A Comparison of Fresh and Saline Spume Droplet Production in High Wind Conditions

Under high wind conditions, sea spray (particularly larger particles or sea spume) plays a significant role in the exchange of heat and momentum across the air-sea interface. It is thus critical for the development of tropical cyclones and other extreme marine boundary layer events that occur on a wide range of spatial and temporal scales over different bodies of water (oceans, rivers, and lakes). While considerable differences are known to exist in spray formation via bubble production between saltwater and freshwater, herein is described the first laboratory experiment quantifying the differences between fresh and saline spume droplet production in hurricane-force wind conditions (U₁₀ around 36-54 ms^-1), conducted in the Air-Sea Interaction Saltwater Tank (ASIST) facility at the University of Miami. Spume droplets were observed at radii from 80 µm to 1400 µm, from which radius-dependent horizontal and scaled height-dependent (z/H_s, where H_s is significant wave height) vertical profiles of number and mass concentration were obtained at five wind speed conditions in the aforementioned range. Number concentrations were found to increase with wind speed in both fresh and salt water, with radius-dependent number concentrations comparatively larger (1.5-2.5x) in the saltwater profiles. Radius-integrated (total) number concentrations from the vertical profiles were observed to vary according to a logistic sinusoid for both fresh and salt water, with increasing amplitude (wider range of r₀ coverage) as wind speed increases. While substantially larger number concentrations were observed universally for salt water (x2 vs. freshwater) near the surface, the vertical profiles converge (regardless of wind speed) at large values of the scaled-height. Radius-dependent differential (saltwater-freshwater) mass concentration profiles were found to exhibit a bimodal distribution, with the first (second) mode peaking near 400 (800) µm. Based on this finding, horizontal mass and number concentrations were segregated into three radius regimes (growth, transition/equilibrium, and fall-off) in an attempt to understand the physical dependence and forcings involved in the evolution of the spume droplets. The extent of each regime was dictated by the bimodal peaks and was quantified by the moving slope with radius of each profile. Normalization of the differences in number concentration between fresh and saltwater ((SW-FW)/SW) suggest spume production and distribution in the growth regime (0 – 400 µm) is determined more strongly by the wind forcing (‘input-driven’), while at the large-radius extreme (800 – 1400 µm), the fall-off rate is governed more by the spume droplet density (‘density-driven’). Spume source functions were formulated from the number concentration profiles to parameterize the spume production in terms of the size (r₀), forcing (U₁₀), and scaled-height (z/H_s) for both types of water. Ultimately, we seek a conversion function to relate fresh and saltwater production under a variety of real-world physical conditions.