A full‐wave investigation of the use of a “cancellation factor” in gravity wave–OH airglow interaction studies

Abstract

[1] Atmospheric gravity waves (GWs) perturb minor species involved in the chemical reactions of airglow emissions in the upper mesosphere and lower thermosphere. In order to determine gravity wave fluxes and the forcing effects of gravity waves on the mean state (which are proportional to the square of the wave amplitude), it is essential that the amplitude of airglow brightness fluctuation be related to the amplitude of major gas density fluctuation in a deterministic way. This has been achieved through detailed modeling combining gravity wave dynamics described using a full-wave model with the chemistry relevant to the airglow emission of interest. Alternatively, others have employed approximations allowing them to derive analytic expressions relating airglow brightness fluctuations to major gas density fluctuations through a so-called ''cancellation factor'' (CF). The effects of these approximations on the derived CF are investigated here using a full-wave model describing gravity wave propagation in a nonisothermal, windy, and viscous atmosphere. This numerical model combined with the chemical reaction scheme for the OH (8, 3) Meinel airglow emission is used to derive fluctuations in the OH* nightglow from which an equivalent CF is calculated. Comparisons are made between the analytically derived CF's and the numerically derived CF's based on using different approximations in the latter model. Differences exist at most wave periods, but they also depend on the horizontal wavelengths of the gravity waves considered. In addition to these different model comparisons, the sensitivity of the numerically derived CF to specific physical processes is examined exclusively using the full-wave model. These sensitivity tests show that the effect of eddy diffusion marginally influences the calculated CF's only for the very slowest gravity waves. Accounting for the effects of a nonisothermal mean state has a significant influence on the calculated CF's, and the CF's calculated assuming an isothermal mean state can be as much as a factor of 2 smaller than those calculated assuming a nonisothermal mean state. The effects of background mean winds also influence the derived CF's, which then become dependent on the azimuth of propagation. In this case the calculated CF's can vary by a factor of $2 from their windless values for gravity waves of short horizontal wavelength with phase speeds less than 100 m s À1 . Finally, reflection from the lower and middle thermosphere in the full-wave model leads to undulations in the calculated CF's as a function of phase speed for gravity waves with horizontal wavelengths of 100 km and phase speeds greater than about 100 m s À1 . These effects that are not reproduced in the analytic model lead to large differences between the CF's calculated with and without winds, but they only occur for fast gravity waves that are not usually observed in the airglow.Citation: Hickey, M. P., and Y. Yu (2005), A full-wave investigation of the use of a ''cancellation factor'' in gravity wave -OH airglow int...

Nonlinear interaction of gravity waves in a nonisothermal and dissipative atmosphere

“…Here, the profiles of the molecular kinematic and thermal diffusivities are almost the same as those of previous studies (Hickey and Yu, 2005;Ortland and Alexander, 2006;Xu et al, 2009b;Yu et al, 2009;Liu et al, 2010). Observational analyses show that the eddy diffusion is strong in a relatively narrow height range in the mesopause region (Balsley et al, 1983;Hocking, 1987;Lübken, 1997).…”

A numerical model characterizing internal gravity wave propagation into the upper atmosphere

“…h is the potential temperature, P is the exner function, p 00 ¼ 1000 mbar (the over-bar represents a horizontal average) is the reference pressure on the ground, j = R/c p , R = R * /M, and R * is the universal gas constant. Other relevant parameters will be discussed in Appendix A and can also be found in Walterscheid and Schubert (1990), Hickey et al (2000Hickey et al ( , 2003 and Hickey and Yu (2005).…”

“…Observational techniques, such as ground-based airglow imaging, can provide the model-needed information regarding the directions of wave propagation, the wave extrinsic periods and their horizontal wavelengths (Walterscheid et al, 1999;Hecht et al, 2001). Combined with models describing the interaction of waves with chemistry and the associated airglow emissions, such measurements provide a useful way of determining the gravity wave amplitudes, momentum fluxes, and energy fluxes (Nakamura et al, 1993;Swenson and Gardner, 1998;Gardner et al, 1999;Hickey, 2001;Hickey and Yu, 2005). Radar observations allow the momentum flux to be calculated using the method described by Vincent and Reid (1983) or by correlating the horizontal and vertical velocity (Fritts and Yuan, 1989).…”

Response of OH airglow emissions to mesospheric gravity waves and comparisons with full-wave model simulation at a low-latitude Indian station

“…We also employ a full-wave model to simulate the effects of wave motions on the OH airglow. This model has been used previously to compare observations and theory of airglow fluctuations (e.g., Hickey et al, 1998;Hickey and Yu, 2005). Here, the model is used to estimate the values of the amplitudes and phases of Krassovsky's ratio which are compared to those derived from the observations, making the present study unique as such model comparison over India has not been done before.…”

“…The model description, including equations, boundary conditions, and method of solution, has been described elsewhere (Hickey et al, 1997;Walterscheid and Hickey, 2001;Schubert et al, 2003). The neutral perturbations are used as input to a linear, steady-state model describing OH airglow fluctuations (Hickey and Yu, 2005). The model solves the equations on a high-resolution vertical grid subject to boundary conditions and generally allows for the propagation in a height-varying atmosphere (nonisothermal mean state temperature and height-varying mean winds and diffusion).…”