Knowledge of the intensity and spatial-temporal distribution of rainfall over the ocean is critical in understanding the global hydrological cycle. However, rain has proven difficult to measure over the ocean due to problems associated with platform motion and flow distortion combined with the spatial and temporal variability of rainfall itself. Underwater acoustical rain gauges avoid these issues by using the loud and distinctive underwater sound generated by raindrops on the ocean surface to detect and quantify rainfall. Here, the physics and operation of and results from an instrument that uses underwater ambient sound to measure rainfall rate and wind speed are presented. Passive Aquatic Listener (PAL) instruments were mounted on a buoy deployed at Ocean Station P and on 13 Argo profilers that were deployed as part of the US National Aeronautics and Space Administration-sponsored Salinity Processes in the Upper-ocean Regional Study (SPURS) field experiment in the North Atlantic Ocean. The PALs provide near-continuous measurements of rain rate and wind speed during the two-year period over the SPURS study region defined by the Argo profilers. Comparisons of PAL data with rain and wind measured by other techniques, including direct in situ observations and satellite measurements, show good agreement for both rain rate and wind speed.
Yang, J., S.C. Riser, J.A. Nystuen, W.E. Asher, and A.T. Jessup. 2015. Regional rainfall measurements using the Passive Aquatic Listener during the SPURS field campaign. Oceanography 28(1):124–133, https://doi.org/10.5670/oceanog.2015.10.
Anderson, J., and S. Riser. 2014. Near-surface variability of temperature and salinity in the near-tropical ocean: Observations from profiling floats. Journal of Geophysical Research 119:7,433–7,448, https://doi.org/10.1002/2014JC010112.
Longuet-Higgins, M.S. 1990. An analytic model of sound production by raindrops. Journal of Fluid Mechanics 214:395–410, https://doi.org/10.1017/S0022112090000179.
Ma, B., and J.A. Nystuen. 2005. Passive acoustic detection and measurement of rainfall at sea. Journal of Atmospheric and Oceanic Technology 22:1,225–1,248, https://doi.org/10.1175/JTECH1773.1.
Medwin, H., and M.M. Beaky. 1988. Bubble sources of the Knudsen sea noise spectra. Journal of the Acoustical Society of America 86:1,124–1,130, https://doi.org/10.1121/1.398104.
Medwin, H., A. Kurgan, and J.A. Nystuen. 1990. Impact and bubble sound from raindrops at normal and oblique incidence. Journal of Acoustical Society of America 88:413–418, https://doi.org/10.1121/1.399918.
Medwin, H., J.A. Nystuen, P.W. Jacobus, L.H. Ostwald, and D.E. Synder. 1992. The anatomy of underwater rain noise. Journal of the Acoustical Society of America 92:1,613–1,623, https://doi.org/10.1121/1.403902.
Nystuen, J.A. 1986. Rainfall measurements using underwater ambient noise. Journal of the Acoustical Society of America 79:972–982, https://doi.org/10.1121/1.393695.
Nystuen, J.A. 1993. An explanation of the sound generated by light rain in the presence of wind. Pp. 659–668 in Natural Physical Sources of Underwater Sound. B.R. Kerman, ed., Kluwer Academic Publishers.
Nystuen, J.A. 2001. Listening to raindrops from underwater: An acoustic disdrometer. Journal of Atmospheric and Oceanic Technology 18:1,640–1,657, https://doi.org/10.1175/1520-0426(2001)018<1640:LTRFUA>2.0.CO;2.
Nystuen, J.A., E. Amitai, E.N. Anagnostou, and M.N. Anagnostou. 2008. Spatial averaging of oceanic rainfall variability using underwater sound: Ionian Sea Rainfall Experiment 2004. Journal of the Acoustical Society of America 123:1,952–1,962, https://doi.org/10.1121/1.2871485.
Nystuen, J.A., and H. Medwin. 1995. Underwater sound generated by rainfall: Secondary splashes of aerosols. Journal of the Acoustical Society of America 97:1,606–1,613, https://doi.org/10.1121/1.412099.
Nystuen, J.A., S.C. Riser, T. Wen, and D. Swift. 2011. Interpreted acoustic ocean observations from Argo floats. Journal of the Acoustical Society of America 129:2,400, https://doi.org/10.1121/1.3587814.
Nystuen, J.A., and H.D. Selsor. 1997. Weather classification using passive acoustic drifters. Journal of Atmospheric and Oceanic Technology 14:656–666, https://doi.org/10.1175/1520-0426(1997)014<0656:WCUPAD>2.0.CO;2.
Oguz, H.N., and A. Prosperetti. 1990. Bubble entrainment by the impact of drops on liquid surfaces. Journal of Fluid Mechanics 218:143–162, https://doi.org/10.1017/S0022112090002890.
Pumphrey, H.C., L.A. Crum, and L. Bjorno. 1989. Underwater sound produced by individual drop impacts and rainfall. Journal of the Acoustical Society of America 85:1,518–1,526, https://doi.org/10.1121/1.397353.
Quartly, G.D., T.H. Guymer, and K.G. Birch. 2002. Back to basics: Measuring rainfall at sea: Part 1. In situ sensors. Weather 57:315–320.
Riser, S.C., J.A. Nystuen, and A. Rogers. 2008. Monsoon effects in the Bay of Bengal inferred from profiling float-based measurements of wind speed and rainfall. Limnology and Oceanography 53(5, part 2):2,080–2,093, https://doi.org/10.4319/lo.2008.53.5_part_2.2080.
Shaw, P.T., D.R. Watts, and H.T. Rossby. 1978. On the estimation of oceanic wind speed and stress from ambient noise measurements. Deep Sea Research 25:1,225–1,233, https://doi.org/10.1016/0146-6291(78)90015-2.
Vagle, S., W.G. Large, and D.M. Farmer. 1990. An evaluation of the WOTAN technique for inferring oceanic wind from underwater sound. Journal of Atmospheric and Oceanic Technology 7:576–595, https://doi.org/10.1175/1520-0426(1990)007<0576:AEOTWT>2.0.CO;2.
This is an open access article made available under the terms of the Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution, and reproduction in any medium or format as long as users cite the materials appropriately, provide a link to the Creative Commons license, and indicate the changes that were made to the original content. Images, animations, videos, or other third-party material used in articles are included in the Creative Commons license unless indicated otherwise in a credit line to the material. If the material is not included in the article’s Creative Commons license, users will need to obtain permission directly from the license holder to reproduce the material.