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On temporal spectra of scintillometers

By Jonathan G. Evans and H.A.R. De Bruin


Clifford (1971) derived expressions for the real and imaginary part of the temporal spectra W(f) for spherical waves propagating through the atmosphere. To a good approximation, the transmitter of a Millimeter-Wave Scintillometer (MWS) can be regarded as a point source and thus Clifford's theory is applicable to a MWS. Nieveen et al. (1998) extended Clifford's theory to large aperture scintillometers (LAS). In both cases a so called corner frequency, fc , can be defined. For frequencies smaller than fc , the real part of W(f) is approximately constant at Wplateau, whereas for f>fc , the real part of W(f) is proportional to f-8/3 for the MWS. It is noted that Wplateau is inversely proportional to the cross wind uc and that fc is proportional to uc, therefore the integral of W(f) over all f is independent of uc; however, this applies for the case where the cross wind does not vary along the path. During a field experiment carried out in mid-summer 2006 at Sheepdrove Organic Farm, UK, over mixed agricultural land use and complex topography, W(f) was measured by a 94 GHz MWS. There were contrasting cool-windy and hot-convective weather conditions during the experiment. With these data the Clifford theory will be validated

Year: 2009
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