Thin Acoustic Scattering Layers Observed in the Low Marine Boundary Layer

Abstract

A novel high-resolution, low minimum-range monostatic sodar, steerable in elevation angle, was tested aboard the Research Vessel Point Sur during Project FIRE (First International Satellite Cloud Climatology Project Regional Experiment), 7?16 July 1987. Unusual sodar echoes were observed in the low marine boundary layer for two hours on 13 July 1987; they took the form of thin, intense echo layers that appeared abruptly at the minimum range of the sodar and increased steadily in range to as much as 150 m, before descending equally rapidly back into the minimum range. In each case, if this primary echo reached more than 20 to 30 m range (dependent upon the antenna elevation angle) it was followed by a second similar echo, which maintained a constant separation in range below the primary echo, before both dropped back in turn beneath the minimum range of the sodar. Such echo patterns have not been reported over the open ocean before, but similar structures have been seen over land and also over an ice shelf. Recent work (Cheung and Little 1990) has indicated that similar sodar patterns were associated with the passage of internal solitary waves with amplitudes comparable to, or even larger than, the depth of the associated stable layer. Such waves transport recirculating fluid; the sodar echoes are attributed to Bragg scatter from ?a/2-scale turbulent eddies produced at the interface between the ambient air and the (traveling) recirculating air. The sharp gradients in temperature, humidity, and velocity implicit in these observations would be likely to have significant effect on quasi-horizontal radio wave propagation and also on the flight of aircraft or helicopters operating at low-levels over the ocean.