Simulating tornado infrasound

Basic theoretical considerations suggest that the natural vibrations of a tornado can produce detectable levels of acoustic radiation. It has been proposed that the infrasound (sound below audible frequencies) of a developing tornado might provide a crucial hint of imminent danger. Accordingly, the Physical Sciences Division of the Earth Systems Laboratory and the National Weather Service Forecast Offices of NOAA are evaluating the use of infrasound detection networks to complement radar based tornado warning systems.

To date, there remains some doubt that tornadoes are behind the exceptionally strong 0.1-10 Hz signals that are observed to radiate from tornadic thunderstorms. Without knowing the acoustic fingerprint of a tornado, there is little hope of clarification. To address this problem, we have begun modeling studies of tornado acoustics with a fully compressible version of the Regional Atmospheric Modeling System (RAMS). Upon comparison to classical analytical results, high resolution RAMS simulations are shown to accurately simulate vortex sound.

Moreover, RAMS simulations are used to compare the acoustic radiation field of a tornado-like vortex (200 meters wide) of variable strength to that of a non-tornadic thunderstorm (several kilometers wide). The thunderstorm simulation includes water vapor, liquid cloud droplets, rain, pristine ice, snow, aggregates, graupel and hail, whereas the tornado simulation is dry. In the tornado simulation, vortex Rossby waves of modest amplitude generate infrasound in the form of spiral waves at frequencies of order 0.1 Hz. The amplitude of this infrasound exceeds the amplitude of the thunderstorm radiation if the maximum tangential wind speed of the tornado is greater than 50 m/s. This basic result provides further evidence that detection of tornado infrasound is plausible. A more detailed study is planned to examine variations of the radiation power spectrum of the tornado with the parameters that describe vortex structure and intensity, such as swirl ratio and Mach number.

Time permitting, the analogy between spiral acoustic radiation from tornadoes and spiral inertia-gravity wave radiation from hurricanes will be addressed. It will be shown that similar conditions on the basic state vortex allow critical layers to inhibit both kinds of radiation.