Ch. 3.4 Scattering and absorption by particlesCh 3.5 Remote sensing by satellite Text: Sections 4.4 and 4.5, p 122-134 Text: Section 4.5.5, 139-144 Liou: Atmospheric Radiation, ch.3.3, p87-94 Reading assignment:
Radiative energy scattering and absorption by the particles in the atmosphere: • As a beam of light passing through the atmosphere, it encounters gas molecules and particles, and its monochromatic intensity decreases at the increment of For gases such as the atmosphere, we have: dIl = -Ilrrklds For particles such as aerosols and cloud liquid and ice particles dIl= - IlKlNsds Where Kl is the extinction efficiency due to either absorption and/or scattering, N: number of particles per unit volume of air, s: the area cross-section of each particle, ds: the differential path length along the incident ray. r: air density, r: mass of the absorbing gas per unit mass of air, kl: extinction coefficient, due to scattering and absorption by gases and particles. The total extinction of the atmosphere is determined by sum of all gases and particles. klNs = kl1N1s1+kl2N2s2+… What determined dIl?
Ch. 3.4 Scattering and absorption by particles Why is clean and clear sky blue? Why is polluted sky white or grey?
Scattering in the atmosphere: You can see clouds clearly You see milky or foggy sky Clear sky
What control scattering and absorption of the light? Rayleigh (1871): Rayleigh scatter explains why clean and clear sky is blue. • For a homogeneous, isotopic, spherical particle whose radius, r, is much smaller than the wavelength of the incident radiation, l (r<<l), the incident radiation (Eo, or applied field) produces a electric field with an oscillating dipole moment (Po) inside and near the particle. Po=aEowhere a is the polarizability of a small particle • The combined field, P, sum of the oscillating Po and modify the applied field by Po, produces a plane-polarized electromagnetic wave, referred to as the scatter wave.
What determines the energy of scatter wave (E)? Er Er: perpendicular to the plane of incident and scattered light El: parallel to the above mentioned plane Q: angle between incident and scattered light El r g Q
Radiativeenegy intensity of the scattered light • Increase with intensity of the incident light, Io • Decrease with 1/r2 • Decrease with angle between incident and scattering light, cos2Q • Decrease with 1/l4. Radiative energy intensity of the scattered light and Rayleigh scattering
OK, why sky is blue? • Blue light is scattered 5.5 time more in clear and clean sky • Why is the sky not violet?
Phase function, scatter cross-section and polarizability Eor Vertical polarized scattered intensity Eor horizontall polarized scattered intensity Eol unpolarized scattered intensity Eo
Scattering by air molecules and particles: • Type of scattering is determined the sizes of the gas molecules or particles relative to the wavelength of the radiation because Ir, Il depends on k or l. • Non-dimensional size parameter: x=2pr/l • x<<1, or r<<l: Rayleigh scattering, Kl~l-4 • X~1 or r ~ l: Mie scattering • X>>1 or r>>l: geometric scattering
Scattering cross section, ss • ss: fraction of the incident energy that is removed by the scattering over the cross section centered at the scatter with radius of r • Polarizability, a: decrease with number of molecules and refractive index of the molecules. • M=mr+imimr: scattering, mi: absorption, for air molecules, mr>>mi
linght remove from the incident beam about 4.5 of it can intercept, strong scattering. For clouds droplets and some aerosols types, absorption can be important Absorption by particles, mi Because rparticles ~ lvs, IR absorption and scattering are governed by Mie theory. • Clouds droplets can be treated as sphere water droplets with mi=1 (complete absorption) and r ~ 10-100 mm. • For both visible (X ~ 1) and infrared (X>>1) lights, extinction of radiation as it pass through cloud layer is all due to absorption. • Even thin layer of clouds can absorb virtually all incident radiation from above and below, especially for longwave radiation (X>>1).
Isotropic scatter forward scatter g(l)=0 g(l)>0 Due to multiple scattering of a parallel beam, solar radiation scattering is much more isotropic than that in cloud-free sky. g(l) ~ 0.5
Angstrom exponent: describe the dependency of the aerosols optical depth or extinction coefficient on wavelength, inversely related to average size of the aerosols. What can we learn from these figures about global distribution of aerosols types and concentration?
Summary • What radiative processes determine the radiative energy extinction as it passes through cloud or aerosol particles? • scattering and absorption of the particles. • What determines scattering? • the size of the particle relative to wave length: Rayleigh, Mie and geometric • Refractive property of the particle, mr, mi • What are the main differences between radiative energy extinction as it passes through aerosols and liquid clouds and air? • Liquid louds: strong absorptive • Aerosols: depends, single scattering albedo, angstrom exponent.
3.5 Principle of atmospheric remote sensing(Text 4.5.5, p139-144) • Reading: Kollias et al. 2007
3.5 Principle of atmospheric remote sensing(Text 4.5.5, p139-144) cloud radar Precip radar
Remote sensing of atmospheric temperature and composition • Compare the spectrum radiance with that of Plancks’ function for a given temperature; • Measure the absorption at different wavelength for different gases.
Observations available for detecting GHG fingerprint in the atmospheric temperature: • Radiosonde: precision: < 1°C in troposphere, 2/day, ~1000 stations since 1958 • Microwave Sounding Units (MSU): precision: 0.25°C, 2/day per location, ~30000, since 1978 Direct measurements of T: Derived T: MSU ~57GHz ~55GHz radiosonde Low trop. T Global trop. T Tropical trop. T
Observed Temperature Changes: • Warming at the surface and in the troposhperic and cooling in the stratosphere are detected by both in- situ and satellite observations. stratosphere Trend, °C/dec Troposphere DTGHGs IPCC AR4
Cloud and precipitation radar: • Cloud radar: CloudSat: 94 GHz, 3.2 mm wavelength, l>> r of air molecules and aerosols, ~ large cloud droplets (10-100 mm or 0.01-0.1 mm). • Rainfall radar-TRMM: 14.3 GHz, 2.14 cm, , l>> r of air molecules, aerosols and cloud droplets, similar to the size of rain droplets (1-10 mm).
Summary • The two most important basic principles for atmospheric remotes sensing are based on • Schwarchilds’ law • Scattering/absorptivity of the atmosphere particles (aerosols and clouds)