic radiation, we need to provideinformation about the amount of energy transferred (quantity),and the type, or quality, of the energy. In the case of radiation, quantity is associated with the height ofthe wave, or its amplitude. Everything else being equal, theamount of energy carried is directly proportional to waveamplitude. The quality, or “type,” of radiation is related to another propertyof the wave, the distance between wave crests (wavelength). The radiation emitted by an object obey some fundamentalphysical laws => the amount of radiation (quantity) emitted andits wavelength (quality) is related.
Electromagnetic Radiation• By analogy a stream of particles can be consideredas a wave.• The chemistry of the atmosphere is dominated byradicals derived from the dissociation of atmosphericspecies.• It is much easier to picture this dissociation as theinteraction of a photon with a molecule than as aninteraction of a wave with the molecular field.• Dissociation requires a threshold of energy, i.e. acertain frequency. enc Only photons with an energy(frequency) above this threshold will causedissociation.
Electromagnetic spectrum• Most of the energy that drives the dynamicscomes from a narrow band of frequenciesknown as the visible spectrum - heats theground• Most the the energy that drives the chemistry pfthe atmosphere comes from the ultraviolet partof the spectrum - high energy photons.• Most of the energy that heats the atmosphere isthermal radiation from the ground
Schematic of a wave
WAVELENGTH• DISTANCE BETWEEN SUCCESSIVE PEAKS• GIVEN THE SYMBOL λ• MANY UNITS USED :• (A) MICRON, 10 -6 METERS• (B) NANOMETER, 10 -9 METERS• (C) ANGSTROM, 10 -10 METERS
FREQUENCYDefined as the number of maxima that pass an observerper secondGiven the symbol c Wave number is also used. It is the reciprocal of the wavelengthIn the textbook it is given the symbol Caution - the use of and are reversed in many textbooks
VELOCITY• WAVE VELOCITY IS DEFINED AS THE DISTANCE APEAK MOVES IN ONE SECOND.• IN VACUO THE WAVE VELOCITY OF ANELECTROMAGNETIC WAVE IS 2.997925x10 8METERS PER SECOND• THE WAVE VELOCITY IS GIVEN THE SYMBOL c.
SOLAR ENERGY• Solar constant – energy from the sun thatfalls on unit surface normal to the line fromthe sun, per unit time, at the outside of theatmosphere at the mean solar distanceS = 1.368 Kw per meter 2• S is integrated over all wavelengths• S varies with the sunspot cycle
Measurements of total solarirradiance 1979-2000
SOLAR SPECTRUM• A blackbody curve for a temperature of6000K matches the observed solarspectrum.• The difference between the 6000Kspectrum below about 400 nm is due toabsorption in the photosphere.• The structures t shown below 400 nm areknown as Fraunhoffer lines
UV-VISIBLE SOLAR SPECTRUMOBSERVED FROM THE GROUND
Absorption spectra of atmospheric gasesUVVisibleInfraredCH 4N 2 OO 2 & O 3ABSOR RPTIVITYCO 2H 2 OatmosphereWAVELENGTH (micrometers)H 2 O dominates >15 µmIR Windows
UV-VISIBLE SOLAR SPECTRUMOBSERVED FROM THE GROUND• Slide shows the absorption of the solarradiation by the atmosphere.• Major absorbers are molecular oxygen(O 2 ), ozone (O 3 ), water vapor (H 2 O) andcarbon dioxide (CO 2 ).• The wavelength is in nm. 1000nm is equalto one micron.
INFRARED FLUX FROM EARTHSAHARA
INFRARED FROM EARTHMEDITERRANEAN
INFRARED FROM EARTHANTARCTIC
EFFECTIVE AREA• If the solar zenith angle (SZA) of the sun is θ,then the effective area is given by1/cos θ.• Hence the energy on unit area at the surface isS divided by the effective area, i.e. S.cos θ.• College Park is at a latitude of 39 degrees• At summer solstice the SZA is 17 degrees, andat the winter solstice 61 degrees• The ratio of the effective areas is 1.97, i.e. twiceas much energy per unit area is incident onCollege Park at summer solstice than at wintersolstice.
Optical line-of-sight pathsNote that the angle defined here is the polar angle
Slant column mass