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Temperature Inversion

Determining the temperature profile of an atmosphere is a difficult subject. As a first approximation brightness temperatures derived from different wavelengths can be assigned to the depths from which they arise. A rigorous approach however requires sounding measurements from as many atmospheric levels as possible. The most basic technique involves find the pressure level where a spectral band achieves unit optical depth and then assigning the band brightness temperature to that level. As figure 3 shows however, the contribution at a particular wavelength can be spread across many pressure levels.

There are three distinct methods of temperature inversion, comparing the measured radiance to the atmospheric model using radiative transfer as a filter, fitting atmospheric models to the measurements, and using an inversion algorithm to retrieve the atmospheric profile directly using the observations (Hanel et al. 1992). The latter choice is the one I will adopt. It consists of solving the radiative transfer equation simultaneously for each wavelength band giving a vertical profile of B(T) which can then be solved for the temperature. The most popular technique for this is a non-linear weighted relaxation method developed by Chahine (1972) that iterates on computed intensities until they match with the observations. Inherently though, temperature inversion is very dependent upon the model used as the best measurements only break the atmosphere down into a P vs. T/ profile. It will be very important to obtain the best profile of atmospheric constituents possible.



Charles Walter
Thu May 18 17:57:23 MDT 1995