Monday, Oct. 2: Clear-sky radiation; solar attenuation, Thermal nomenclature
Sun Earth Y-axis: Spectral radiance, aka monochromatic intensity units: watts/(m^2*ster*wavelength) Blackbody curves provide the envelope to Sun, earth emission
Sun Earth visible
Depth of penetraion into earth s atmosphere of solar UV 1 Angstrom= 10-10 m. Photoionization @ wavelengths < 0.1 micron (1000 angstroms) Photodissociation @ wavelengths < 0.24 microns: O 2 -> 2O Ozone dissociation @wavelengths < 0.31 micron Visible spectrum 0.39 to 0.76 micron
Thermal Radiation: scattering negligible absorption,emission is what matters Math gets complicated: thousands of absorption lines, each varying individually with pressure, temperature Natural Doppler broadening: Half-width goes as T 1/2 Lorentz (Pressure) broadening: Half-width goes as P/T (0.5-1.0) absorption natural < 20 km, pressure broadening > 50 km Doppler broadening (Freq shift)/half-width
Continuing efforts to improve database on line absorption strengths and Halfwidths: H20 continuum, Microwave lines, are examples 16 micron 7 micron
Thermal Radiation transmits through an atmospheric layer According to: +J ds I = intensity = air density r = absorbing gas amount k =mass extinction coeff. rk = volume extinction coeff. Inverse length unit emission Path length ds Extinction=scattering+absorption ~ 0
T = e- sec Langley plot Ln (I inf /I ) = sec Beer s Law used to assess solar constant in pre-satellite days, now used to calibrate instrumentation & determine aerosol&cloud optical depth from ground
Transmission through a layer, ignoring scattering and emission: After integration: di = -I k abs sec dz T = e- sec Beer s Law or Lambert s Law T = transmissivity; = optical depth, or thickness Consequence: most radiation is absorbed/emitted at an optical depth of 1.
brightening Limb Effects darkening affects ALL terrestrial remote sensing
Limb Sounding as a Remote Sensing Technique: first get the temperature from Planck function radiance then use radiance in an absorbing/emitting wavelength to get atmospheric concentration at that height HIRDLS 10 1 Wavelength [µm] 25 18 14 12 10 9 8 7 6 5 4.55 20-km Tangent Height 10 2 CO 2 Gas Only 217-K Planck Function 10 3 O 3 10 4 N 2 O N 2 O CFC 11 N 2 O 5 H 2 O NO 2 HNO 3 ClONO 2 10 5 CFC 12 CH 4 Aerosol Aerosol Aerosol HIRDLS Channel Locations 10 6 400 600 800100012001400160018002000220 Wave Number [cm 1 ]
To calculate the broadband infrared emission, One simplification is to group lines together, Use spectral-band-average values for absorption - band models. A more elegant solution is to group lines by their absorption lines strengths, and integrate over that. Only works in infrared
Full radiative transfer equation for infrared/microwave (I.e. ignores scattering): attenuation emission Plane-parallel approximation: the earth is flat. -> the temperature, atmospheric density is a function of height (or pressure) alone. Curvature of earth ignored, atmosphere assumed to be horizontally homogeneous. Flux density with flux transmissivity
Radiative heating rate profiles: -or- Cooling to space approximation: Ignore all intervening layers Manabe & Strickler, 1965 Rodgers & Walshaw, 1966, QJRMS
Remote temperature sensing CO 2 particularly suited (well-mixed & emissive) (what part of the Earth is this from?)
Weighting function
If scattering is also included: 3 radiatively-important scatterer parameters: optical depth (how much stuff Is there?) single-scattering albedo : k sca /(k scat + k abs ) (how much got Scattered rather than absorbed?) asymmetry parameter g, or phase function P(cos ): (describe how it scatters)
Wednesday: results from top of atmosphere radiation Balance questions up to 4.40 some other aerosol, greenhouse gas, results
Whether/how solar radiation scatters when it impacts gases,aerosols,clouds,the ocean surface depends on 1. ratio of scatterer size to wavelength: Size parameter x = 2*pi*scatterer radius/wavelength X large Sunlight on a flat ocean Sunlight on raindrops Microwave (cm) X small Scattering neglected IR scattering off of air, aerosol Microwave scattering off of clouds