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METO 621 Lesson 16

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Transmittance For monochromatic radiation the transmittance, T, is given simply by The solution for the radiative transfer equation was:

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Transmittance Which is equivalent to similarly

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Transmittance If we switch from the optical depth coordinate to altitude, z, then we get The change of sign before the integral is because the integral range is reversed when moving from to z

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Transmission in spectrally complex media So far we have defined the monochromatic transmittance, T We can also define a beam absorptance b ( ), where b ( ) =1- T But in radiative transfer we must consider transmission over a broad spectral range, (Often called a band) In this case the mean band absorptance is defined as Here u is the total mass along the path, and k is the monochromatic mass absorption coefficient.

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Schematic for A

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Transmission in spectrally complex media Similarly for the transmittance

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Transmission in spectrally complex media If in the previous equation we replace the frequency with wavenumber then the product The relationship of W to u is known as the curve of growth

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Absorption in a Lorentz line shape

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Transmission in spectrally complex media Consider a single line with a Lorentz line shape. As u gets larger the absorptance gets larger. However as u increases the center of the line absorbs all of the radiation, while the wings absorb only some of the radiation. At this point only the wings absorb. We can write the band transmittance as;

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Transmission in spectrally complex media Let be large enough to include all of the line, then we can extend the limits of the integration to ±∞. Landenburg and Reichle showed that

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Transmission in spectrally complex media For small x, known as the optically thin condition,

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Elsasser band model

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Elsasser Band Model Elsasser and Culbertson (1960) assumed evenly spaced lines (d apart) of equal S that overlapped. They showed that the transmittance could be expressed as

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Statistical Band Model (Goody) Goody studied the water-vapor bands and noted the apparent random line positions and band strengths. Let us assume that the interval contains n lines of mean separation d, i.e. =nd Let the probability that line i has a line strength S i be P(S i ) where P is normally assumed to have a Poisson distribution

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Statistical Band Model (Goody) For any line the most probable value of S is given by The transmission T over an interval is given by

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Statistical Band Model (Goody) For n lines the total transmission T is given by T=T 1 T 2 T 3 T 4 …………..T n

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Statistical Band Model (Goody)

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We have reduced the parameters needed to calculate T to two These two parameters are either derived by fitting the values of T obtained from a line-by-line calculation, or from experimental data.

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Summary of band models

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K distribution technique

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