HF radar detection of internal waves in the ocean

HF radar is an established technology for observing gravity waves on the ocean surface. The dominance of the Bragg scattering mechanism and the acute Doppler sensitivity of such radars enable them to measure both the ocean roughness spectrum and its transformation under advection by surface currents. Several hundred such radars are presently in operation around the world, most delivering maps of surface currents and significant waveheight in near-real time. Some provide estimates of the directional wave spectrum. While the utility of the radar products is unquestioned, it is noteworthy that they are properties of the ocean´s surface layer, not of its interior. In many applications it is desirable to know something of the prevailing dynamics of the entire water body, in particular the presence of energetic phenomena which may impact on offshore facilities or otherwise interfere with maritime operations. At present this can be accomplished only by costly in situ point measurements such as thermistor chains and expendable bathythermographs, or inferred under favorable conditions from satellite imagery. Accordingly the question arises, can HF radar, with its vast coverage and persistent surveillance capability, observe and perhaps characterize any of the interior dynamics? In this paper we investigate the possibility of detecting large-scale internal waves via the `signature´ of their interaction with the surface gravity wave field.