Retrieval of atmospheric parameters

In spectral regions where the atmosphere is transparent and for objects without an atmosphere, the temperature, emissivity, and scattering properties of the solid surface determine the measured intensity, while, in opaque portions of the spectrum, atmospheric properties are dominant. In this section we treat the atmospheric retrieval problem and defer discussion of surface properties to Section 8.5. 

To illustrate the principles of atmospheric parameter retrieval, let us consider an atmosphere in local thermodynamic equilibrium. The emerging radiance at a 

The extent to which various atmospheric parameters can be unambiguously retrieved from a set of measurements depends on the nature of the measurements, including spectral range and resolution, observational geometry, and signal-to-noise ratio. The fundamental problem associated with the quantitative interpretation of infrared planetary spectra is the formulation of effective methods for the extraction of the maximum information possible from a given set of measurements. A wide variety of approaches to this problem have been developed here we will consider only a selected few. 

The atmospheric parameters to be retrieved are physically related to the measurements through the radiative transfer process described in Chapter 2. The nature of this process imposes limitations on the information that can be extracted, even from error-free measurements, while measurement errors (noise) further reduce the information content. Both factors must be taken into consideration when choosing an appropriate method for a given retrieval problem. 

For convenience the computational methods can be divided into two general classes, direct modeling and inversion. The two approaches are illustrated schematically in Fig. 8.1.1 along with the general radiative transfer process. The latter, depicted in part (A), can be regarded as a low-pass filter, permitting only information on the larger spatial scales in the vertical atmospheric structure to be preserved 

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