Crosswind from a Single Aperture Scintillometer Using Spectral Techniques

In this study we are interested in obtaining the wind component perpendicular to a path, the so called crosswind, using scintillometer measurements. A scintillometer is a device that consist of a transmitter and receiver. The transmitter emits a light beam which is refracted in the turbulent atmosphere, causing light intensity fluctuations that are recorded by the receiver. A scintillometer obtains a path-averaged estimate of the crosswind. For certain applications this can be advantageous, e.g. monitoring the crosswind along airport runways.

Past research mainly focused on dual aperture scintillometers that use the time delay between the two signals to estimate the crosswind (Wang et. al., 1981). In this study we use spectral techniques, which are also applicable to single aperture scintillometer measurements. The essence of the spectral techniques lie in the fact that the scintillation power spectrum shifts linearly along the frequency axes as a function of the crosswind. Therefore, a salient point in the spectrum also shift linearly across the frequency axis.

Three algorithms with different salient points were used, namely the corner frequency, maximum frequency, and the cumulative spectrum algorithm, which all used different representation of the scintillation power spectrum. Clifford (1971) described a theoretical model of the scintillation spectrum. We used this model to calibrate our algorithms, instead of relying on experimental calibration.

The algorithms were examined with data, of a boundary layer scintillometer and sonic anemometer, collected at the Haarweg (The Netherlands). The scintillation spectra were obtained with Fast Fourier Transformations and wavelets. Wavelets were used to obtain a well-defined spectra for short time intervals (< 1 minute).

The spectrally derived crosswinds compared well with sonic anemometer estimates (see Figure 1). The cumulative spectrum algorithm, a new algorithm we introduced, obtained the best results for the crosswind when compared with the sonic anemometer. However, the maximum frequency algorithm was most robust in obtaining the crosswind. Over short time intervals (< 1 minute) the crosswind can be obtained with the cumulative spectrum algorithm using wavelets to calculate the scintillation power spectra.

Figure 1: Scatter plots of 10 minute crosswinds averages with on the x-axis that of the sonic anemometer and on the y-axis that of the scintillometer for corner frequency (a), maximum frequency (b) and cumulative spectrum (c) algorithm with in colors the signal to noise ratio.

Literature

Clifford, S. F., 1971: Temporal-frequency spectra for a spherical wave propagating through atmospheric turbulence. Journal of the Optical Society of America, 61, p. 1285–1292.

Wang, T. I., G. R. Ochs, and R. S. Lawrence, 1981: Wind measurements by the temporal cross-correlation of the optical scintillations. Applied optics, 20, p. 4073–4081.