In 2006, CASA completed the deployment of a four radar test bed network in southwestern Oklahoma. The ability of CASA's radar technology to detect and track weather hazards such as tornadoes makes it an ideal instrument for use in evaluating/validating weather hazard detection and tracking technologies. One such technology is infrasound. Infrasound is very low frequency sound, typically defined as sound less than 20Hz. It is known that tornadoes generate infrasound at a frequency that is a function of the size and strength of the tornado. In a project called ISNet, NOAA researchers demonstrated the potential for using arrays of infrasound sensors for tornado detection. The ISNet consisted of three infrasound arrays located at Boulder, CO; Pueblo, CO; and Goodland, KS, respectively. Each ISNet array consisted of four infrasound sensors arranged in a square topology 40-80 meters on a side. By measuring the time delays between when a given infrasound signal arrived at each sensor in an array, the azimuth-of-arrival and apparent horizontal velocity of the infrasound signal was determined. With detections of the same signal at two or more arrays, it was then possible to geo-locate the source of the infrasound. Repeating this process over a sliding window of infrasound data, the source of the infrasound could be tracked.
NOAA's motivation for ISNet was to improve tornado lead times by overcoming the update rate, resolution, and low-altitude blockage limitations of its WSR-88D NEXRAD radars. Being a passive sensing technology, infrasound gives a continuous omni-directional surveillance that could provide a synergistic adjunct to weather radar. By integrating the detection and warning infrastructure with infrasound, infrasound detections could fill the temporal and low-level gaps in WSR-88D coverage. One of the difficulties encountered by NOAA in realizing this vision is that infrasound sensing technology also has a coverage gap. The coverage gap in infrasound is not due to earth curvature and scanning strategies as in radar, but rather due to the way that infrasound propagates. Initially, infrasound propagates upwards as it moves away from the source. This creates an acoustic shadow at ground level starting at about 30km from the source and extending to several hundreds of kilometers from the source (depending on weather conditions) at which point the infrasound ducts back down to the earth's surface. Because the separation between the ISNet sites was ~250 km, acoustic shadowing meant that it was generally not the case that the same tornado was simultaneously detected at more than one ISNet array. Because infrasound propagates through the atmosphere with essentially no amplitude attenuation, ISNet was giving single array detections of tornadoes many hundreds of kilometers distant. As a result, it was not possible for forecasters to verify in real-time whether or not a tornado was occurring somewhere along the back-azimuth direction. As a result, the majority of ISNet results are from post-analysis cross-correlation of archived infrasound data detections with radar data and not from real-time diagnosis.
It is our thesis that just as a dense network of radars is needed to overcome the radar coverage gap, a dense network of infrasound arrays is needed to overcome the acoustic shadow induced infrasound coverage gap. The goal of the present project is to co-locate infrasound arrays with the radars in the CASA Oklahoma test bed. By collocating infrasound arrays with CASA's radars, a 30 km spacing between infrasound arrays is obtained which we believe will overcome the acoustic shadow induced coverage gap. To test this hypothesis, infrasound detections that geo-locate inside the CASA radar network will be cross-correlated with radar weather detections, and vice-versa. The ultimate goal of our study is to attempt to go beyond a proof of the potential of the technology to a proof of its operational value in improving tornado lead-time. This paper will detail the infrasound array hardware and software being developed for a 2011 deployment in Oklahoma.