2.2. Wind Waves and Surface Tension

Wind wave interactions are a particularly complex problem since we still do not have a complete theory of wind wave generation and growth [ Donelan and Hui, 1990]. This further exacerbates the problem of remote sensing (see section 5) using techniques that depend on backscatter from the surface to determine wind fields [ Hsiao and Shemdin, 1983], or the stress [ Plant, 1982]. Geernaert [1990] summarized work on the effect of wind waves on the transfer coefficient of momentum. Fetch, wave age, the wave spectrum and water depth, all modify the transfer coefficient; however, there is still considerable uncertainty even as to the sign of the trend between the drag coefficient and sea state [ Smith et al., 1992].

Le Méhauté and Khangaonkar [1990] established the dynamic effect of intense rain on water waves, demonstrating that vertically falling rain dampen wind waves, but rainfall in high winds contributes to the growth of the amplitude of waves. The effect of surface slicks has also received a lot of attention in the past few years [e.g., Wei and Wu, 1992; Tang and Wu, 1992]. Their results indicate that films intensify the dampening of the short waves, which in turn reduces the surface roughness and consequently the wind stress. Gas and heat exchange is also retarded due to the presence of surface films. Recently, Walsh et al. [1994b] demonstrated that surface slicks observed in the tropical ocean may be related to tidally-forced internal waves. These surface slicks reduce surface tension and dampen the short waves. They also correspond with higher sea surface temperatures associated with surface convergence. The transfer of scalars across the air-sea interface also have a component that is dependent on sea state in the form of the production of spray and foam injected into the atmosphere (for a more detailed discussion, see Fairall et al. [1990a]). Sea spray affects the fluxes by increasing evaporation at the cost of the sensible heat flux and thereby increasing the stability of the air adjacent to the sea surface. These factors must also be considered in a complete model of air-sea exchange processes.

The Surface Wave Dynamics Experiment (SWADE), which took place between October 1990 and March 1991, was designed to understand the dynamics of the wave field in the open ocean, to determine the effects of waves on the air-sea transfer of momentum, heat and mass, to explore the response of the upper ocean to atmospheric forcing, and to explore the effect of waves on microwave remote sensing systems [ Weller et al., 1990]. Scanning Radar Altimeter (SCR) measurements made in the vicinity of the Gulf Stream and a warm core ring indicate that surface current fields strongly influence the propagation and refraction of low frequency surface gravity waves ( Shay et al., [1994] and Walsh et al., Surface wave-current interaction during SWADE, Part I: Observations submitted to Global Atmos. Ocean System). Using the sam Phillips et al. [1993a]. They developed an approximate theory relating the spatial and temporal configuration surrounding an extreme wave crest to the space-time autocorrelation function that compared well with the SWADE altimeter measurements [ Phillips et al., 1993b].