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Shoot for the moon and if you miss you'll land among the stars...
6. KIRCHHOFF’S LAW: Emissivity T ) = Absorptivity Illustrative example: Kirchhoff’s law allows determination of the emission spectrum of any object solely from knowledge of its absorption spectrum and temperature For any object: … very useful!
8. TERRESTRIAL RADIATION SPECTRUM FROM SPACE: composite of blackbody radiation spectra for different T Scene over Niger valley, N Africa
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10. 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: where A is the albedo (reflectivity) of the Earth = 255 K
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12. NORMAL VIBRATIONAL MODES OF CO 2 forbidden allowed allowed IR spectrum of CO 2 bend asymmetric stretch
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16. 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
18. TERRESTRIAL RADIATION SPECTRUM FROM SPACE: composite of blackbody radiation spectra emitted from different altitudes at different temperatures
19. HOW DOES ADDITION OF A GREENHOUSE GAS WARM THE EARTH? 1. 1. Initial state 2. 2. 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
20. 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.
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22. CLIMATE CHANGE FORCINGS, FEEDBACKS, RESPONSE Positive feedback from water vapor causes rough doubling of
23. CLIMATE FEEDBACK FROM HIGH vs. LOW CLOUDS convection T o T cloud ≈ T o Clouds reflect solar radiation ( A > 0) cooling; … but also absorb IR radiation ( f > 0) warming WHAT IS THE NET EFFECT? T o 4 T cloud 4 ≈ T o 4 LOW CLOUD: COOLING T cloud 4 < T o 4 T o 4 HIGH CLOUD: WARMING
26. TERRESTRIAL RADIATION SPECTRUM FROM SPACE: composite of blackbody radiation spectra emitted from different altitudes at different temperatures
27. ORIGIN OF THE ATMOSPHERIC AEROSOL Soil dust Sea salt Aerosol: dispersed condensed matter suspended in a gas Size range: 0.001 m (molecular cluster) to 100 m (small raindrop) Environmental importance: health (respiration), visibility, radiative balance, cloud formation, heterogeneous reactions, delivery of nutrients…
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29. EVIDENCE OF AEROSOL EFFECTS ON CLIMATE: Observations NASA/GISS general circulation model Temperature decrease following large volcanic eruptions Mt. Pinatubo eruption 1991 1992 1993 1994 -0.6 -0.4 -0.2 0 +0.2 Temperature Change ( o C)
30. 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
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32. EVIDENCE OF INDIRECT EFFECT: SHIP TRACKS 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
33. SATELLITE IMAGES OF SHIP TRACKS AVHRR, 27. Sept. 1987, 22:45 GMT US-west coast NASA, 2002 Atlantic, France, Spain
Need to supplement with one of those diagrams…indicating that total radiation flux at the ground includes a lot of longwave.
Question: How many watts/m 2 radiated to space on a clear night in the Niger with the surface still at 320K? (200 cm -1 x 150x10 -3 W/m 2 ) x 2 = 6 x (30+20) = 300 Wm -2 . [ + 200x100x10 -3 ?] Question: What would be the emission rate if T=280 instead of 320 (a cool clear winter night)? Estimate that it goes at T 4 : (7/8) 4 = .58 => 100 Wm -2 . How much does that cool the air? Assume snow cover (perfect insulator), effect through 100 m depth of air. Heat capacity of air = 1005 J/kg/K. In 1 hour each sq. meter loses 36 KJ. 7000m of air = 10,000 kg/m 2 , so 200m is 300 kg. DelT= 360KJ/300/1.005 = 1.2 K/hr. In a night, 12 hr, 14K; 16 hr, 19K T decline!
At the root of any climate change must be a perturbation of the rad eq of the Earth, a perturbation that we call radiative forcing. The concept of radiative forcing is central to research and policy on climate change, and it is not a difficult concept to understand. The Earth is a thermal engine. A stable climate reflects a close balance between the absorption of solar radiation, indicated here by Fin, and the blackbody emission of IR terrestrial radiation, indicated here by Fout. Aerosols and clouds reflect solar radiation, reducing Fin; greenhouse gases with IR absorption features absorb the terrestrial radiation and reemit it at lower temperatures, decreasing Fout. Perturbations to the levels of aerosols or greenhouse gases thus produces a radiative imbalance which we call radiative forcing. Greenhouse gases, absorbing aerosols result in a positive radiation forcing and the Earth warms; scattering aerosols result in negative radiative forcing and the Earth cools. Eventually, on a time scale of decades limited by the thermal inertia of the ocean, the Earth adjusts to a new radiative equilibrium. For example, the warming resulting from a positive radiative forcing increases the IR terrestrial emission and hence Fout. Many complications and feedbacks are involved in this climate adjustment, involving in particular the effect on the hydrological cycle. Calculations of climate response to a radiative forcing are done by GCMs, which are first-principles physical models for the Earth’s climate. The climate sensitivity factor lambda, defined as the global chance in surface air temperature in response to a unit radiative forcing, varies by a factor of 4 between GCMs, reflecting the uncertainty in climate change calculations. However, for a given GCM, it is found that lambda is relatively insensitive to the type or magnitude of the forcing. Because the radiative forcing can be calculated with much better reliability than the ultimate climate response, it is a widespread metric for use in science and policy.
“ official chart”
Question: How many watts/m 2 radiated to space on a clear night in the Niger with the surface still at 320K? (200 cm -1 x 150x10 -3 W/m 2 ) x 2 = 6 x (30+20) = 300 Wm -2 . [ + 200x100x10 -3 ?] Question: What would be the emission rate if T=280 instead of 320 (a cool clear winter night)? Estimate that it goes at T 4 : (7/8) 4 = .58 => 100 Wm -2 . How much does that cool the air? Assume snow cover (perfect insulator), effect through 100 m depth of air. Heat capacity of air = 1005 J/kg/K. In 1 hour each sq. meter loses 36 KJ. 7000m of air = 10,000 kg/m 2 , so 200m is 300 kg. DelT= 360KJ/300/1.005 = 1.2 K/hr. In a night, 12 hr, 14K; 16 hr, 19K T decline!
Accumulation mode—happens to be in visible range; also repirable!. Growth rate is proportional to 1/r.
Highest when particle r = wavelength (pi*d)—surface wave, diffraction. Rayleigh=inefficient
Absorbing (right panel) ; Jfk jr; particle size and composition.