Observations of the Marine Atmosphere Using a Ship-Mounted Acoustic Echo Sounder

Abstract

A monostatic acoustic echo sounder with a vertically-pointed antenna was installed aboard the NOAA research vessel Oceanographer and was tested during a recent cruise in the Pacific and Caribbean Oceans. Acoustic returns produced by turbulence-induced temperature fluctuations were received from up to 300 m in height. The data revealed the existence of a rich variety in the structure of the marine atmosphere. Thermal convective plumes were usually observed when the sea water temperature exceeded the air temperature by as little as 1°C. Under more stable conditions the echo sounder detected the presence of layered structures associated with temperature inversions that were often perturbed by gravity waves or wind shear. Doppler frequency shift of the returned echo signals was used to estimate the vertical velocity of atmospheric scatterers. The results of the tests indicate that it is possible to probe remotely the lower layers of the marine atmosphere from a moving ship using acoustic echo sounding techniques. Wind-generated noise, ambient acoustic noise, and structure-borne vibrations proved to be the major limitations on the performance of the echo sounder.