Surface wave modulation of atmospheric refractivity and remote sensing over the ocean

Over the ocean low wind conditions offer the remarkable opportunity to observe dominant wave driving of the of the air flow as well as the wave-induced fluctuations of scalars. In particular, the wave-induced fluctuations of atmospheric humidity can have a profound influence on the pattern of radar signals propagation in the oceanic refractive duct.

The data analyzed in this work were collected from the Air-Sea Interaction Tower observatory located South of Martha's Vineyard, Massachusetts in a water depth of 15 meters. A mast in the tower was instrumented with 4 sonic anemometers measuring wind turbulence and air temperature, 3 hygrometers registering water vapor fluctuations, and two pressure sensors. Water surface elevation was measured directly beneath the instruments mast by a microwave sensor. The refractive index was calculated from these data.

At wind speeds below 4 m/s the wind velocities from all the instruments were clearly modulated by the the underlying wave field. In such conditions the low turbulent intensity is associated with little mixing which allows vertical gradients of atmospheric humidity to be formed and maintained. The wave-induced velocity fluctuations lead to a vertical displacement of the mean wind stream lines and an associated vertical displacement of the sheared humidity profiles. As observed, a sensor at a fixed hight registers wave-induced fluctuations of humidity. For a radio signal propagating over the ocean, such fluctuations effectively create semi-periodic refractive medium bending the trajectory of the radar beam. We determine that in case of uncorrelated waves and atmospheric turbulence, the structure function of the atmospheric refractivity conveniently splits into a turbulent and wave-induced parts. From the wind-wave interaction theory (wave-mean flow interaction only is considered) we evaluate the wave influence on the refractive structure function.