Photochemical-dynamical modeling of the measured response of airglow to gravity waves: 1. Basic model for OH airglow

A photochemical-dynamical model for the OH Meinel airglow is developed and used to study the fluctuations in OH emission due to atmospheric gravity waves propagating through the mesosphere. The linear response of the OH Meinel emission to gravity wave perturbations is calculated assuming realistic photochemistry and gravity wave dynamics satisfying Hines (1960) isothermal windless model. The current model differs from prior models in that it considers fluctuations in vibrationally excited hydroxyl populations [OH(ν)] instead of fluctuations in the production rate of OH(ν). Two types of correction terms to the latter class of models are found, one involving advection of excited-state populations by the gravity wave and one involving quenching of OH(ν) by collisions with perturber molecules. Effects of these additional terms are expressed in terms of the so-called Krassovsky ratio η, which relates relative fluctuations in the column intensity measured by a passive optical instrument to relative fluctuations in the ambient temperature. The extra wave advection term is found to be unimportant under typical conditions, but quenching is important and has two major effects: (1) It makes η a vibrational-level-dependent quantity, and (2) it can lower η by more than 50% depending on the wave period. A typical range for η over a reasonably chosen range of wave parameters was found to be from less than 1 up to 9. The measuring instrument was also explicitly considered in the model formulation. Instead of simply assuming that the instrument measured the brightness-weighted temperature, as is commonly done in gravity wave response models, two common instruments for determining temperature from passive column-integrated measurements were explicitly modeled. The instruments modeled consisted of (1) a moderate-resolution instrument, such as a Michelson interferometer, which infers the temperature from the ratio of two rotational lines in a vibrational band (the rotational temperature) and (2) a high-resolution instrument, such as a Fabry-Perot interferometer, which uses the Doppler width of a single line to infer the temperature (the Doppler temperature). For gravity waves with large phase velocity (large-scale waves), calculations by both of these methods are found to be generally in agreement with each other and with the brightness-weighted temperature. However, for gravity waves with small phase velocity (small-scale waves) the two realistic simulations can differ from simulations using the brightness-weighted temperature by as much as 35%. The effect of vertical standing waves is considered by modifying the Hines model to include a rigid ground boundary. It is found that the standing waves have a profound effect on the phase of the gravity wave response. Values of η generated from the model are compared with published ground-based OH Meinel measurements of a quasi-sinusoidal short-period gravity wave by Taylor et al. (1991) from Sacramento Peak, New Mexico, at 15° elevation, as well as with the Svalbard polar-night data of Viereck and Deehr (1989). The agreement was found to be reasonable in both amplitude and phase for standing waves.