Investigation into the sources of infrasound emitted by severe storms

The term "infrasound" refers to acoustic waves with frequencies less than the lower limit of unimpaired human hearing (20 Hz). Observational studies have shown that severe storms can emit abnormally strong, sustained infrasound in the 0.5-5 Hz frequency range. There is reason to believe that the infrasonic emissions come from developing and mature tornadoes. However, some ambiguity remains in the interpretation of the data. It is fair to say that we do not yet fully understand the conditions for which a vortex signal is discernible from the infrasound of non-tornadic sources within a storm. There is a pressing need to advance our fundamental understanding of the different mechanisms that generate infrasound in atmospheric convection.

To this end, numerical modeling may be the best method of investigation. We are exploring this avenue of research with a version of the Regional Atmospheric Modeling System (c-RAMS) that has been customized to simulate acoustic phenomena. By comparison to analytical results, we have shown that c-RAMS adequately generates the infrasound of tornado-like vortices, and of simple diabatic cloud processes such as droplet evaporation. The basic credibility of the model justifies pushing ahead (cautiously) with simulations of infrasound generated by realistic convective systems.

In this presentation, we will expound a convenient method for diagnosing the primary sources of infrasound in complex storm simulations. The method is based on an exact acoustic wave equation for the perturbation Exner function Π'. Scale estimates suggest that two source terms in the Π' equation are especially important contributors to the infrasound of a severe storm. The first is commonly associated with adiabatic vortex fluctuations, whereas the second is connected to the heat (and mass) generated or removed during phase transitions of moisture. We will examine these sources and their infrasonic emissions in various simulations, including one of a towering cumulonimbus with state-of-the-art microphysics. In addition, we will discuss computational evidence suggesting that the 0.1-1 Hz infrasound of a moderate-to-strong tornado can significantly exceed that of a generic, non-rotating storm.

This work is supported by NSF grant ATM-0832320.