Detection of gravity waves across the USArray: A case study

• We divide the USArray Transportable Array (TA) up into 576 small arrays to study wave propagation.
• We study how wave properties that are constrained locally by each array vary across the TA.
• We demonstrate the method by studying slow-moving gravity waves associated with a swarm of tornadoes.
• The method can be applied as is to any coherent wave phenomena or with modifications to incoherent waves.
• The method can be used to detect waves and study their statistics over long time-periods.

Barometers deployed as part of the USArray Transportable Array (TA) recorded large (300 Pa peak-to-peak) gravity waves, or gust fronts, that originated near a severe tornadic storm system in the southern United States on April 27, 2011. We present a new method in which the TA is divided into a large number of elemental sub-arrays. Each 3-element sub-array (triad) is sufficiently closely spaced so that the long-period gravity wave signal is coherent, but large enough to provide a robust estimate of the signal's direction and speed. The results from each triad are combined to follow the progress of gravity waves as they propagate across the TA over 2 days in late April, 2011. We observe a large, high-amplitude gravity wave, spanning a region over 200,000 km2, progressing to the NNW away from the tornadic storm region. We also observe gravity waves with lower amplitudes and smaller spatial extent along the gulf coast, propagating southward, away from the storm region. This study demonstrates the functionality of the USArray pressure sensors for analyzing gravity wave dynamics over periods greater than 40 min and across a wide region. In principle the method could be applied to study other long-period wave phenomena recorded by any dense large-scale network and is not limited to gravity waves.