Radiative Damping in the Upper Mesosphere

Xun Zhu Department of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, Maryland

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Darrell F. Strobel Department of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, Maryland

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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.

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.

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