Sabine von Hünerbein

Simulating acoustic scattering from atmospheric temperature fluctuations using a k-space method

This paper describes a numerical method for simulating far-field scattering from small regions of inhomogeneous temperature fluctuations. Such scattering is of interest since it is the mechanism by which acoustic wind velocity profiling devices (Doppler SODAR) receive backscatter. The method may therefore be used to better understand the scattering mechanisms in operation and may eventually provide a numerical test-bed for developing improved SODAR signals and post-processing algorithms. The method combines an analytical incident sound model with a k-space model of the scattered sound close to the inhomogeneous region and a near-to-far-field transform to obtain far-field scattering patterns. Results from two test case atmospheres are presented: one with periodic temperature fluctuations with height and one with stochastic temperature fluctuations given by the Kolmogorov spectrum. Good agreement is seen with theoretically predicted far-field scattering and the implications for multi-frequency SODAR design are discussed.

A doppler sodar case study of top-down convection and convective dissipation of stratus

Abstract Stratus dissipation over the Swiss Plateau has long been a forecasting problem. A case study of stratus structure and development was conducted during the winter of 1997/98. Measurements were performed with the monostatic Doppler sodar of the Swiss Federal Institute of Technology, Zurich, to supplement conventional observation methods. A pyranometer, a second sodar, and a sonic anemometer were also intermittently available. Sodar echo patterns and vertical velocity data have been investigated with respect to the inversion height, especially during stratus dissipation. Typical cloud-dissipating processes, such as convection, top-down convection, and advection have been discerned. The results show that sodar measurements can supplement other observation methods in a useful way, especially when the data analysis of echo patterns and high-resolution vertical velocities are combined.

Beam Geometry Calibration of Sodars without Use of a Mast

AbstractA new method for calibration of sodar wind speed measurements is described. The method makes no assumptions whatsoever about the sodar operation and its hardware and software, other than the assumption that only one beam is transmitted at a time. Regardless of the complexity of the actual beam shape, the effective beam zenith angle is accurately estimated: this is the angle that must be used in estimations of velocity components. In a very simple experiment, the effective beam zenith angle has been found to within around 0.2°, which is as good as is required in the most stringent sodar calibration procedures. It has been found, even for such a short data run, that the estimated beam angle is very close to that calculated from the sodar array geometry. The main limitation is the requirement for horizontally homogeneous flow, since the regression methods use both a tilted beam and a vertical beam. Note that this is also a fundamental limiting assumption in the normal operation of ground-based wind l...

The effect of microphone wind noise on the amplitude modulation of wind turbine noise and its mitigation

Microphone wind noise can corrupt outdoor recordings even when wind shields are used. When monitoring wind turbine noise, microphone wind noise is almost inevitable because measurements cannot be made in still conditions. The effect of microphone wind noise on two amplitude modulation (AM) metrics is quantified in a simulation, showing that even at low wind speeds of 2.5 m/s errors of over 4 dBA can result. As microphone wind noise is intermittent, a wind noise detection algorithm is used to automatically find uncorrupted sections of the recording, and so recover the true AM metrics to within ±2/±0.5 dBA.

Using blind signal processing algorithms to remove wind noise from environmental noise assessments : a wind turbine amplitude modulation case study

Microphone wind noise can corrupt outdoor measurements and recordings. It is a particular problem for wind turbine measurements because these cannot be carried out when the wind speed is low. Wind shields can be used, but often the sound level from the turbine is low and even the most efficient shields may not provide sufficient attenuation of the microphone wind noise. This study starts by quantifying the effect that microphone wind noise has on the accuracy of two commonly used Amplitude Modulation (AM) metrics. A wind noise simulator and synthesized wind turbine sounds based on real measurements are used. The simulations show that even relatively low wind speeds of 3 m/s can cause large errors in the AM metrics. Microphone wind noise is intermittent, and consequently, one solution is to analyze only uncorrupted parts of the recordings. This paper tests whether a single-ended wind noise detection algorithm can automatically find uncorrupted sections of the recording, and so recover the true AM metrics. Tests showed that doing this can reduce the error to ±2 dBA and ±0.5 dBA for the time and modulation-frequency domain AM metrics, respectively.

Atmospheric sound scattering model to test signal coding methods for acoustic wind profiling.

This presentation concerns simulation of atmospheric sound propagation, including the backscatter of acoustic waves from moving temperature fluctuations. The model has been developed to evaluate the performance of acoustic wind speed profiling techniques, in particular, the use of pulse coding methods which aim to increase range resolution, accuracy, and data availability. Stepped frequency chirps are propagated through the model and the results used to optimize matched filtering algorithms. Results from simulations are presented and the performance of several different signal detection algorithms evaluated. This methodology provides insight into particular problems encountered by the signal detection algorithm and will be utilized to aid the design of an experimental acoustic wind speed profiler. [Work funded by the Engineering and Physical Sciences Research Council (EPSRC).]

RASS sound speed profile (SSP) measurements for use in outdoor sound propagation models

The performance of outdoor sound propagation models depends to a great extent on meteorological input parameters. In an effort to improve speed and accuracy, model output synthetic sound speed profiles (SSP) are commonly used depending on meteorological classification schemes. In order to use SSP measured by RASS in outdoor sound propagation models, the complex profiles need to be simplified. In this paper we extend an investigation on the spatial and temporal characteristics of the meteorological data set required to yield adequate comparisons between models and field measurements, so that the models can be fairly judged. Vertical SSP from RASS, SODAR wind profiles as well as mast wind and temperature data from a flat terrain site and measured over a period of several months are used to evaluate applicability of the logarithmic approximation for a stability classification scheme proposed by the HARMONOISE working group.

Application of pulse coding to low‐level atmospheric turbulent scattering

A SODAR uses atmospheric turbulence backscatter from several narrow inclined beams to estimate turbulence and wind profiles. Simple averaging‐over‐range and averaging‐over‐frequency schemes have been used in some SODAR systems, but generally code techniques are problematic because of the high fractional Doppler shift of 0–0.04. Results are given of detailed simulations, using weather‐like targets, of a sequenced comb of frequencies, a chirp, and phase‐encoding methods. Three Doppler‐adaptive matched filters are described and evaluated against the simulated noisy atmosphere. It is found that the comb of frequencies produce the least variance in estimated Doppler wind speed, and under expected typical turbulence levels, the filter should provide Doppler winds to about 1%. These methods are evaluated in a compact SODAR configuration, with the emphasis on extracting high spatial resolution data over the lowest 20–40 m altitude. This height range has applications in pollutant and agricultural investigations.