Other
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmsa43b1756m&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #SA43B-1756
Other
[3384] Atmospheric Processes / Acoustic-Gravity Waves
Scientific paper
Acoustic-gravity waves generated below the mesopause can be trapped between the ground and the mesosphere, and can form resonance. This mechanism is expected from a lot of theoretical and numerical studies. The resonance has two (or three) modes with frequencies of about 3.8 and 4.5 mHz, which are consistent with many observational results, such as ionospheric disturbances, geomagnetic pulsations, atmospheric pressure perturbations and solid Earth’s free oscillations. There are, however, still a lot of unknown problems concerned with the resonance such as the formation process of the modes, effects of evanescent regions and horizontal characteristics, and it is difficult to clarify them only through observations. In this study, to investigate the problems, a three-dimensional nonhydrostatic compressible neutral atmosphere model is used. Here, input as the source of an atmospheric disturbance is a sinusoidal wave at a certain height. The atmospheric densities and temperatures are taken from the empirical model, MSISE-90. Calculated vertical velocities directly above the center of the source at different altitudes show that propagating waves reflect around the mesopause and the ground (or tropopause), and two types of vertical standing waves are formed between them. One has the half wavelength of about 40 km, the frequency of about 4.5 mHz and the short duration. The other has the half wavelength of about 110 km, the frequency of about 3.8 mHz and the long duration. These two types of standing waves correspond to the resonance modes expected from previous studies. Above the mesopause only propagating waves are seen and the frequencies of them are about 4.5 mHz in the earlier time of the calculation, and about 3.8 mHz in the later time. For comparison simulations are also performed for isothermal atmospheres and waves with frequencies near the Brunt-Vaisala frequency, independent of input ones are excited. This means that acoustic-gravity waves are excited at some altitudes as the input wave propagates and trapped waves are not ones propagating directly from sources. As for regions away from the center of the source in the horizontal direction, similar analyses are performed and it is shown that two resonance modes are formed also when the acoustic-gravity waves are propagating in inclined directions although the appearance time is later and the amplitude is smaller.
Iyemori Toshihiko
Matsumura Masahiro
Shinagawa Hiroyuki
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