Spray Concentration Measurements from ASIST for Freshwater and Seawater

The size-dependent vertical distribution of spume particles in high wind conditions is necessary to understand their effect on air-sea fluxes of heat and momentum. The predominant focus of previous studies of spray dynamics has been on the marine environment. Spray dynamics in non-seawater bodies have not been extensively studied, and any significant differences between sea and freshwater remain unquantified. To address this gap, we have conducted the first laboratory experiment directly comparing spume concentrations above fresh and real seawater for 10-m equivalent wind speeds of 36-54 m/s. Droplets in the air above the intensely breaking wind-waves were directly observed and their distribution as functions of wind speed, height, and droplet radius was compared between the two water types. Spume droplets were imaged using a Dantec Dynamics particle image velocimetry (PIV) acquisition system modified to be used in a shadow imaging mode. A camera was positioned outside of the tank and oriented to be looking into a high intensity strobe, also mounted outside of the tank, but directly opposite the camera. For each of the vertical levels (3 total), the wind was allowed to ramp up and time (120 s) was allowed for the tank conditions to become stationary. Then at least 7 consecutive sets of 250 images were acquired for all five wind speeds. Substantially higher concentrations of seawater spume were observed as compared to freshwater across all particle sizes and wind speeds. The seawater particles’ vertical distribution was concentrated near the surface, whereas the freshwater droplets were more uniformly distributed. Seawater and freshwater height-dependent distributions exhibited different wind-speed dependences. These findings were generally unexpected and point to an unanticipated role of physiochemical processes in the spume generation mechanism which may impact spray-mediated flux parameterization over water bodies of different salinities. This dataset (.mat variable and a .m script) is associated to the above article “A Laboratory Investigation of Spume Generation in High Winds for Fresh and Seawater” by Mehta et al. (2019), submitted in JGR-Atmospheres. Specifically, this is the data used to create figures 2 through 8 in the paper.