The Excitation of Secondary Gravity Waves From Local Body Forces: Theory and Observation

Abstract

We examine the characteristics of secondary gravity waves (GWs) excited by a localized (in space) and intermittent (in time) body force in the atmosphere. This force is a horizontal acceleration of the background flow created when primary GWs dissipate and deposit their momentum on spatial and temporal scales of the wave packet. A broad spectrum of secondary GWs is excited with horizontal scales much larger than that of the primary GW. The polarization relations cause the temperature spectrum of the secondary GWs generally to peak at larger intrinsic periods τIr and horizontal wavelengths λH than the vertical velocity spectrum. We find that the one‐dimensional spectra (with regard to frequency or wave number) follow lognormal distributions. We show that secondary GWs can be identified by a horizontally displaced observer as “fishbone” or “>” structures in z − t plots whereby the positive and negative GW phase lines meet at the “knee,” zknee, which is the altitude of the force center. We present two wintertime cases of lidar temperature measurements at McMurdo, Antarctica (166.69°E, 77.84°S) whereby fishbone structures are seen with zknee=43 and 52 km. We determine the GW parameters and density‐weighted amplitudes for each. We show that these parameters are similar below and above zknee. We verify that the GWs with upward (downward) phase progression are downward (upward) propagating via use of model background winds. We conclude that these GWs are likely secondary GWs having ground‐based periods τr=6–10 hr and vertical wavelengths λz=6–14 km, and that they likely propagate primarily southward.

Numerical Modeling of the Generation of Tertiary Gravity Waves in the Mesosphere and Thermosphere During Strong Mountain Wave Events Over the Southern Andes

“…This results in time‐dependent, spatially and temporally localized horizontal accelerations of the background flow (i.e., local body forces) in the direction the MWs were propagating (in the intrinsic reference frame) before breaking. These local body forces cause the background flow to become unbalanced, which results in the creation of counterrotating mean wind cells and the generation of secondary GWs (Vadas et al, , ). These secondary GWs are created with concentric ring structure, propagate upward and downward away from the body force, have their largest amplitudes in and against the force direction, and propagate in all azimuths except those perpendicular to the force direction.…”

“…Vadas et al () calculated the theoretical spectra of secondary GWs from MW breaking during the winter over McMurdo. They found that these spectra are quite broad and have λ H ∼500 to thousands of kilometers, λ z ∼10–150 km, and c H ∼50–250 m/s.…”

“…Using SABER data, Liu et al () showed that GW amplitudes in the Southern Hemisphere during the wintertime peak over the Southern Andes, with a westward tilt of the maximum with increasing altitude at z >65 km; they argued that these GWs were likely secondary GWs from MW breaking. Finally, several fishbone structures were identified in lidar data at McMurdo near the winter stratopause and were shown to contain secondary GWs (Vadas et al, ). As corroboration of these results, analysis of wintertime lidar data at McMurdo inferred that λ H of the GWs was much larger in the MLT than in the stratosphere (Chen et al, ; Chen & Chu, ; Zhao et al, ), in excellent agreement with model results (BV18; VB18).…”

“…Because the secondary GW spectrum excited by a body force is broad (Fritts et al, ; Vadas & Fritts, ; Vadas et al, , ), it is expected that some of the smaller‐amplitude secondary GWs with intrinsic horizontal phase speeds of c IH ∼100m/s can propagate above the turbopause to z ∼130 km (Vadas, ). Above the turbopause, GWs dissipate from kinematic viscosity, thermal diffusivity, ion drag, wave‐induced diffusion, and wave breaking (if the GW amplitudes are large enough), with kinematic viscosity ( ν ) and thermal diffusivity ( , where is the Prandtl number) being the most important dissipation mechanism for linear‐amplitude GWs with observed periods of τ r < few hours (DelGenio & Schubert, ; Gossard & Hooke, ; Heale et al, ; Hickey & Cole, ; Lund & Fritts, ; Miyoshi & Fujiwara, ; Pitteway & Hines, ; Yiğit et al, , ; Vadas, ; Vadas & Fritts, , ).…”

Modeled Gravity Wave‐Like Perturbations in the Brightness of Far Ultraviolet Emissions for the GOLD Mission

“…Gravity waves in the upper atmosphere can still be primary waves (Vadas & Fritts, ; Vadas, ). In addition, there is evidence that gravity waves observed in the upper mesosphere and in the thermosphere are also generated from the body forces that result from the dissipation of the primary gravity waves (Vadas, ; Vadas et al, , ). These gravity waves have large scales and high horizontal phase speeds; they can induce traveling ionospheric disturbances (Francis, ; Richmond, ; Vadas & Nicolls, ; Vlasov et al, ).…”