Re-Imagining Sea-Spray Generation in Extreme Winds

The precise role of sea-spray generation in hurricanes and the associated heat and momentum fluxes has long been considered to be one of the important challenges in understanding the process of rapid intensification. Although considerable strides have been made in this regard, the field has been hampered by a basic conceptual gap in the understanding of spray production processes in hurricane winds. Spray generation has been largely attributed to a combination of breaking wave generation, bubble bursting and/or direct tearing of the water surface by waves. Andreas and Fairall (2010) provided a description of sea spray generation due to breaking waves and correctly noted that it was concentrated on the windward side of the wave crest and therefore had an initial upward component. However they did not provide a physical explanation for this phenomenon. Troitskaya et al. (2018) described the bag break-up mechanism for spray generation and Soloviev et al. (2017) and Vanderplow et al. (2020) pointed to the role of Kelvin-Helmholtz instabilities in extreme winds. Here, based on direct observation of the interface in the extreme winds generated in the University of Miami’s SUSTAIN laboratory and by direct numerical simulations we argue that the three distinct mechanisms are closely linked, but are mechanistically distinct from previously descriptions and that this has considerable implications for spray production processes. We will show that steeping of short waves on the windward crest of large waves due to hydrodynamic modulation leads to explosive K-H type instabilities that generate sheets of spray that are advected upward well above the crest height of the large waves. This leads to much longer residence times for large spume droplets than previously assumed. The second key observation is that the energetic breaking on the forward face of the wave entrains large amounts of air which initiates the “bag” formation and breakup also on the windward face of long waves. These two mechanisms constitute what has previously been referred to as direct tearing by wind and critically both occur where upward advection of the generated spray by the wind occurs.