Detection of Rain in Tropical Cyclones by Underwater Ambient Sound

Rain in tropical cyclones is studied using eight time series of underwater ambient sound at 40-50 kHz with wind speeds up to 45 m s-1 beneath three tropical cyclones. At tropical cyclone wind speeds, rain-and wind-generated sound levels are comparable, and therefore rain cannot be detected by sound level alone. A rain detection algorithm that is based on the variations of 5-30-kHz sound levels with periods longer than 20 s and shorter than 30 min is proposed. Faster fluctuations (<20 s) are primarily due to wave breaking, and slower ones (>30 min) are due to overall wind variations. Higher-frequency sound (>30 kHz) is strongly attenuated by bubble clouds. This approach is supported by observations that, for wind speeds < 40 m s-1, the variation in sound level is much larger than that expected from observed wind varia-tions and is roughly comparable to that expected from rain variations. The hydrophone results are consistent with rain esti-mates by the Tropical Rainfall Measuring Mission (TRMM) satellite and with Stepped-Frequency Microwave Radiometer (SFMR) and radar estimates by surveillance flights. The observations indicate that the rain-generated sound fluctuations have broadband acoustic spectra centered around 10 kHz. Acoustically detected rain events usually last for a few minutes. The data used in this study are insufficient to produce useful estimation of rain rate from ambient sound because of limited quantity and accuracy of the validation data. The frequency dependence of sound variations suggests that quantitative rain-fall algorithms from ambient sound may be developed using multiple sound frequencies.