Radiative Damping in the Upper Mesosphere

Abstract

Radiative damping rates of atmospheric temperature perturbations can be calculated by either an eigenvalue method or a scale-dependent Newtonian cooling method, which we show are equivalent in two limits. One limit is an infinite, homogeneous atmosphere based on Spiegel's model. The other, corresponding to an empirical scale-independent Newtonian cooling coefficient, is the transparent limit to radiation. In the upper mesosphere the damping rate is calculated by both methods using a non-LTE Curtis matrix. If the atmospheric application requires only thermal damping in a narrow altitude region for waves of small vertical wavelength or damping in a thick layer for large vertical wavelength waves, then one of these limits is a valid approximation. Under these circumstances the easily calculated, scale-dependent, Newtonian cooling rate gives a good approximation to the radiative damping rate. Scale-dependent radiative damping rates calculated with non-LTE Curtis matrices and an exact line-by-line integration scheme are presented over the region 60–93 km and supersede the widely used damping rates of Fels in 1984.