Spray Generation by a Plunging Breaker

An experimental investigation of droplet generation by a plunging breaking wave is presented. In this work, simultaneous measurements of the wave crest profile evolution and of droplets ranging in radius down to 50 mu m for a mechanically generated plunging breaker during many repeated breaking events in freshwater are performed. We find three distinct time zones of droplet production, first when the jet impacts the free surface upstream of the wave crest, second when the large air bubbles entrapped by the plunging jet impact reach the free surface and burst, and third when smaller bubbles burst upon reaching the free surface later in the breaking process. These subprocesses account for 22%, 44%, and 34%, respectively, of the average of 653 droplets produced per breaking event. The probability distributions of the ranges of large and small droplet radii are well represented by power law functions that intersect at a radius of 418 mu m. Plain Language Summary Sea spray is known to dramatically enhance the transfer of mass, momentum, and energy between the ocean and the atmosphere. Previous investigations have reported measurements of temporally averaged droplet size distributions and fluxes at fixed heights in wind-wave systems at sea and in laboratory-scale experiments. It is commonly accepted that droplets are primarily produced by three breaking subprocesses: wind shear (primarily at the wave crest), splashing, and the popping of breaker-entrained air bubbles that rise to the free surface. However, there are little experimental data that quantitatively link local droplet generation characteristics to these subprocesses. Here we present the results of simultaneous spatially and temporally resolved measurements of the crest profile evolution and similarly resolved measurements of droplets ranging in radius down to 50 mu m for a mechanically generated plunging breaker during many repeated breaking events. We find three distinct time zones of droplet production, first when the jet impacts the free surface, second when the large air bubbles entrapped by the plunging jet impact reach the surface and burst, and third when smaller bubbles reach the surface later in the breaking process. The results of this research can be used in steps to improve models of droplet motion and evaporation.