Physics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmsa21b..06h&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #SA21B-06
Physics
[0355] Atmospheric Composition And Structure / Thermosphere: Composition And Chemistry, [3384] Atmospheric Processes / Acoustic-Gravity Waves
Scientific paper
Walterscheid and Hickey (2001) showed that the accuracy of gravity wave models for waves propagating in a diffusively separated atmosphere depends sensitively on how compositional effects are included. Gases in the thermosphere are diffusively separated, so the mean molecular weight (M) varies with altitude. Conventional models of gravity wave propagation in the thermosphere treat the atmosphere as a single gas. These models include the height variation of M while neglecting fluctuations of M due to gravity wave motions. This is equivalent to assuming an instantaneous balance between the perturbing effects of vertical motion and the restoring effects of mutual diffusion. Using a one-gas full-wave model Walterscheid and Hickey (2001) examined the two limiting cases where an instantaneous balance prevails (M’=0) and where M is conserved following a parcel (M’ reflects only vertical advection). It was found that the two limits gave significantly different results. We have recently developed a two-gas model (N2 and O) describing gravity wave motions in the thermosphere and have calculated the total gas fluctuation accounting fully for the variation of M due to both vertical advection and mutual diffusion. This model has allowed us to evaluate the accuracy of one-gas models with fixed M (M’=0) and with conserved M (dM/dt =0) for a wide range of wave parameters. We have explicitly calculated both the perturbing effects of vertical advection and the restoring effects of mutual diffusion and have compared the two competing effects. Our results show that the error in ignoring mutual diffusion can be significant and that in general it is best to assume that M is conserved following parcels rather than to assume that M’ is zero. The conservation of M is easily implemented in one-gas models. The implications for future studies of acoustic-gravity wave motions in the thermosphere are discussed. Walterscheid, R., and M. Hickey (2001), One-gas models with height-dependent mean molecular weight: Effects on gravity wave propagation, J. Geophys. Res., 106(A12), 28831-28839.
Hickey Michael P.
Schubert Gerald
Walterscheid Richard L.
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