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G. Mastrantonio and G. Fiocco

Abstract

The measurement of the radial wind velocity component in sodar systems is generally carried out by application of the first moment integral to the spectrum of the received echoes. In unfavorable signal-to-noise conditions this leads to a large systematic bias and lack of precision. In this paper the use of a two-step procedure is shown, whereby the spectrum of the echo is first localized and then the actual value of its center frequency is determined. Large improvements in accuracy and precision are thus obtained.

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D. Contini, G. Mastrantonio, A. Viola, and S. Argentini

Abstract

Accurate measurements of vertical wind velocity w by sodar are useful for investigating dynamic phenomena in the PBL such as convection, subsidence, divergence, and vertical movements associated with local circulation. Moreover, its statistics may be used to derive some micrometeorological parameters. When very low vertical movements need to be measured the estimation accuracy of this parameter becomes crucial. A method is proposed to estimate the accuracy of the mean vertical velocity as measured by sodar. This method is used to evaluate the improvement that can be achieved if the loudspeaker transfer function is used to correct the spectra. The transfer function is obtained by averaging the spectra obtained when the sodar is passively recording the background environmental acoustic noise. Results show that the sodar system can be used to measure mean vertical velocities averaged over periods of 3 h or more with an uncertainty better than 0.05 m s−1, providing that a two-step procedure is used to retrieve the Doppler shift. The transfer function correction improves the accuracy by a percentage between 12% and 20%, depending on the averaging period. The improvement may even be better if care is used in designing the antennas so that the influence of atmospheric parameters on the electroacoustic transducer decreases and their alignment is suitably accurate.

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G. Mastrantonio, F. Einaudi, D. Fua, and D. P. Lalas

Abstract

The characteristics of internal gravity waves generated by tropospheric jet streams are analyzed and discussed. By solving numerically the equations of motion in the linear, inviscid and Boussinesq limit, it is shown that a modal structure exists. Some of these modes have the ability to propagate vertically away from the jet and are likely to he responsible for some of the observed wave activities in the ionosphere as well as at the ground. For selected values of the minimum Richardson number of the flow, growth rates and horizontal phase velocities are given as functions of the horizontal wavenumber, for jet streams of varying width. Finally, a brief study of the stability of the so-called low-level jet, whose spectrum of generated waves undoubtedly will contribute to the dynamics of the nocturnal boundary layer, is undertaken.

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G. Mastrantonio, J. Naithani, P. S. Anderson, S. Argentini, and I. Petenko

Abstract

While some people involved in the acoustic remote sensing field are aware of the possibility of receiving dot echoes from nonatmospheric targets, most of the papers available in the scientific literature dealing with this phenomenon associate them to atmospheric targets, such as clusters of water vapor inhomogeneity, thermodynamical processes of condensation and reevaporation of water vapor, anisotropic irregularities localized in thin layers, etc. At present, dot echoes are defined by their appearance on the echogram and are not differentiated by causative processes. As such, they share similar characteristics, such as being randomly distributed and having a time length that is similar to the time length of the emitted tone. In this paper dot echoes conforming to this definition are investigated through the analysis of the signal in both the time and frequency domain. The timescale of a dot signature along with the configuration of the sodar system provide an upper limit to the size of the targets producing these echoes. The spectral characteristics and the first and second momenta of the echoes are compared with clear-air echoes as well as with echoes produced by pilot balloons released from nearby sodar antennas. The conclusion is that the dot echoes analyzed in this paper are reflections from birds and are not due to atmospheric effects.

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