[Paper Review] Infrasound generation by turbulent convection

Maia Akhalkatsi, G. Gogoberidze+1 more|arXiv (Cornell University)|Sep 15, 2004

Underwater Acoustics Research3 citations

TL;DR

This paper investigates infrasound generation in supercell thunderstorms by extending Lighthill's acoustic analogy to include stratification, velocity profile inhomogeneity, and temperature fluctuations. It finds that temperature-induced dipole radiation is at least as strong as Lighthill’s quadrupole source, and stratification contributes significantly, while velocity inhomogeneity sources are negligible for low Mach number flows.

ABSTRACT

Low frequency acoustic wave generation is studied taking into account the effect of stratification, inhomogeneity of background velocity profile and temperature fluctuations. It is shown that for the typical parameters of convective storms the dipole radiation related to temperature inhomogeneities is at least of the same order as radiation of Lighthill's quadrupole source. It is also shown that the source related to stratification could have valuable contribution whereas some other sources are shown to be inefficient.

Motivation & Objective

• To understand the physical mechanisms behind infrasound radiation from convective storms, particularly supercells, where observed acoustic power exceeds Lighthill’s quadrupole estimates.
• To assess the role of background flow inhomogeneities—stratification, velocity shear, and temperature fluctuations—in generating low-frequency acoustic waves.
• To evaluate the relative contributions of various acoustic sources (quadrupole, dipole from temperature, dipole from stratification, etc.) in turbulent convection using acoustic analogy theory.
• To determine whether commonly neglected sources like temperature inhomogeneities or stratification could explain the observed high infrasound power in storms.
• To provide a theoretical framework that improves upon Lighthill’s original model by incorporating realistic atmospheric stratification and thermal fluctuations.

Proposed method

• Constructs a simplified background flow model for updrafts in a stratified atmosphere, assuming isentropic or isothermal conditions with specified temperature and pressure profiles.
• Applies the generalized acoustic analogy by decomposing flow variables into mean and fluctuating parts, deriving a linear wave equation with source terms on the right-hand side.
• Identifies four distinct source terms: Lighthill’s quadrupole (S_l), dipole from stratification (S_s), dipole from velocity inhomogeneity (S_V), and dipole from temperature fluctuations (S_T).
• Uses low Mach number approximation to neglect convective terms in the wave equation, enabling analytical estimation of acoustic power from each source term.
• Derives power estimates using dimensional analysis: N ∼ ρv̄⁸/(l c_s⁵) for quadrupole, and similar forms for dipole sources involving v̄⁶, ΔT², or velocity gradients.
• Compares relative contributions by scaling each source term with respect to Lighthill’s quadrupole, using typical storm parameters (e.g., v̄ ∼ 3–10 m/s, ΔT ∼ 3–10 K, T ∼ 270 K, H ∼ 10⁴ m).

Experimental results

Research questions

• RQ1 How do temperature inhomogeneities in turbulent updrafts contribute to infrasound radiation in supercell storms?
• RQ2 What is the relative importance of stratification-induced dipole sources compared to Lighthill’s quadrupole source in convective infrasound generation?
• RQ3 To what extent do inhomogeneities in the background velocity profile contribute to acoustic radiation in low Mach number flows?
• RQ4 Why is the observed infrasound power from supercells higher than predictions based on Lighthill’s quadrupole model?
• RQ5 Can the combined effect of multiple sources (especially temperature fluctuations and stratification) explain the observed broadband infrasound spectrum?

Key findings

• The dipole radiation sourced by temperature inhomogeneities is at least of the same order of magnitude as Lighthill’s quadrupole source for typical supercell storm parameters.
• The acoustic power from the stratification-induced dipole source is estimated at approximately 10% of the Lighthill quadrupole power (N_s ∼ 0.1 N_l), indicating a potentially significant contribution.
• The source term related to velocity profile inhomogeneity (S_V) is shown to be negligible, with acoustic power scaling as M⁴ N_s, where M ∼ 0.1–0.15, making it four orders of magnitude smaller than the stratification dipole.
• The thermo-acoustical source from small-scale temperature fluctuations (S_T) yields acoustic power comparable to the Lighthill source when realistic storm parameters (ΔT ∼ 3–10 K, v̄ ∼ 3–10 m/s) are used.
• Shear-induced noise and other secondary sources are found to be inefficient in low Mach number flows, consistent with previous studies on shear noise being weaker than Lighthill’s quadrupole.
• The model neglects humidity effects and rotational dynamics, but these are expected to have minimal impact on the main conclusions, as they would only add small, negligible contributions to the acoustic output.