Use of an Ultrasonic Sodar to Sense Raindrop Size Distributions

Abstract

The design of a sodar that uses ultrasound to remotely sense drop size distributions in rainfall is described. The Doppler shift is proportional to a drop's terminal velocity and gives a measure of the drop diameter, whereas the intensity of scatter gives a measure of the number of drops of each diameter. Since rain drops cause Rayleigh scattering in the usable range of sound frequencies, the intensity of this scattering is proportional to the fourth power of the frequency. Because attenuation rises with frequency, a 40-kHz operating frequency was found to maximize the power reflected back from rain drops. Verification of design factors was undertaken in an anechoic chamber. Field tests showed that Doppler frequency spectra and drop size concentrations matched expected profiles. Rainfall intensities inferred from the Doppler spectrum over time were within 20% of the values obtained by conventional rain gauges. The instrument was found to be useful for obtaining information in the lowest 10 m of the atmosphere, which makes it an attractive alternative to ground-based sensors.