Beamwidth Effects on Acoustic Backscatter in the Planetary Boundary Layer

Abstract

An ?excess attenuation? experiment is described that uses an acoustic sounder (or echosonde) operating at simultaneous frequencies of 1250 and 2500 Hz. The relative error in the received powers at these two frequencies is shown to be a beam-width-dependent effect. The magnitude of this effect at 150 m ranged typically from 50% in free-convection conditions to 400% with wind speeds of 10 m s?1, consistent with a number of previous comparison experiments. The results of this experiment are interpreted in terms of models of the broadening of the beam by small-scale turbulence and the refraction of the beam by a transverse wind.