Page 66 - Summer2022

RAINFALL AT SEA Figure 3. The average sound spectra for rainfall rates of 2-5 (left) and 5-10 (right) mm/h are decomposed into various wind speeds. Gray area, wind-only spectra; dashed-dotted (left), dotted (center), and dashed (right) black lines, average spectra for wind speeds at 2-4, 4-6, and 6-8 m/s, respectively; numbers in box, number of data points in each category (Ma and Nystuen, 2005b). Increasing wind speed causes decreasing sound levels in a predicable way. greater effect on smaller raindrops, so the sound signal from drizzle is highly sensitive to wind speed. The wind appears to suppress the bubble creation mechanism of small raindrops, with the rain-generated sound at around 15 kHz inversely proportional to the wind speed (Figure 3). For larger raindrops that generate sound in the 2- to 8-kHz frequency band, the sound level is relatively insen- sitive to the wind speed. The bubble-trapping mechanism for large drops appears to be insensitive to the angle of impact (Ma and Nystuen, 2005b). To develop corrections for wind speed in acoustic rain gauges, rainfall spectra have been classified for various wind speeds using coin- cidental acoustic and wind speed data obtained during rain events. Acoustic Rain Gauges The need for better measurements of rainfall rates at sea and the ability to make such measurements by recording ambient-noise spectra led to the development of acous- tic rain gauges (ARGs), later renamed Passive Aquatic Listeners (PALs) by Nystuen et al. (2015) (Figure 4). Nystuen received the Medwin prize from the Acoustical Society of America (ASA) for this work in 2003 (Ma and Leopold, 2021). The PAL was a self-contained, low-power acoustic recorder that could estimate and store acoustic spectra every minute over year-long periods. The PAL data-collection sequence consisted of first obtaining four 10- to 24-ms time series sampled at 100 kHz at 5-s inter- vals, from which power spectra were computed (0-50 kHz). These spectra were averaged and compressed to 64 frequency bins, forming the 1 rain measurement. The PAL recorded spectra at 1-min intervals during rain- fall events and 8-min intervals otherwise. When rain or drizzle signals were detected, determined by obtaining Figure 4. Jeffrey Nystuen and a self-contained Passive Aquatic Listener designed for deployment on a mooring in 2001. 66 Acoustics Today • Summer 2022