Abstract:
Low-frequency (200--1000 Hz) surface backscatter measurements reported by Percy et al. and Brown et al. show levels that are 10 to 20 dB less than the levels measured by Chapman--Harris, Chapman--Scott, and Ogden--Erskin under similar wind-speed/sea-state conditions. The Percy/Brown measurements were generally made under conditions in which the surface water was warmer (less dense) than the underlying water and consequently was gravitationally stable (buoyant). The other experiments cited were generally done in winter conditions where the surface water was either isothermal or colder than the underlying water, and, as a result, the water column was either marginally stable or unstable. Preliminary low-angle backscatter predictions using inputs estimated from Thorpe's upward-looking sonar measurements give a level difference of over 10 dB between stable and unstable conditions. Since scattering from the sea surface and high void fraction bubble plumes should not depend strongly on the water column stability, an experimental demonstration of a marked dependence of low-angle backscatter on the gravitational stability of the water column would point to advected microbubble clouds as the primary scattering mechanism. The details of the calculations and planned experiments will be discussed. [Work supported by the Office of Naval Research.]