A Wave Follower for Field Study of Air-Sea Interactions

In the study of wave generation by wind, an exploration of the wind (or pressure) field close to the water surface is critical. An electrohydraulic servomechanism, a "wave follower," has been designed to hold a probe at small distances above the sea surface while following the surface. The follower, mounted underwater, senses the location of the water surface with a capacitance wave probe. A hydraulic system adjusts the position of the probe so as to seek a null. The follower\´s response to the surface extends to 4 Hz with good coherence, and it can follow fully developed water waves up to 30 cm in height. A hot-film anemometer probe senses the wind speed, while the wave probe records waves up to 12 Hz. The follower is remote controlled at distances up to 60 m. Two field trips were undertaken in 1969 at Miles River, Chesapeake Bay. Wind speed data were taken at 2 to 40 cm above the surface, with wind speeds of 5 to 15 knots and fetches of 1 to 6 nmi. The waves ranged approximately 10 to 30 cm in height. Preliminary investigation of the data shows, among other things, presence of the sheltering effect and a small but definite coherence between wind and waves at the wave peak frequency.