Gravity Waves Emitted From Kelvin‐Helmholtz Instabilities

Abstract

It has been recognized for many years that gravity waves (GWs) play fundamental roles in a wide range of atmospheric processes from the surface to very high altitudes (Fritts & Alexander, 2003). Understanding these processes and their influences requires more complete quantification of the mechanisms by which GWs are generated, together with their characteristics, distributions, and responses in a wide range of environments. Of the recognized GW sources, secondary GWs (GWs) are important because they extend the vertical range of GW influences into the thermosphere and can do so quickly because of their often-large scales and vertical group velocities. Importantly, however, their generation mechanisms are the least rigorously studied and understood to date. This is because their sources are challenging to quantify observationally, and their associated dynamics are intrinsically nonlinear.Our focus in this paper is on GWs excited by Kelvin-Helmholtz instabilities (KHI), which have been less studied to date. Theory and modeling have suggested that GWs are excited by two types of instabilities. First, GW "self-acceleration" (SA) instability dynamics due to localized and transient GW/mean-flow interactions excite GWs having spatial scales dictated by the geometry and timescale of the local induced body forcing (Dong et al., 2020(Dong et al., , 2021(Dong et al., , 2022Fritts et al., 2020). Importantly, this mechanism can lead to GW generation prior to, and perhaps in the absence of, primary GW instabilities. Second, where KHI arise in an unstable shear due in part to inertia gravity waves (IGWs), they can radiate smaller-scale and higher-frequency GWs. As examples, previous studies have suggested that GWs can be emitted from small-scale KHI, localized KHI "packets," and turbulent wakes (e.g.,

Accelerating Atmospheric Gravity Wave Simulations Using Machine Learning: Kelvin‐Helmholtz Instability and Mountain Wave Sources Driving Gravity Wave Breaking and Secondary Gravity Wave Generation

“…We explore here the potential of CAM‐Net for modeling GWs emitted from KHI described by Dong et al. (2023). We use CGCAM to generate training and testing data for CAM‐Net.…”

“…We employ the Complex Geometry Compressible Atmospheric Model (CGCAM) to create the training datasets for CAM-Net. CGCAM is a finite volume model that has been used extensively to study GW dynamics and their instabilities in the Mesosphere and Lower Thermosphere (MLT) at very high resolution (see, e.g., Dong et al, 2020Dong et al, , 2021Dong et al, , 2022Dong et al, , 2023Fritts et al, 2020Fritts et al, , 2021Fritts et al, , 2022aFritts et al, , 2022bLund et al, 2020). CAM-Net is based on the Adaptive Fourier NO (AFNO) proposed by Guibas et al, 2021.…”

Generation Mechanism for Stratospheric Gravity Waves by Unbalanced Flow in Tropical Cyclones

“…There is a compensating inflow below the outflow layer in P‐SI, which is absent in P‐RI. The vertical shear of the radial speed along the interface is ∼7 m s −1 km −1 in P‐SI, which is larger than that in P‐RI (<3 m s −1 km −1 ), and the large shear may generate the GWs (Dong et al., 2023; Fritts, 1982; Fritts & David, 1984). The azimuth‐mean profiles of the hourly precipitation during each period are shown as green lines in Figures 2c and 2d.…”

“…In addition to the diabatic heating and mechanical oscillation resulting from convection around the eyewall and rainband, the role of radial wind vertical shear ($$\frac{\partial {u}_{r}}{\partial z}$$) caused by the TC outflow layer should not be ignored above the rainband because $$\frac{\partial {u}_{r}}{\partial z}\ne 0$$ is a necessary condition for K–H instability. The threshold of the K–H instability is that the Richardson number should be larger than 0 but less than 0.25 (Dong et al., 2023). The areas where there is always K–H instability at each hour throughout P‐SI are shown by green shading in Figure 5c.…”

Sub-Hourly Variations of Wind Shear in the Mesosphere-Lower Thermosphere as Observed by the China Meteor Radar Chain

“…Gravity waves (GWs) have a crucial function in atmospheric processes from the ground to higher altitudes. To gain insights into these processes and their impacts on background atmosphere, it is essential to quantify the mechanisms of GW generation as well as their characteristics, distributions, and responses in various environments [1,2]. Of the recognized mechanisms that generate and propagate GWs, the GW-Shear interaction is quite important.…”