This chapter is devoted to the physical characterization of the various processes that determine the apparent emittance Eap of targets and backgrounds. For this purpose, the problem is split into two parts:
(1) the conduction of heat within the target, and
(2) the heat exchange in the surface atmospheric boundary layer.
It is emphasized that parameters will not be discussed at full length. Discussion will be limited to the functional relations between parameters and the definition of practical usable expressions. Therefore, the origin of these parameters are only given limited attention. To be able to evaluate the behavior of ÎEap in a theoretical model for a given situation, it is necessary to determine:
(1) the physical processes involved in creating a radiation contrast;
(2) the parameterization of these processes;
(3) the relative importance of these parameters (sensitivity analysis); and
(4) the availability and required accuracy of these parameters for model input.
The latter has no particular meaning in a theoretical survey as such, but in the end it determines the practical usefulness (and validation possibilities) of the model.
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