The publisher regrets to inform readers that, owing to the incorrect pagination of an earlier paper, the paper by Renner et al.

With the increase in the use of small uncrewed aircraft systems (UAS) there is a growing need for real-time weather forecasting to improve the safety of low-altitude aircraft operations. This will require integration of measurements with autonomous systems since current available sampling lack sufficient resolution within the atmospheric boundary layer (ABL). Thus, the current work aims to assess the ability to measure wind speeds from a quad-copter UAS and compare the performance with that of a fixed mast. Two laboratory tests were initially performed to assess the spatial variation in the vertically induced flow from the rotors. The horizontal distribution above the rotors was examined in a water tunnel at speeds and rotation rates to simulate nominally full throttle with a relative air speed of 0 or 8 m/s. These results showed that the sensor should be placed between rotor pairs. The vertical distribution was examined from a single rotor test in a large chamber, which suggested that at full throttle the sensor should be about 400 mm above the rotor plane. Field testing was then performed with the sensor positioned in between both pairs of rotors at 406, 508, and 610 mm above the rotor plane. The mean velocity over the given period was within 5.5% of the that measured from a fixed mast over the same period. The variation between the UAS and mast sensors were better correlated with the local mean shear than separation distance, which suggests height mismatch could be the source of error. The fluctuating velocity was quantified with the comparison of higher order statistics as well as the power spectral density, which the mast and UAS spectra were in good agreement regardless of the separation distance. This implies that for the current configuration a separation distance of 5.3 rotor diameters was sufficient to minimize the influence of the rotors.

Earthquakes have repeatedly been shown to produce inaudible acoustic signals (<20 Hz), otherwise known as infrasound. These signals can propagate hundreds to thousands of kilometers and still be detected by ground-based infrasound arrays depending on the source strength, distance between source and receiver, and atmospheric conditions. Another type of signal arrival at infrasound arrays is the seismic induced motion of the sensor itself, or ground-motion-induced sensor noise. Measured acoustic and seismic waves produced by earthquakes can provide insight into properties of the earthquake such as magnitude, depth, and focal mechanism, as well as information about the local lithology and atmospheric conditions. Large earthquakes that produce strong acoustic signals detected at distances greater than 100 km are the most commonly studied; however, more recent studies have found that smaller magnitude earthquakes (Mw<2.0) can be detected at short ranges. In that vein, this study will investigate the ability for a long-term deployment of infrasound sensors (deployed as part of the Source Physics Experiments [SPE] from 2014 to 2020) to detect both seismic and infrasonic signals from earthquakes at local ranges (<50 km). Methods used include a combination of spectral analysis and automated array processing, supported by U.S. Geological Survey earthquake bulletins. This investigation revealed no clear acoustic detections for short range earthquakes. However, secondary infrasound from an Mw 7.1 earthquake over 200 km away was detected. Important insights were also made regarding the performance of the SPE networks including detections of other acoustic sources such as bolides and rocket launches. Finally, evaluation of the infrasound arrays is performed to provide insight into optimal deployments for targeting earthquake infrasound.

Tornado producing storms have been shown to emit infrasound (sound below 20 Hz) before and after tornadogenesis. This infrasound can be detected over large distances due to the low atmospheric attenuation of sound signals at low frequencies. The ability for infrasound signals to travel large distances could allow for the use of infrasound microphone arrays to assist with tornado detection and improve tornado warnings. The current work will focus on investigating the effects of the local atmosphere on a propagating infrasound signal by running simulations utilizing an atmospheric modeling code known as AVO-G2S and a collection of numerical models for the propagation of infrasound known as NCPAprop. This work will report the results from these simulations which investigated tornado and hail producing storms that occurred in Oklahoma between 2017 and 2020. Particularly, the impact of acoustic and atmospheric models on emitted infrasound signals will be investigated. [This work was funded by NOAA under Grant No. NA19OAR4590340.]

Tornado warnings have not improved over the past 20 years, which is especially true in hilly terrain where radar cannot see near the ground due to the curvature of the earth. While Tornado Alley is best known for tornadoes, most tornado related deaths occur in the southeastern US where hilly terrain is more prevalent. Tornadoes emit sounds at frequencies below what humans can hear (infrasound), and there is strong evidence that these sounds carry information about the forming of the tornado as well as its size. In addition, these very low frequencies can travel well beyond the line-of-sight. Currently, this information is not used to guide warnings because we do not understand what makes it. Our team has been working on identifying the fluid mechanism for the past few years and there are four commonly proposed mechanisms that are consistent with observations; radial oscillation, latent heat effect, pressure relaxation, and shear instability. In this presentation each mechanism will be discussed relative to available observations. In addition, an overview of current field and laboratory testing motivated by the success and limitations of each proposed mechanism will be discussed.