Numerical Simulation of Infrasound Generated by Severe Storms

Recent observations and longstanding theoretical considerations suggest that a tornado can have a detectable signature in the infrasound of a severe storm [A.J. Bedard Jr., Mon. Wea. Rev., 133, 241 (2005)]. In order to reliably distinguish the vortex signal from extraneous noise, we must advance our current understanding of the various mechanisms that produce infrasound in atmospheric convection. Without detailed observations of the acoustic sources within a storm, numerical modeling may be the best method of investigation. We are exploring this avenue of research with the Regional Atmospheric Modeling System (RAMS), customized to simulate aeroacoustics. By comparison to analytical results, we have shown that the customized model adequately generates the infrasound of tornado-like vortices, and of basic diabatic cloud processes [D.A. Schecter et al., J. Atmos. Sci., 65, 685 (2008); D.A. Schecter and M.E. Nicholls, J. Appl. Meteor. Clim., 49, 664 (2010)]. Encouraged by the fundamental credibility of the model, we have (tentatively) pushed ahead to simulate the infrasound of various convective systems, including a basic cumulonimbus and a non-supercell tornado storm.

Provisional results suggest that the 0.1-3 Hz infrasonic radiation of a generic, non-rotating cumulonimbus is substantially weaker than what a moderate-to-strong tornado is capable of emitting. Nevertheless, a severe rotating storm may have additional sources of infrasound (within vigorous, diabatic cloud turbulence) that rival a tornado. For cases in which tornado infrasound does not clearly dominate, we have developed a novel technique for differentiating and comparing the emissions of all relevant sources. Our technique is built upon a generalized Lighthill analysis, and is currently under evaluation. On another front, we are investigating the infrasound of developing and mature "tornadoes" that are created by artificial buoyancy forcing in dry, rotational environments. This simplified setup allows us to easily isolate and understand the infrasound associated with quasi realistic vortex fluctuations in various parameter regimes.

This work is supported by NSF grant ATM-0832320.