Acoustic Doppler Sounding of Falling Snow

Abstract

Acoustic echoes received during snowfall have been investigated using a vertically directed echo sounder. Doppler shift analysis of the echoes indicates consistent downward motion in the range 1?2 m s?1 and thus demonstrates that the snowflakes are the cause of the observed backscattering. Consideration of the mean scattered power yields a radar reflectivity for the snow of 35?42 dBZ, and a precipitation rate of 1.2?2.7 mm h?1. The latter figure may be compared with the actual mean precipitation rate of 0.9 mm h?1 averaged over the duration of the showers, and intervening periods of little or no snow. The observed probability distribution of instantaneous echo power is log-normal. This is in contrast to the theoretically predicted exponential function. The standard deviation of the log-normal distribution is close to 6.1 dB, which is the saturation value expected for acoustic waves interacting strongly with the turbulent atmosphere. Multiple scattering may thus be significant in these observations and also generally in atmospheric echo sounding.