Acoustic Radar Studies of Rain Microphysics

Abstract

Raindrop size distributions are obtained from the Doppler frequency spectrum of an acoustic radar. Number concentrations of 12 drop diameters with a minimum diameter 0.14 cm are obtained and averaged over 3?15 min at 20-m range gates from 20 to 220 m. The last three range gates are used to estimate rain intensity?dependent background noise, which is dynamically subtracted from the signals. Multifrequency sounding is also used. Intercomparisons with the vertical rain intensity profile from an X-band radar and with drop size distributions from an impact disdrometer show general agreement between instruments and demonstrate the usefulness of the acoustic profiler in giving vertical continuity below the range of electromagnetic radars. Temporal variations in raindrop size distributions are found to have an essentially flat spectrum for periodicities shorter than 12 min, although the step response to a sudden change in rainfall rate is a function of drop size. Principal component analysis applied to a time series of drop spectra shows that nearly all the variation is at the large-drop end. The utility of the acoustic radar is demonstrated for examining the microphysics of rain through time-dependent changes.