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[AAS10-06] Contribution of gravity waves to the universal vertical wavenumber (~m-3) spectra revealed by a gravity-wave-permitting general circulation model
Keywords:Gravity wave, Middle atmosphere, Atmospheric dynamics
Gravity waves (GWs), having small spatial and temporal scales, play crucial roles in the dynamics of the middle atmosphere. Observations with high vertical resolution have revealed that power spectra of horizontal wind and temperature fluctuations versus the vertical wavenumber m have a universal shape with a steep slope in a high m range, which is proportional to ~m-3 at most latitudes. Several theoretical models explaining this spectral slope were proposed under an assumption of the GW saturation. However, little evidence has been obtained to show that these universal spectra are fully composed of GWs. Thus, to confirm the validity of the assumption, spectral analysis of GWs in the middle atmosphere was conducted using outputs from a GW-permitting high-top general circulation model. GWs were extracted as fluctuations having total horizontal wavenumbers of 21–639. Spectra of fluctuations, i.e., deviation from the linear trend (hereafter “all fluctuations”) in the vertical, which were often examined by previous observational studies, were compared with the GW spectra.
The model-simulated spectra of all fluctuations have steep spectral slopes similar to observations. Comparison of the spectra of GWs with those of all fluctuations shows that the two spectra well accord only in a higher m range of the steep-slope part, whereas significant departure is observed in a lower m range of that. This fact indicates significant contribution of the disturbances other than GWs to the lower m range even in the steep-slope part. The departure in the lower m range is especially large in the low-latitude region. Equatorially-trapped waves are plausible candidates contributing to the spectra in the lower m range.
Moreover, vertical and geographical distributions of the characteristic wavenumbers, slopes, and spectral densities of GW spectra were examined. Characteristic wavenumbers mg*, the lower end of the spectral range with the steep slope, are smaller, and spectral densities (F0/2) at mg* are larger at higher altitudes. This feature is consistent with the theoretical inference that GWs are amplified while propagating upward due to exponential decrease in atmospheric density with height, which leads to the wave saturation even at low ms. However, there is not the only mechanism that can explain the vertical change in the spectral shape. Steep spectral slopes are observed near the cores of the eastward and westward jets in the middle atmosphere. It is shown that the strong vertical shear below the jets is also responsible for the formation of the steep slopes of the GW spectra.
Reference:
Okui, H., Sato, K., and Watanabe, S., Contribution of gravity waves to universal vertical wavenumber (~m-3) spectra revealed by a gravity-wave-permitting general circulation model, submitted to Journal of Geophysical Research: Atmospheres.
The model-simulated spectra of all fluctuations have steep spectral slopes similar to observations. Comparison of the spectra of GWs with those of all fluctuations shows that the two spectra well accord only in a higher m range of the steep-slope part, whereas significant departure is observed in a lower m range of that. This fact indicates significant contribution of the disturbances other than GWs to the lower m range even in the steep-slope part. The departure in the lower m range is especially large in the low-latitude region. Equatorially-trapped waves are plausible candidates contributing to the spectra in the lower m range.
Moreover, vertical and geographical distributions of the characteristic wavenumbers, slopes, and spectral densities of GW spectra were examined. Characteristic wavenumbers mg*, the lower end of the spectral range with the steep slope, are smaller, and spectral densities (F0/2) at mg* are larger at higher altitudes. This feature is consistent with the theoretical inference that GWs are amplified while propagating upward due to exponential decrease in atmospheric density with height, which leads to the wave saturation even at low ms. However, there is not the only mechanism that can explain the vertical change in the spectral shape. Steep spectral slopes are observed near the cores of the eastward and westward jets in the middle atmosphere. It is shown that the strong vertical shear below the jets is also responsible for the formation of the steep slopes of the GW spectra.
Reference:
Okui, H., Sato, K., and Watanabe, S., Contribution of gravity waves to universal vertical wavenumber (~m-3) spectra revealed by a gravity-wave-permitting general circulation model, submitted to Journal of Geophysical Research: Atmospheres.