Conclusion   Page 2

The sound of rain underwater is a loud and distinctive signal that can be used to detect and measure rain at sea. Individual raindrops make sound underwater by two distinct mechanisms: the impact of the raindrop onto the ocean surface and sound radiating from any bubbles trapped underwater during the splash. For most raindrops, the sound radiation by bubbles is, by far, the louder sound source. Because the geometry of their splashes regularly traps a bubble of uniform size, small raindrops (0.8-1.2 mm diameter) are unexpectedly loud underwater. These drops are responsible for the remarkably loud "sound of drizzle" heard between 13-25 kHz. Medium raindrops (1.2-2.0 mm diameter) are relatively quiet, while large (2.0-3.5 mm diameter) and very large (> 3.5 mm) raindrops have energetic splashes which can trap larger bubbles. These bubbles radiate sound at frequencies as low as 1 kHz. Because the different raindrop sizes produce sound with distinctive features, the sound field can be "inverted" to measure the raindrop size distribution within the rain. This is a good measure of rainfall rate, or other interesting features of rainfall.

Although there are sometimes man-made or biological noises that are loud and could potentially interfere with the acoustical measurement of rain, these noises are generally intermittent or geographically localized. When rain is present, the sound from rain dominates the underwater sound field. There are two features of rain- and drizzle-generated sound that allow detection of rain at sea. These are the relative level (very loud) and the relatively higher sound levels at higher frequency (over 10 kHz) when compared to wind. By monitoring for these distinctive spectral features, it is possible to detect and then quantify rainfall at sea.

New ARGs are currently being deployed on several of the moorings that form the Tropical Atmosphere Ocean (TAO) deep-ocean mooring array deployed by NOAA in the tropical Pacific Ocean (McPhaden et al. 1998). Data from these ARGs should become available for scientists beginning in the year 2000. By learning to listen to the ocean, we can make important rainfall observations that will help meteorologists, oceanographers and climatologists to better understand the distribution and intensity of this important component of climate.


S.P. Anderson, S.P., R.A.Weller and R. Lukas, 1996: Surface buoyancy forcing and the mixed layer of the western Pacific warm pool: Observations and 1–D model results. J Climate 9, 3056–3085.

C.S. Clay and H. Medwin, 1977, Acoustical Oceanography, Chapter 6, Wiley, New York, 544p.

D.M. Farmer and D.D. Lemon, 1984: The influence of bubbles on the ambient noise in the ocean at high wind speeds. J. Phys. Oceanogr. 14, 1762–1778.

M.J. McPhaden, A.J. Busalacchi, R. Cheney, J.R. Donguy, K.S. Gage, D. Halpern, M. Ji, P. Julian, G. Meyers, G.T. Mitchum, P.P. Niiler, J. Picaut, R.W. Reynolds, N. Smith and K. Takeuchi, 1998: The Tropical Ocean–Global Atmosphere (TOGA) observing system: A decade of progress. J. Geophys. Res, 103, 14,169–14,240.

H. Medwin and M.M. Beaky, 1989: Bubble sources of the Knudsen sea noise spectrum. J. Acoust. Soc. Am. 83, 1124–1130.

H. Medwin, J.A. Nystuen, P.W. Jacobus, L.H. Ostwald and D.E. Synder, 1992: The anatomy of underwater rain noise. J. Acoust. Soc. Am. 92, 1613–1623.

M. Minnaert, 1933: On musical air bubbles and the sounds of running water. Philos. Mag. 16, 235–248.

J.A. Nystuen, 1996: Acoustical rainfall analysis: Rainfall drop size distribution using the underwater sound field. J. Acoust. Soc. Am. 13, 74–84.

J.A. Nystuen and D.M. Farmer, 1989: Precipitation in the Canadian Atlantic Storms Program: Measurements of the Acoustic Signature. Atmosphere–Ocean 27, 237–257.

J.A. Nystuen, M.J. McPhaden and H.P. Freitag 1999: Surface Measurements of Precipitation from an Ocean Mooring: The Underwater Acoustic Log from the South China Sea", submitted to J. Applied Meteor.

J.A. Nystuen and H.D. Selsor, 1997: Weather classification using passive acoustic drifters. J. Atmos. and Oceanic Tech., 14, 656–666.

H.C. Pumphrey, L.A. Crum and L. Bjorno, 1989: Underwater sound produced by individual drop impacts and rainfall. J. Acoust. Soc. Am. 85, 1518–1526.

S. Vagle, W.G. Large and D.M. Farmer, 1990: An evaluation of the WOTAN technique for inferring oceanic wind from underwater sound. J. Atmos. and Ocean. Tech. 7, 576–595.

back Detection and Measurement of Rain at Sea

Print this entire article