ATMOSPHERIC RADIATION

Here  is the radiation flux emitted in [  is the flux distribution function characteristic of the object Total radiation flux emitted by object: EMISSION OF RADIATION Radiation is energy transmitted by electromagnetic waves; all objects emit radiation One can measure the radiation flux spectrum emitted by a unit surface area of object:

BLACKBODY RADIATION Objects that absorb 100% of incoming radiation are called blackbodies For blackbodies,   is given by the Planck function:     k 4 /15c 2 h 3 is the Stefan-Boltzmann constant max = hc/5kT Wien’s law Function of T only! Often denoted B(  T) max

KIRCHHOFF’S LAW: Emissivity  T) = Absorptivity For any object:…very useful! Illustrative example: Kirchhoff’s law allows determination of the emission spectrum of any object solely from knowledge of its absorption spectrum and temperature

SOLAR RADIATION SPECTRUM: blackbody at 5800 K

TERRESTRIAL RADIATION SPECTRUM FROM SPACE: composite of blackbody radiation spectra for different T Scene over Niger valley, N Africa

RADIATIVE EQUILIBRIUM FOR THE EARTH Solar radiation flux intercepted by Earth = solar constant F S = 1370 W m -2 Radiative balance  effective temperature of the Earth: = 255 K where A is the albedo (reflectivity) of the Earth

ABSORPTION OF RADIATION BY GAS MOLECULES …requires quantum transition in internal energy of molecule. THREE TYPES OF TRANSITION –Electronic transition: UV radiation (<0.4  m) Jump of electron from valence shell to higher-energy shell, sometimes results in dissociation (example: O 3 +h   O 2 +O) –Vibrational transition: near-IR (  m) Increase in vibrational frequency of a given bond requires change in dipole moment of molecule –Rotational transition: far-IR (  m) Increase in angular momentum around rotation axis Gases that absorb radiation near the spectral maximum of terrestrial emission (10  m) are called greenhouse gases; this requires vibrational or vibrational-rotational transitions

NORMAL VIBRATIONAL MODES OF CO 2 forbidden allowed IR spectrum of CO 2 bend asymmetric stretch

GREENHOUSE EFFECT: absorption of terrestrial radiation by the atmosphere Major greenhouse gases: H 2 O, CO 2, CH 4, O 3, N 2 O, CFCs,… Not greenhouse gases: N 2, O 2, Ar, …

SIMPLE MODEL OF GREENHOUSE EFFECT Earth surface (T o ) Absorption efficiency 1-A in VISIBLE 1 in IR Atmospheric layer (T 1 ) abs. eff. 0 for solar (VIS) f for terr. (near-IR) Incoming solar Reflected solar Surface emission Transmitted surface Atmospheric emission Atmospheric emission Energy balance equations: Earth system Atmospheric layer Solution:T o =288 K  f=0.77 T 1 = 241 K VISIBLE IR

RADIATIVE AND CONVECTIVE INFLUENCES ON ATMOSPHERIC THERMAL STRUCTURE In a purely radiative equilibrium atmosphere T decreases exponentially with z, resulting in unstable conditions in the lower atmosphere; convection then redistributes heat vertically following the adiabatic lapse rate

The ultimate models for climate research

EQUILIBRIUM RADIATIVE BUDGET FOR THE EARTH

TERRESTRIAL RADIATION SPECTRUM FROM SPACE: composite of blackbody radiation spectra emitted from different altitudes at different temperatures

HOW DOES ADDITION OF A GREENHOUSE GAS WARM THE EARTH? Initial state Add to atmosphere a GG absorbing at 11  m; emission at 11  m decreases (we don’t see the surface anymore at that  but the atmosphere) 3. At new steady state, total emission integrated over all ’s must be conserved  Emission at other ’s must increase  The Earth must heat! 3. Example of a GG absorbing at 11  m

EFFICIENCY OF GREENHOUSE GASES FOR GLOBAL WARMING The efficient GGs are the ones that absorb in the “atmospheric window” (8-13  m). Gases that absorb in the already-saturated regions of the spectrum are not efficient GGs.

RADIATIVE FORCING OF CLIMATE CHANGE Incoming solar radiation Reflected solar radiation (surface, air, aerosols, clouds) F out F in IR terrestrial radiation ~ T 4 ; absorbed/reemitted by greenhouse gases, clouds, absorbing aerosols EARTH SURFACE Stable climate is defined by radiative equilibrium: F in = F out Instantaneous perturbation  Radiative forcing  F = F in – F out Different climate models give  = K m 2 W -1, insensitive to nature of forcing; differences between models reflect different treatments of feedbacks Increasing greenhouse gases   F > 0 positive forcing The radiative forcing changes the heat content H of the Earth system: where T o is the surface temperature and is a climate sensitivity parameter eventually leading to steady state

CLIMATE CHANGE FORCINGS, FEEDBACKS, RESPONSE Positive feedback from water vapor causes rough doubling of

CLIMATE FEEDBACK FROM HIGH vs. LOW CLOUDS convection ToTo T cloud ≈ T o Clouds reflect solar radiation (  A > 0)  cooling; …but also absorb IR radiation (  f > 0)  warming WHAT IS THE NET EFFECT? To4To4  T cloud 4 ≈  T o 4 LOW CLOUD: COOLING  T cloud 4 <  T o 4 To4To4 HIGH CLOUD: WARMING

IPCC [2007]

ORIGIN OF THE ATMOSPHERIC AEROSOL Soil dust Sea salt Aerosol: dispersed condensed matter suspended in a gas Size range:  m (molecular cluster) to 100  m (small raindrop) Environmental importance: health (respiration), visibility, radiative balance, cloud formation, heterogeneous reactions, delivery of nutrients…

SCATTERING OF RADIATION BY AEROSOLS: “DIRECT EFFECT” By scattering solar radiation, aerosols increase the Earth’s albedo Scattering efficiency is maximum when particle radius =  particles in  m size range are efficient scatterers of solar radiation 2 (diffraction limit)

Mt. Pinatubo eruption Temperature Change ( o C) Observations NASA/GISS general circulation model Temperature decrease following large volcanic eruptions EVIDENCE OF AEROSOL EFFECTS ON CLIMATE:

SCATTERING vs. ABSORBING AEROSOLS Scattering sulfate and organic aerosol over Massachusetts Partly absorbing dust aerosol downwind of Sahara Absorbing aerosols (black carbon, dust) warm the climate by absorbing solar radiation

AEROSOL “INDIRECT EFFECT” FROM CLOUD CHANGES Clouds form by condensation on preexisting aerosol particles (“cloud condensation nuclei”)when RH>100% clean cloud (few particles): large cloud droplets low albedo efficient precipitation polluted cloud (many particles): small cloud droplets high albedo suppressed precipitation

 Particles emitted by ships increase concentration of cloud condensation nuclei (CCN)  Increased CCN increase concentration of cloud droplets and reduce their avg. size  Increased concentration and smaller particles reduce production of drizzle  Liquid water content increases because loss of drizzle particles is suppressed  Clouds are optically thicker and brighter along ship track N~ 100 cm -3 W~ 0.75 g m -3 r e ~ 10.5 µm N~ 40 cm -3 W~ 0.30 g m -3 r e ~ 11.2 µm from D. Rosenfeld EVIDENCE OF INDIRECT EFFECT: SHIP TRACKS

AVHRR, 27. Sept. 1987, 22:45 GMT US-west coast NASA, 2002 Atlantic, France, Spain SATELLITE IMAGES OF SHIP TRACKS

Aircraft condensation trails (contrails) over France, photographed from the Space Shuttle (©NASA). OTHER EVIDENCE OF CLOUD FORCING: CONTRAILS AND “AIRCRAFT CIRRUS”