The inversion approach aims at retrievals of trace gas concentration and temperature profiles of atmospheric state, namely the modeled state vector, based on the measured radiance transmitted or reflected or scattered (SCIAMACHY spectrometer) by the Earth-Atmosphere system. Satellite instruments measure the radiance L that reaches the top of the atmosphere at given frequency v. The measured radiance is related to geophysical variables of Earth's atmosphere (e.g. temperature vertical profiles and chemical composition, aerosols, clouds, rain, etc.) and surface (e.g. temperature, spectral emissivity and reflectance, etc.) by the Radiative Transfer Equation (RTE). In RTE measured spectral radiances are assumed as the result of different contributions: a) thermal emission from the different layers (at heigt z) of atmosphere at temperature T(z) modulated by the atmospheric transmittance from z to the sensor heigt. It depends on both temperature profile T(z) and trace gas concentration along the optical path; b) Surface emission. It depends mostly on Eart's surface temperature T(0) and spectral emissivity c) Surface reflection/scattering. It depends on spectral reflectance and local properties like surface rugosity Others, more complex contributions comes from: cloud/rain, aerosols, etc. In its simplified form, terms a) and b) dominate as far as InfraRed (IR) radiances are considered. Term a) can be neglected in those bands where atmosphere is transparent (atmospheric windows). Term b) can be negletcted in the IR spectral bands (sounding channels) where it is fully adsorbed by some specific constituent of the atmosphere. Among the IR sounding channels some ones are selected being associated to atmospheric constituents (like CO2 or oxygen) whose mixing ratio in the atmosphere is known to be constant. For radiances measured in these bands term a) in RTE depends only on T(z) (through a Fredholm equation of the first kind) that can be then retrieved by inversion methods. When T(z) are known trace gas concentrations survive as the only unknown of term a) and can be retrieved by inversion methods using radiances measured in their corresponding sounding channels. Similar inversion strategies have been suggested as far as radiances (emitted, transmitted, reflected, adsorbed) measured in different spectral ranges (from the Visible to the Microwaves) are considered.
Explain how it is possible to retrieve atmospheric temperature and trace gases profiles form multi/iper spectral radiances
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