One outstanding lesson of the previous chapter is that wind waves play a central role in air-sea transfer processes. In this chapter, we examine how wind waves first arise on a wind-blown water surface, what laws govern their growth, what processes are responsible for their decay, and how exactly they facilitate the transfer of momentum and of scalar properties across the air-sea interface.

The Origin of Wind Waves

In a triumph of early twentieth century fluid mechanics, a combination of theory and laboratory experiment elucidated the origin of turbulence, as Schlichting (1960) has authoritatively chronicled. The theoretical advance was the development of stability theory for laminar shear flowin a boundary layer, with the “Orr-Sommerfeld Equation” (OSE) as its centerpiece and principal tool for analytical investigations. This equation describes the behavior of small-amplitudewave-like disturbances on laminar shear flow that spontaneously grow to large amplitude (“instability waves”). Instability waves in laminar boundary layer flow became known as “Tollmien-Schlichting waves,” after the theoreticians who explored their properties.

For some time, such waves eluded efforts at observing them in the laboratory, because laminar flow free of residual turbulence is almost impossible to maintain. Finally, in a wind tunnel made painstakingly clear of other disturbances, the experimentum crucium of Schubauer and Skramstad (1947) demonstrated that Tollmien-Schlichting waves of highest theoretical growth rate were the initially observed surface undulations. This verified the theory on the supposition that minute random disturbances of all other wavelengths were present from the beginning, but only the most unstable ones grew to an observable amplitude.