Fence Facilitates All-Weather Tornado Detection Potential Using Infrasound

Contact: Alfred Bedard

During summer 2003, NOAA/ETL and the NOAA/NWS collaborated to evaluate a prototype infrasonic tornado-detection network. Infrasonic sensor arrays were located at ETL's Boulder Atmospheric Observatory (BAO) site in Erie, Colorado, and two NWS sites in Pueblo, Colorado, and Goodland, Kansas. Data from these sensors are continuously uplinked, processed, and provided to the National Weather Service. The network is being used to evaluate the potential of infrasound for early detection and location of tornados.

Infrasonics is the study of sound below the range of human hearing. Such low-frequency sounds are produced by a variety of geophysical processes including earthquakes, severe weather, volcanic activity, geomagnetic activity, ocean waves, avalanches, turbulence aloft, meteors, and by some man-made sources such as aircraft and explosions. Infrasonic detection systems have the potential to provide advanced warning and remote monitoring of extreme events.

However, strong winds at instrument sites cause pressure noise that can mask tornado and other infrasonic signatures. To reduce such noise an innovative wind fence was recently developed to minimize pressure variations caused by wind gusts. The wind fence is constructed using a 6-ft.-tall chain-link fence with a 65% porous windscreen. To break the wind parcels into smaller scales a series of 2-ft tall plywood triangles are fastened around the periphery at the top of the fence. The fence design produces at most light winds having random directions within the enclosed area, even in the presence of high winds outside. A dramatic (~100-fold) reduction of the wind noise signal has been observed, thus facilitating the detection of tornadic infrasonic signals under all weather conditions. One wind fence surrounds each 50-ft.-diameter infrasonic detector.

Infrasonic detection complements NEXRAD's radar-based tornado detection capabilities, and has potential to improved detectability in situations that confound NEXRAD. One such example is when an obstacle blocks the radar beam; another is detection at long ranges where radar resolution is degraded; and a third is at short ranges where rotation aloft produces little Doppler shift.

The new wind fence design has facilitated the collection of an extensive high-quality data set from the infrasonic network. ETL scientists and NWS forecasters are currently reviewing those data to assess the effectiveness of an infrasound network in NOAA operations.