Radiation transport effects and the interpretation of infrared images of gravity waves and turbulence

Radiation transport modulates the spatial frequencies of atmospheric structures, acting as a low pass filter, which causes the power spectra of the accumulated radiance to have different power spectral slopes than the underlying atmospheric structure. Additional effects arise because of the non-stationarity of the atmosphere. The SHARC atmospheric radiance code is used to model both equilibrium and non-equilibrium radiance and radiance fluctuation statistics. It predicts two dimensional radiance spatial covariance functions and power spectral densities, PSDs. Radiance power spectral slopes for paths through isotropic Kolmogorov turbulence are predicted to vary from –5/3 to –8/3 depending on the length of the path through the turbulence. The input gravity wave 3-D covariances and PSDs of atmospheric temperature are consistent with current gravity wave theory, having vertical and horizontal power spectral indices of -3 and –5/3, respectively. Altitude profiles of variances and correlation lengths account for the non-stationary of the gravity wave structure in the atmosphere. The radiance covariance and PSD power spectral slopes differ from the atmospheric gravity wave temperature model values of -3 and –5/3. These modulations depend on LOS orientations, and scale lengths of the sampled altitudes along the LOS.