Research On High-frequency Power Behavior Of The Temporal Spectrum By Large Aperture Scintillometer

Large aperture scintillometer(LAS)is widely used in the measurement of optical turbulence in near surface layer.The characteristics of atmospheric turbulence are obtained based on the analysis of power spectral density(PSD)of scintillation logarithmic light intensity fluctuation.Spectrum power is an important parameter in the expression of PSD.In the process of experimental observation,there are many cases where the measured spectrum power is inconsistent with the-13/3 predicted by the theory,which will cause the measurement error of the LAS based on the theory.In this paper,the spectrum power characteristics of logarithmic light intensity PSD of LAS are analyzed from experimental measurement and numerical calculation,and the reasons for the variation of spectrum power are explained.Long-term experimental observations were carried out at three locations in Beijing,Baoding and Hefei using three self-made LASs.The results show that the measured spectrum power is inconsistent with the theoretical calculation.The spectrum power of Hefei,Baoding and Beijing stations are generally distributed around-8/3,-11/3 and-10/3,respectively.A new variation law of spectrum power is proposed.Wdrop is used to represent the PSD where the PSD begins to decrease with frequency,while Wnoise is used to represent the PSD of high-frequency noise,then the absolute value of the spectrum power increases with the increase of Wdrop/Wnoise,and finally the spectrum power approaches to about-11/3.Based on the analysis of optical transmission theory,instrument performance and data acquisition process,the reason why the spectrum power of logarithmic light intensity fluctuation spectrum of LAS deviates from the theoretical expected value is analyzed.The numerical calculation results show that when the parameters on the light wave transmission path remain unchanged,the spectrum power is a constant of-13/3.The spectrum power is calculated using the measured transverse wind speed probability distribution.Whether the transverse wind speed is small or large,the calculated spectrum power is always-13/3.The reason why the measured spectrum power deviates from theoretical value is analyzed from the perspective of instrument noise.For the same Wdrop/Wnoise,different light source noise,background light and scattered light noise cause the measured spectrum power to fluctuate up and down in the overall change trend.Different magnification,extinction and turbulence intensity,superimposed with the influence of data acquisition card bits,form the variation trend of spectrum power with Wdrop/Wnoise.If the spectrum power is inconsistent with the theoretical value,the refractive index structure parameter measured by large aperture scintillator will be larger than the actual value.