Contribution of Gravity Waves to Universal Vertical Wavenumber (∼m−3) Spectra Revealed by a Gravity‐Wave‐Permitting General Circulation Model. Issue 10 (20th May 2022)

• Record Type: Journal Article
• Authors: Okui, Haruka; Sato, Kaoru; Watanabe, Shingo
• Is Part Of: Journal of geophysical research. Volume 127:Issue 10(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-05-20
• Subjects: gravity waves -- middle atmosphere -- spectral analysis; Atmospheric physics -- Periodicals; Geophysics -- Periodicals; 551.5
• DOI: 10.1029/2021JD036222
• Languages: English
• ISSNs: 2169-897X

Abstract

Abstract: Observations with high vertical resolution have revealed that power spectra of horizontal wind and temperature fluctuations versus vertical wavenumber m $m$ have a universal shape with a steep slope in a high m $m$ range, approximately proportional to m − 3 ${m}^{-3}$ . Several theoretical models explaining this spectral slope were proposed under an assumption of gravity wave (GW) saturation. However, little evidence has been obtained to show that these universal spectra are fully composed of GWs. To confirm the validity of this assumption, two kinds of m $m$ spectra are calculated using outputs from a GW‐permitting high‐top general circulation model. One is the spectra for GWs designated by fluctuations having total horizontal wavenumbers of 21–639. The other is the spectra of fluctuations unfiltered except extracting the linear trend in each vertical profile that were often analyzed in the observational studies. A comparison between the two shows that GWs dominate the observed spectra only in a higher m $m$ part of the steep slope, whereas disturbances other than GWs significantly contribute to its lower m $m$ part. Moreover, geographical distributions of the characteristic wavenumbers, slopes, and spectral densities of GW spectra are examined for several divided height regions of the whole middle atmosphere. It is shown that strong vertical shear below the zonal wind jets as well as wave saturation are responsible for the formation of the steep slopes of GW spectra. Plain Language Summary: Radar, lidar, and radiosonde observations have revealed that vertical wavenumber power spectra of horizontal wind and temperature fluctuations have a common shape with a steep slope. Several theoretical studies have explained this universality by assuming that these spectra are composed of saturated gravity waves. To confirm the validity of this assumption, a spectral analysis of gravity waves, having short horizontal wavelengths, was conducted using a gravity‐wave‐permitting high‐top general circulation model. A comparison of the spectra of gravity waves with those of all model‐simulated disturbances showed that disturbances other than gravity waves significantly contribute to the spectra in a low vertical‐wavenumber part of the steep slope. Moreover, vertical and geographical variations in the characteristics of gravity wave spectra were described. It is inferred that strong vertical shear below the eastward and westward jets as well as the wave saturation are responsible for the formation of the steep slopes of gravity wave spectra. Deepening our understanding of gravity waves in the middle atmosphere, these findings may provide useful guidelines for improving parameterizations of gravity waves in climate models. Key Points: Using a gravity‐wave‐permitting general circulation model, vertical wavenumber ( m $m$ ) spectra in the whole middle atmosphere were examined Characteristics of the model‐simulated m $m$ spectra of gravity waves are broadly consistent with observations Disturbances other than gravity waves contribute substantially to the lowest m $m$ part of the ∼ m − 3 ${m}^{-3}$ range