Gravity Waves Emitted From Kelvin‐Helmholtz Instabilities

• Published in: Geophysical research letters Vol. 50; no. 8
• Main Authors: Dong, Wenjun, Fritts, David C., Liu, Alan Z., Lund, Thomas S., Liu, Han‐Li
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 28.04.2023; Wiley
• Subjects: Altitude; Amplitude; Amplitudes; Atmosphere; Atmospheric models; Balances (scales); Billows; Compressibility; Density; Direct numerical simulation; Dynamic stability; Dynamics; Emissions; gravity wave; Gravity waves; High frequencies; Inertia; Initial conditions; Instability; Kelvin-helmholtz instability; Mathematical models; Numerical simulations; Resolution; Shear layers; Simulation; Wave propagation; Wavelengths; Wind shear
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2022GL102674

Fritts, Wang, Lund, and Thorpe (2022, https://doi.org/10.1017/jfm.2021.1085) and Fritts, Wang, Thorpe, and Lund (2022, https://doi.org/10.1017/jfm.2021.1086) described a 3‐dimensional direct numerical simulation of interacting Kelvin‐Helmholtz instability (KHI) billows and resulting tube and knot (T&K) dynamics that arise at a stratified shear layer defined by an idealized, large‐amplitude inertia‐gravity wave. Using similar initial conditions, we performed a high‐resolution compressible simulation to explore the emission of GWs by these dynamics. The simulation confirms that such shear can induce strong KHI with large horizontal scales and billow depths that readily emit GWs having high frequencies, small horizontal wavelengths, and large vertical group velocities. The density‐weighted amplitudes of GWs reveal “fishbone” structures in vertical cross sections above and below the KHI source. Our results reveal that KHI, and their associated T&K dynamics, may be an important additional source of high‐frequency, small‐scale GWs at higher altitudes. Plain Language Summary A high‐resolution compressible atmosphere model is applied to explore gravity wave emissions from a shear with Kelvin‐Helmholtz Instability initiated by a three‐dimensional, small‐amplitude initial noise field in velocity, such as must always occur in the atmosphere. Simulations reveal that a wind shear with an amplitude of 65 m/s and a half‐width of 0.8 km can induce strong Kelvin‐Helmholtz Instability dynamics, which can further emit gravity waves having periods of ∼10–20 min and horizontal wavelengths of ∼20 km. These gravity waves have high frequencies and small horizontal scales. The density‐weighted amplitudes of gravity waves created a “fishbone” structure in z‐t plots due to upward‐ and downward‐propagating gravity waves arising at the layer of Kelvin‐Helmholtz Instability. Our results demonstrate that Kelvin‐Helmholtz Instability and the resulting instability dynamics may be a prevalent source of gravity waves impacting higher altitudes. Key Points Kelvin‐Helmholtz instabilities (KHI) generated by a stratified shear layer induce gravity waves (GWs) that penetrate to high altitudes KHI‐radiated GWs may be a major influence of near‐stationary shears at high altitudes to which they cannot readily propagate directly GWs generated by KHI can account for “fishbone” structures seen in vertical profiling