Passive acoustic detection and measurement of rainfall at sea and an empirical ocean ambient sound model
Rainfall over the ocean is one of the most important climatic parameters for both oceanic and atmospheric science. Traditional accumulation-type rain gauges are difficult to operate at sea, and so an alternate technique for measuring rain using underwater sound has been developed. Over 90 buoy-months of ambient sound spectra have been collected on the Tropical Atmosphere Ocean Project (TAO) array since 1998. By applying the Vagle et al. (1990) wind speed algorithm, the instrument noises and sensitivity bias for the absolute calibration of each Acoustic Rain Gauge (ARG) are obtained. An acoustic discrimination process is developed to retrieve the pure geophysically generated signals. A new single frequency rainfall rate algorithm is proposed. The acoustic rainfall accumulation comparisons with ancillary data show the comparable results for both long (seasonal) and short (hours) time scales. By identifying the characteristics of sound spectra generated from wind and rain, an empirical ocean ambient sound model is constructed. This model has five components for different frequency bands, each physically based on the physical interaction of the sound generating mechanisms for wind and rain. The results show significant improvements predicting sound levels from 1 to 50 kHz when compared with a previous model, APL-UW 9407. Using the temporal separation of rainfall detection to define an individual rainfall event, a typical rainfall event in the study region (at 10°, 12°N, 95°W) has a mean duration of 30 minutes and a mean accumulation of 8mm, respectively. However, large events with durations over than an hour, contribute 50% of the total accumulation. In contrast, events in the Western Tropical Pacific Ocean (at 0°, 165°E) are smaller and of shorter duration, 4.6 mm and 18 minutes, respectively, with fewer large events present. The mean rainfall rate when it is raining is the same for both locations, 15mm/hr. Locally, rainfall events produce patchiness of relatively cool and fresh water that is mixed vertically. The fresh patch of water is also advected horizontally. The mean rate of vertical mixing is 25--200 cm/hr to a depth of 1 meter and 65 cm/hr to depths of 5 and 10 meters.
- Oceanography