Generation of Acoustic and Gravity Waves by Turbulence in an Isothermal Stratified Atmosphere
Abstract
Lighthill's method of calculating the aerodynamic emission of sound waves in a homogeneous atmosphere is extended to calculate the acoustic and gravity-wave emission by turbulent motions in a stratified atmosphere. The acoustic power output is Pac ≈ 103θouo3/loM5 ergs/cm3 sec, and the upward gravity wave flux is Fzgr ≈ 102θoUo3/lo (lo ergs/cm3 sec. Here u0 is the turbulence velocity scale, l0 is its length scale, and H the scale height at the atmosphere. M = u0/c0 is the Mach number of the turbulence. The acoustic power output is proportional to the maximum value of the turbulence spectrum, and inversely to its rate of falloff at high frequencies. The stratification cuts off the acoustic emission at low Mach numbers. The gravity emission occurs near the critical angle to the vertical θc = cos−1ω/ω2, where ω22 = (γ - 1)/γ2 (c0/H), and at very short wavelengths. It is proportional to the large wave number tail of the turbulence spectrum. On the sun, gravity-wave emission is much more efficient than acoustic, but can occur only from turbulent motions in stable regions, whereas acoustic waves are produced by turbulence in the convection zone.
- Publication:
-
Solar Physics
- Pub Date:
- December 1967
- DOI:
- 10.1007/BF00146490
- Bibcode:
- 1967SoPh....2..385S
- Keywords:
-
- Mach Number;
- Acoustic Emission;
- Gravity Wave;
- Sound Wave;
- Critical Angle