The wind power industry has seen tremendous growth over the past decade and with it has come the need for clutter mitigation techniques for nearby radar systems. Wind turbines can impart upon these radars a unique type of interference that is not removed with conventional clutter-filtering methods. Time series data from Weather Surveillance Radar-1988 Doppler (WSR-88D) stations near wind farms were collected and spectral analysis was used to investigate the detailed characteristics of wind turbine clutter. Techniques to mask wind turbine clutter were developed that utilize multiquadric interpolation in two and three dimensions and can be applied to both the spectral moments and spectral components. In an effort to improve performance, a nowcasting algorithm was incorporated into the interpolation scheme via a least mean squares criterion. The masking techniques described in this paper will be shown to reduce the impact of wind turbine clutter on weather radar systems at the expense of spatial resolution.
Weather station barograph records as well as infrasonic recordings of the pressure wave from the Mount St. Helens eruption of May 18, 1980, have been used to estimate an equivalent explosion airblast yield for this event. Pressure amplitude versus distance patterns in various directions compared with patterns from other large explosions, such as atmospheric nuclear tests, the Krakatoa eruption, and the Tunguska comet impact, indicate that the wave came from an explosion equivalent of a few megatons of TNT. The extent of tree blowdown is considerably greater than could be expected from such an explosion, and the observed forest damage is attributed to outflow of volcanic material. The pressure‐time signature obtained at Toledo, Washington, showed a long, 13‐min duration negative phase as well as a second, hour‐long compression phase, both probably caused by ejecta dynamics rather than standard explosion wave phenomenology. The peculiar audibility pattern, with the blast being heard only at ranges beyond about 100 km, is explicable by finite amplitude propagation effects. Near the source, compression was slow, taking more than a second but probably less than 5 s, so that it went unnoticed by human ears and susceptible buildings were not damaged. There was no damage at Toledo (54 km), where the recorded amplitude would have broken windows with a fast compression. An explanation is that wave emissions at high elevation angles traveled to the upper stratosphere, where low ambient air pressures caused this energetic pressure oscillation to form a shock wave with rapid, nearly instantaneous compression. Atmospheric refraction then returned part of this wave to ground level at long ranges, where the fast compressions were clearly audible.
Observations of sound attenuation in the out of doors have shown an attenuation factor approximately dependent on the five-fourths power of frequency, rather than the square. Both power laws have been applied to calculations of yield-scalable pressure signatures from explosions to allow comparison of results with measurements of explosion-wave compression rise times. It appears that the five-fourths law better explains the long rise times observed, but there are still serious underpredictions. Nevertheless, this model has been applied to the problem of determining the requirements for pressure-gauge frequency response. At the low overpressures of concern in environmental monitoring, it appears that a 1-kHz instrument response is more than adequate for recording explosion waves.
Seismological exploration companies and Defense Department proving grounds occasionally experience liability suits for damage wrought by explosions. Good forecasts of troposphere temperature and wind structure can be used to predict where shocks will strike, and with fair accuracy whether or not the shock will crack windows. Formulas are derived by which these predictions may be made.
Acoustic attenuation theory with coefficients obtained by experiment has been used to calculate losses from long range propagation along various paths in a standard atmosphere. Frequency dependence of attenuation has been incorporated into Fourier series approximations of positive phase waves from explosions to obtain net attenuations in terms of explosion yield and burst altitude. For some applications this incorporation improves on estimations from assumption of a single frequency of input. Attenuation results are presented for small 0.45-kg grenades burst at 30-to 90-km altitudes and for ton and kiloton explosives burst near the ground with waves propagated to 50-km altitudes and back to the ground near 200-km ranges. Acoustic attenuation has usually been neglected in making predictions or calculations of the propagation of low-frequency waves (1 Hz from large explosions. In general, this neglect was justified, but some doubts have arisen regarding certain applications involving small explosives with 1-100-YIz frequencies and very long propagation paths, or paths through very low atmospheric densities at high altitudes.Silver Spring, Md., 1967. . Morse, P.M., and K. V. Ingar d, The. oretical Acoustics, McGraw-Hill, New York, 1968. Reed, J. W., Amplitude variability of explosion waves at long ranges, J. Acoust. Soc. Amer., 39(5), Part. 1, 9•0-981, 1966. Sadwin, L. D., and E. A. Christian, Characteristics of the shock wave generated in air by a blasting cap, NOLTR 71-105, U.S. Naval Ordnance Laboratory, Silver Spring, Md., June 1971. SchrSdinger, E., Zur Akustik der Atmosph•ire, Phys.
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